This is to certify that the

thesis entitled

RODENT SCHISTOSOMIASIS : PREVALENCE

OF INFECTION AND PATHOLOGIC CHARACTERISTICS
presented by

Anne M. Zajac

has been accepted towards fulfillment
of the requirements for

Master of Science degree in Microbiology and Public
Health

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RODENT SCHISTOSOMIASIS: PREVALENCE

OF INFECTION AND PATHOLOGIC CHARACTERISTICS

BY

Anne M. Zajac

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
MASTER OF SCIENCE

Department of Microbiology and Public Health

1978

ABSTRACT

RODENT SCHISTOSOMIASIS: PREVALENCE OF
INFECTION AND PATHOLOGIC CHARACTERISTICS

BY

Anne M. Zajac

Studies were undertaken to determine the extent of natural
infection with Schistosomatium douthitti (Fam. Schistosomatidae)
in a local rodent population and to examine pathologic characteristics
of both natural and experimental infections with the parasite.
Wild meadow voles, Microtus pennsylvanicus, were trapped and examined
fer the presence of the blood fluke. Seventy percent of the 47
voles captured were found to be infected, in some cases with over
200 worms. S. douthitti infections were also found in 2 other
small rodents in the area: Peromyscus Zeucopus, the white footed
mouse and Zapus hudsonius, the jumping mouse.

Comparison of experimentally infected laboratory mice and
meadow voles showed that levels of infection which produced fatal
schistosomiasis in mice did not produce signs of clinical disease
in voles. Examination of histologic changes in the tissues of
infected animals revealed that both species produced a granulomatous
response to schistosome ova, but that differences occurred between
mice and voles in the cellular composition of the host reaction.

Naturally infected voles also appeared unaffected by infection with

S. douthitti. The tissue response of these wild voles resembled

that of experimentally infected voles.

ACKNOWLEDGEMENTS

I am especially grateful to Dr. J. F. Williams for his
enthusiastic support of all aspects of my degree program. His
discerning appreciation of current parasitological problems has
immeasurably increased my understanding not only of my own area
of research, but of the whole spectrum of host-parasite relationships.

I would also like to thank the members of my graduate committee,
Dr. Harold Miller and Dr. Donald Twohy, for their interest and
encouragement.

Special thanks are due to Dr. Harvey Blankenspoor and members
of the Schistosome Unit at the University of Michigan for providing
us with S. douthitti. I would like to acknowledge, too, the staff
of the Rose Lake Wildlife Research Station of the Michigan Department
of Natural Resources, for permitting field work at Rose Lake and
for their encouragement and advice.

I am also grateful to Dr. Rollin Baker and both former and
current students in the Mammalogy Division of the Department of
Zoology for their kindness in providing live traps for field work
and for their enthusiastic interest in this study.

Finally, I would like to express my appreciation to my
colleagues in the laboratory for their friendship and constant
helpfulness; I am particularly indebted to Alma Shearer whose
unflagging sympathy frequently made the disappointment of negative

results bearable.

ii

TABLE OF CONTENTS

INTRODUCTION .
LITERATURE REVIEW

Schistosome Life Cycle and Clinical Schistosomiasis .
Life cycle . . .
Mammalian schistosomes .
Clinical schistosomiasis .
Schistosome dermatitis
Acute schistosomiasis .
Chronic schistosomiasis .

Immunopathology of Chronic Disease

Biology and Epidemiology of Non-Human Mammalian
Schistosomes

Schistosomes of domestic animals . .
Other schistosomes of domestic animals .
Schistosomes of wild mammals . .
Other schistosomes of wild mammals .

Schistosomatium douthitti .
Natural infection
Laboratory infections
Life cycle . . .
Histological studies .
Immunological studies

.Microtus pennsylvanicus .
REFERENCES .

ARTICLE 1— PREVALENCE AND INTENSITY OF INFECTION OF THE
MEADOW VOLE (MJCROTUS PENNSYLVANICUS) WITH
SCHISTOSOMATIUM DOUTHITTI (FAM. SCHISTOSOMATIDAE)
IN MICHIGAN . . . . . . . . . . . . . .

ARTICLE 2- PATHOLOGIC CHARACTERISTICS OF INFECTION WITH
SCHISTOSOMHTIUM DOUTHITTI (FAM. SCHISTOSOMATIDAE)

IN THE LABORATORY MOUSE AND THE MEADOW VOLE,
MICROTUS PENNSYLVANICUS

iii

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26
28
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32

34
34
37
38
39
4O

42

45

59

LIST OF TABLES

Table
ARTICLE 1

1. Relationship between age of meadow vole
and worm burden .

iv

Page

63

Figure

10.

ll.

12.

13.

14.

15.

LIST OF FIGURES

Vole. Typical mononuclear cell periportal
infiltrate in the liver. x 433 .

Vole. Diffuse inflammatory reaction surrounding
ova in the liver. x 200.

Vole. Intense eosinophil response surrounding
ova in the liver. x 176 . . .

Vole. Hepatic giant cell granuloma. x 186 .

Vole. Typical intestinal giant cell granuloma.
Darkly staining plasma cells are present on the
periphery. x 440 .

Vole. Mature schistosome egg in cecal lamina propria

partially associated with giant cell reaction.
x 338 . . .

Vole. Complex hepatic reaction to schistosome
ova. x 160 .

Vole. Fig. 9 detail. x 433

Wild vole. Adult schistosomes and ova in cecum of
a heavily infected animal. x 40

Wild vole. Mature schistosome ova in the small
intestine unaccompanied by host response. X 64 -

Wild vole. Typical giant cell granuloma in the
liver. x 360 . . . . . .

Mouse. Mononuclear cell periportal infiltrate in
the liver. X 421 . . . . . .

Mouse. Undifferentiated ova in the intestine.
x 400

Mouse. Extensive inflammatory reaction in the liver.
Eosinophils are prominent. X 200 .

Mouse. Unstructured host response to ova. Numerous
cell types are present. X 205 .

Page

74

74

75

75

78

78

79

79

81

81

82

84

84

85

8S

Figure Page

16. Mouse. Epithelioid granuloma with lymphocytes and 88
eosinophils present on the periphery. x 400 .

17. Mouse. Granuloma showing prominent deposition of
connective tissue. x 400 . . . . . . . . . . . . . . . 88

18. Mouse. Mature ova in the lamina propria of the
small intestine. x 200 . . . . . . . . . . . . . . . . 89

19. Mouse. Intestinal granuloma containing mature
ova. x 150 . . . . . . . . . . . . . . . . . . . . . . 89

20. Mouse. Schistosome ova and extensive host reaction
in the small intestine. x 45 . . . . . . . . . . . . . 90

21. Diffuse host reaction in the submucosal tissue of
the small intestine. x 64 . . . . . . . . . . . . . . 90

vi

Introduction

 

Infection with schistosomes is currently estimated to occur in
some 200 million people throughout the world (Dorozynski, 1976).
The range of these blood flukes, digenetic trematodes of the
Family Schistosomatidae, is presently expanding as the extension
of irrigation systems in underdeveloped nations provides new
areas suitable for the snail intermediate host of the parasite
(Dorozynski, 1976). The widespread morbidity resulting from
the infection and its potentially fatal consequences have resulted
in the past two decades in the recognition of schistosomiasis
as a leading public health problem in portions of Africa, Asia
and South America. The World Health Organization of the United
Nations has listed schistosomiasis as one of six parasitic diseases
to be attacked in a special program of Research and Training in
Tropical Disease (WHO, 1978). Efforts to date to control and
eliminate the parasite have revolved around chemotherapy and
snail eradication. Both methods are not only costly, but labor
intensive and emphasis has currently been placed on the development
of vaccines and more effective drugs for mass treatment (WHO, 1978).
Although the importance of human schistosomes has overshadowed
other aspects of infection with this group of trematodes, blood

flukes are not uncommon parasites of other non-human mammals

and birds (Yamaguti, 1958). Infection rates amongst wild and
domestic animals may be high and Lawrence and Condy (1970) for
example, have reported infection rates of Schistosoma mattheei
of up to 92% in Rhodesian cattle. Outbreaks of severe disease
may result in substantial economic loss to the farmer (Strydom,
1963). Hussein et a1 (1975) have referred to a report of an
outbreak of Schistosoma bovis in the Sudan in which 8,000 head of
livestock died. While schistosomes in animals are therefore important
in their own right, their relevance to human disease should not be
overlooked. Cases of human infection with schistosomes of other
mammals have been reported. Pitchford and Visser (1962), in a
survey of a South African community, found 80% of the cattle and
40% of the women and children infected with S. mattheei. Infections
of both wild and domestic animals with S. mansoni and S. japonicum
in some areas may act as important reservoirs of infection for
the human population (Martins,l958). Finally, although the species
of host and parasite may vary, the pathogenesis of the disease
appears to be largely similar, and these animal systems may thus
provide useful model systems for the study of the mechanism of
disease in human schistosomiasis.

This last consideration may prove to be especially significant
in the light of increasing interest in the immunological response
of the host to the parasite and the potential for vaccine development.
Much of the current understanding of schistosomiasis has grown
from experimental studies conducted primarily with infections
of laboratory mice with the human parasite S. mansoni. Warren (1964)

has maintained that the mouse provides an experimental system in

which the parasite behaves as it does in the human, but whether

this correspondence is exact in all aspects is questionable.

Cheever (1965b), in a study performed with several different

species of rodents, found that the cellular reaction to the

parasite varied and suggested that caution be used in interpretation
of results in laboratory animals. Von Lichtenberg (1968) points

not only to the difference in cellular reaction between mice and
humans, but also to differences in the hepatic vascular anatomy

of the two species.

The situation is further complicated by apparent strain
differences within the species of fluke. Infections of S. mansoni
in wild rodents in Africa appear to be incidental and unlikely
to be maintained independently of the human cycle of infection
(Pitchford and Visser, 1962). In South America, however, infection
rates in wild rodents may be high and the parasite seems to be
successfully established in and transmitted by these animals
(Antunes et a1, 1973).

This type of qualitative difference in reaction to natural
infection with the human parasite in rodents can also be seen,
although less strikingly, in laboratory infections (Warren, 1967;
Hsu and Hsu, 1960), and may be especially misleading in attempts
to establish the degree of immunological resistance developing
in infected individuals. The presence of immunity to reinfection
in man has never been satisfactorily established (Warren, 1973a)
and it is probably dangerous to attempt to generalize from the
reactions elicited by the parasite in a host for which it is not

primarily adapted. It may therefore be more valuable to develop

an understanding of the development of disease and the formation
of immunological resistance in the natural host-schistosome
systems.

Knowledge of the epidemiology of schistosome infections,
like that of the pathogenesis of the disease, has been derived
exclusively from studies with the human schistosome species.
Workers in the field, frequently medical personnel, have produced
many surveys of rates of infection in endemic areas. These reports,
while useful in directing eradication and treatment efforts to
areas of high endemicity, have done little to answer questions
about the frequency and intensity of exposure encountered by
individuals in these areas, and the actual extent of debility
due to disease in a community. MacDonald (1965) and Hairston
(1965), amongst others (see Cohen, 1977), have proposed mathematical
models for human schistosome populations but these have proven
largely impractical as predictive tools in field situations.

Prohibitions against human experimentation contribute to the
difficulty in understanding the mechanisms of transmission of the
parasites. Cultural factors which influence water contact and
perceptions of infection and disease, further obstruct accurate
interpretation of data. Warren (1973a) has suggested that evidence
of decreasing egg production with increasing age, which has been
commonly accepted as indirect evidence of resistance to infection
in humans, may be as convincingly explained by reduced contact
with water as individuals grow older. The enumeration and elim-
ination of these various social and cultural factors in a field

situation present an almost insurmountable difficulty. Here too,

the study of the non-human mammalian schistosomes may prove to
be an exceptionally valuable means of developing an appreciation
of the basic parameters of schistosome infection.

The following pages contain the report of preliminary work
on attempts to utilize one of these mammalian schistosomes---
Schistosomatium dbuthitti. Both basic epidemiological and path-
ological studies are presented. The Literature Review provides
a brief comparative picture of schistosomiasis in man and animals
and a short review of the current knowledge of the non-human

mammalian schistosomes.

LITERATURE REVIEW

Schistosome Life Cycle and Clinical Schistosomiasis

Life Cycle

 

Adults of the species belonging to the Family Schistosomatidae
parasitize the blood vessels of various organs. Unlike other
Trematodes which are hermaphroditic, adult schistosomes are dioecious.
Eggs passed in the feces or urine (and, in the case of Schistosoma
nasale, in the nasal secretions) of the definitive host hatch
when they contact water to release a swimming, non-feeding larval
stage---the miracidium. During its short life span of a few hours,
this larval form must locate a suitable snail intermediate host.
Metabolic products of the snail in the water act as attractants
and stimulants to the miracidium which penetrates through the skin
of the snail (Chernin, 1974). In the sporocyst form within the
snail, the parasite undergoes a phase of repeated asexual division
to produce the next phase of the life cycle, the cercaria. Under
the appropriate environmental stimuli, which vary with the species
of schistosome, cercariae are released into the water (Wright, 1971).
Although other digenetic trematodes must pass through a further
stage of larval development (the metacercaria), within the schistosomes
it is the cercaria which is infective for the definitive host. The
cercaria, responding to chemical stimuli like the miracidium,

penetrates through the skin of the host with the aid of enzymatic

secretions (Smithers, 1976). After a period of migration and
maturation the schistosome finally comes to rest in its appropriate
location within the circulatory system. This simple life cycle,
combined with the primitive sanitary facilities prevalent in

many developing nations, make infection in endemic areas difficult,

if not impossible to avoid.

Mammalian Schistosomes

 

Three species of schistosome are important in the generation
of disease in humans. S. mansoni, found in the mesenteric venous
system, is distributed over a wide area, including parts of Africa,
the Middle East, the West Indies and South America. S. japonicum,
which, like S. mansoni, lives in the mesenteric veins of the host,
occurs throughout much of the Far East. The range of the third
species of human schistosome, S. haematobium, extends through
large areas of Africa and the Middle East. Unlike the adults
of the other two species, those of S. haematobium are found
in the vesical and pelvic venous plexuses. Although human infections
with other species of schistosome do occur (for example, S. mattheei
and S. intercalatum), they are not considered of widespread importance
as causes of disease and will not be further discussed here as
human parasites (Hunter et a1, 1976).

A variety of species of schistosomes occur in non-human
mammals ranging from rodents (Price, 1931) to hippopotami (McCully
et a1, 1967) and elephants (Rao and Hiregandar, 1953). The
difficulty involved in examining dead animals for the presence of

schistosomes, and the failure of schistosome eggs to float in

standard flotation solutions, have probably led to an underestimation
of the number and range of species. These mammalian schistosomes,

as a group, appear to be highly successful and are distributed

over a large part of the world. With only a few exceptions, adult
parasites are found in the mesenteric veins of the host. The
following discussion will concentrate on those schistosomes,

of both man and animals which occur in the intestinal venous system.

Clinical Schistosomiasis

 

Schistosome Dermatitis

Several distinct clinical syndromes are associated in both
man and animals with the different stages of the parasite life
cycle and with variation in duration of infection. The first
form of the parasite to be encountered is the cercaria, and response
by the host will depend on the species of parasite and the immuno-
logical experience of the host. That non-human parasites will
readily penetrate human skin was first appreciated by Cort in
Michigan in 1928. Further work in Michigan has shown that the
cercariae of Trichobilharzia and Gigantobilharzia, parasites
of birds, and of Schistosomatium douthitti, a schistosome of
rodents, will produce the familiar condition known as "swimmer's
itch" (Cort, 1950). This dermatitis has been shown by Olivier
(1949) to be a sensitization phenomenon. Initial exposure to
the cercaria results in mild itching directly associated with
penetration. Macules may develop, but are not long-lived, rarely
lasting twenty-four hours. These primary exposures commonly go

unnoticed or are attributed to other causes. Subsequent penetration

by cercariae, however, will result in a characteristic allergic
reaction with intense and long lasting itching, erythema and
production of papules which may last as long as a week (Olivier,
1949). Histologically, in both man and experimental animals,
secondary exposure is marked by congestion and infiltration at
the site of penetration by polymorphonuclear cells, lymphocytes
and macrophages. Primary exposure produces this same infiltrate
but at a much reduced intensity and with a smaller component of
monocytes (MacFarlane, 1949, Olivier, 1949; Kagan and Meranze,
1955; Batten, 1956). In massive infections of S. douthitti
in rhesus monkeys, Kagan (1953) was able to demonstrate that a
few abnormal adults developed in the liver, but the normal fate
of incompatible cercaria is entrapment and death in the dermal
and basal layers of the skin. Sensitized individuals show enhanced
clearance of the dead cercariae by neutrophils and phagocytes
(MacFarlane, 1949).

Whether or not the human schistosomes produce this same
syndrome of sensitization and dermatitis in their natural host
is not clear. In early work in an area in Egypt endemic for
S. haematobium, Barlow (1936) reexposed local peasants, known
to be infected, to cercariae of S. haematobium and was able to
show the development of an apparently typical schistosome dermatitis.
The local population seemed familiar with the condition and associated
it with schistosomiasis. Barlow also infected himself with S.
haematobium and reported characteristic signs of' schistosome
dermatitis (Barlow and Meleney 1949). Later authors (Elwi, 1967;

Jordan and Webbe, 1969) however, have suggested that dermatitis

10

due to human schistosomes is uncommon in endemic areas, although
Europeans seem to develop the condition following exposure to
cercariae. The widely documented dermatitis known as "Kabure"
which occurs in Japan and other areas of the Far East endemic
fbr S. japonicum, appears to be caused largely by penetration

of cercariae of an avian schistosome (Miyake, 1967).

In animals also, penetration by cercariae of schistosomes
specific for the host does not seem to provoke a significant
dermatitis. Redness at the site of penetration has been observed
by Lawrence (1977a) with S. mattheei and by Hussein (1971) and
Massoud (1973) in experimental infections of calves with S. bovis.
A mild maculopapular reaction to experimental infections of

S. douthitti in laboratory mice was described by Batten (1956).

Acute Schistosomiasis

The path followed by the immature worm, or schistosomulum,
once the skin is penetrated has not been thoroughly documented.
It is most commonly believed that after a short period in the skin,
the young worm enters a capillary and is carried to the heart and
lungs and thence to the systemic circulation. An alternative
hypothesis suggests that upon reaching the lungs the schistosomulum
leaves the circulatory system and migrates through the diaphragm.
Kagan (1958) found large numbers of S. deuthitti in the thoracic
cavities of experimentally infected mice. In massive infections,
respiratory symptoms may be associated with this phase although
lower levels of infection are rarely associated with clinical

disease. Olivier (1952) found rapidly resolving focal hemorrhages

11

in the lungs of mice infected with S. douthitti, S. japonicum and
S. haematobium. Interestingly, far fewer hemorrhages were produced
by the S. mansoni infections, the schistosomula were found in the
lung over a much longer period of time and a smaller proportion of
cercariae administered eventually established as adults than

with the other two species. Olivier suggested that these obser-
vations may reflect the behaviour of a parasite in an abnormal

host as compared to that of S. douthitti and S. japonicum, both
schistosomes normally occurring in rodents.

Following this migratory phase adult worms are found in the
mesenteric veins (or the veins of the vesical plexus in the case
of S. haematobium) where copulation occurs and egg deposition
begins. Female worms lie within the depression formed by the
body of the male known as the gynecophoric canal, leaving it only
to migrate as far as possible into the capillaries where the ova
are deposited. Estimates of the number of eggs produced per day
are as high as 150-300 for female S. mansoni, while S. japonicum
females may produce some ten times that number (Warren, 1973b).

To reach the external environment and continue the transmission
cycle, the egg must penetrate the capillary wall and pass through
the tissue of the intestine, finally breaking through into the
lumen of the gut where it is incorporated into the fecal material.
Ova of S. haematobium correspondingly penetrate through the wall
of the bladder and are passed out of the body in the urine. This
passage through the tissue occurs over a period of several days
and may be aided by the production of proteolytic enzymes as well

as by the peristaltic movements of the gut, in the case of parasites

12

of the mesenteric veins (Jordan and Webbe, 1969). Miracidia within

the ova are immature when the eggs are deposited and develop to

maturity during this phase of tissue migration. The period required

for the infection to become patent (determined by the presence

of eggs in the feces or urine) varies from as little as 28 days

for S. douthitti (Price, 1931) to 109 days described in a voluntary

infection with S. haematobium (Barlow and Meleney, 1949).
Clinically, the period of final migration of worms and

increasing production of metabolic products coupled with the in-

itiation of ovoposition is associated with an acute or toxemic

disease syndrome. In steers heavily infected with S. mattheei,

Lawrence (1977a) observed severe diarrhea, with blood in the

feces, abdominal pain and straining coincident with the beginning

of egg production by the parasites. Similar observations were

made by Massoud (1973) in calves, sheep and goats infected with

S. bovis. Chimpanzees heavily infected with S. mansoni showed

an elevation in temperature and bloody diarrhea six weeks after

infection (Sadun et a1, 1966). Acute signs of S. mansoni infection

in humans, like the cercarial dermatitis, are reported primarily

from Europeans (Jordan and Webbe, 1967). Beginning 3-8 weeks

after exposure, the toxemic stage of infection generally takes

the form of a mild febrile illness. Acute schistosomiasis is

a much more frequent manifestation of infection with S. japonicum.

This characteristic acute syndrome, known as Katayama Fever,

occurs 2-12 weeks following exposure and varies in severity in

relation to the intensity of infection. Fever, diarrhea and

generalized or abdominal pain and nausea are the most common

l3

presenting symptoms (Hunter et al, 1961; Edington and Gilles,
1976).

With the possible exception of Katayama Fever and isolated
cases of infection of Europeans with the African schistosomes,
the acute form of schistosomiasis accompanies only heavy exposure
to the parasite. Mild experimental infection in general produces
either no acute disease or disease of such a mild nature that
its identification in field situations would be difficult (Jordan

and Webbe, 1969).

Chronic Schistosomiasis

The estimated average life span of the human schistosomes
is 5-10 years with some convincing evidence that indiviual worms
may live as long as twenty years (Warren et a1, 1974). The presence
of resistance to reinfection in humans has not yet been proven
(Warren, 1973a), and in areas endemic for the disease individuals
can probably expect to be infected throughout much of their
lives. Consequently, schistosomiasis is most commonly presented
as a chronic disease.

For a complete discussion of the clinical signs and symptoms
associated with chronic schistosomiases the following references
may be consulted: Edington and Gilles, 1976; Hunter et al, 1976;
Jordan and Webbe, 1969 and Mostofi, 1967. Briefly, signs and
symptoms associated with chronic schistosomiasis mansoni and
japonicum include varying degrees of abdominal pain, nausea, and
diarrhea containing blood and mucus. Grossly, the intestinal

tract appears reddened with occasional petechial hemorrhages and

14

tortuous mesenteric veins. In some cases, particularly in infections
of S. japonicum, ulcers may develop. Inflammation of the submucosa
leads to epithelial hyperplasia resulting in the formation of polyploid
lesions. Liver, spleen and mesenteric lymph nodes are often enlarged
and discolored by the accumulation of schistosome pigment. The

liver is frequently spotted with small white foci. In severe

cases Symmer's clay pipe-stem fibrosis with its characteristic
lesions along the portal tracts may be observed. Portal hypertension
develops and leads to ascites and may be accompanied by the develop-
ment of esophageal varices. Rupture of these varices may result

in rapidly fatal hemorrhage. In occasional cases dyspnea or
pneumonia may develop as a consequence of infection. If symptoms

are severe enough death may result, but a fatal outcome of infection
is rare.

S. haematobium, because of its location in the urinary tract,
produces disease of a different nature. The most characteristic
symptom of infection is hematuria. In areas of Egypt where the
prevalence of disease is high, the appearance of blood in the urine
was, until recently, considered an important sign of virility
(Makar, 1967). Further symptoms reported are fever, headache,
pain at micturition and muscular pain. Gross organ lesions are
not as apparent as in the hepatosplenic schistosomiases. Macroscopic
lesions of the bladder most commonly take the form of so-called
"sandy patches", in which the mucosa is roughened and raised and
the overlying epithelium is thickened. Ureters, the reproductive
system, liver and lungs may be involved in the infection. Lesions

of the ureters may be severe enough to result in obstruction with

15

consequent hydronephrosis. Involvement of the gastrointestinal
tract is rare, although some lesions may be found in the rectum.
An association between infection with S. haematobium and carcinoma
of the bladder has been frequently recognized in endemic areas.
The naturally occurring schistosomes of other mammals produce
lesions of the same nature as the human parasites. The hepato-
splenic form of the disease results in fibrotic changes in large
areas of the liver (Basson et a1, 1970; Hussein et a1, 1973;
Bartsch and Ward, 1976; Lawrence, 1978a) Hemorrhagic lesions
of the intestine have been reported (Hussein et al, 1973). However,
the importance of the disease in natural infections is difficult
to assess. Fatal infections in experimental animals exposed to
large numbers of cercariae occur at the same time as, or shortly
after, egg deposition begins (McCully and Kruger, 1969; Malherbe,
1970). Descriptions of field outbreaks of schistosomiasis in
domestic animals also appear to be more closely related to acute
rather than chronic disease (Strydom, 1963; Hurter and Potgieter,

1967; Lawrence and Condy, 1970; Reinecke, 1970; VanWyck et al, 1974).

Immunopathology of Chronic Disease

 

For many years the etiology of the lesions of chronic schis-
tosomiasis was poorly understood. The prevalence of the disease
in areas where malnutrition and other parasitic diseases are
widespread made it difficult to establish which pathological
changes could be specifically attributed to schistosome infections.
Malnutrition was frequently invoked by early workers as the cause

of the hepatomegaly and cirrhotic liver lesions seen in patients

16

(Warren, 1972).

As awareness grew of the potential severity of schistosomiasis
attention was directed to the adult fluke as the cause of disease.
However, while the toxemic stage of schistosomiasis seems to be
related, at least in part, to the migration and metabolic products
of the worms themselves, the flukes normally do not appear to play
a major part in the production of the chronic lesions. McCully et a1
(1967) have suggested that the generalized endophlebitis seen in
infected hippopotami may be due entirely to the presence of adult
S. hippopotami. At the same time, they point out that the hippopot-
amus may be an abnormal host of the parasite. However, the cause
of the endophlebitis seen in cattle infected with S. mattheei
(McCully and Kruger, 1969; Hussein, 1971; Lawrence, 1978b)and
S. bovis (Massoud, 1973) may be associated with the presence of
adult worms. Photomicrographs of the human schistosome species
show adult worms eliciting no response in the walls of the veins
surrounding them (Smith and von Lichtenberg, 1974). Similarly,
the existence of a toxin elaborated by adult worms and producing
chronic hepatic lesions has not been demonstrated. The absence
of disease in laboratory animals with massive unisexual infections
(Warren, 1961) and the finding by Raslavicius (1965) that hepatic
lesions did not develop in the uninfected parabiotic partner of
an infected animal also point to the relatively nonpathogenic
nature of the living worms. It has also been suggested that it
is instead the dead flukes and the reaction that they elicit
in the liver that produces the chronic fibrosis and portal hyper-

tension. However, treatment of infection and death of large numbers

17

of worms in mice before the initiation of ovoposition does not
result in the production of disease (Warren, 1961).

The importance of the schistosome egg in the generation
of schistosomiasis has come to be appreciated only within the
last two decades, largely, as will be discussed below, through
studies by Kenneth Warren and his coworkers. Following the deposition
of ova in the capillaries by the female worms, many eggs are able
to migrate successfully through the intestine and reach the
external environment. Others are surrounded by the tissue reaction
of the host, and die within it. A significant number are swept
up by the circulation and carried to other organs where they are
trapped and killed by the host response. In S. japonicum and
S. mansoni infections the presinusoidal capillaries of the liver
capture the majority of these eggs, which are transported in the
portal circulation. Only in long-lasting infections are ova seen
in significant numbers in the lungs and other organs (Edington
and Gilles, 1969).

As the ova become lodged in small vessels they form a mechanical
barrier to blood flow. In mice with severe hepatosplenic disease,
however, Bloch et al (1972) demonstrated that even large numbers
of eggs alone would not significantly affect portal flow or
parenchymal perfusion. It is rather the granulomatous host reaction
surrounding the ova which interrupts portal circulation and leads
eventually to portal hypertension and arterial compensation
(Andrade and Cheever, 1971). Cheever (19653) found a direct
relationship between the size of the granuloma surrounding the

eggs and the development of portal hypertension. The damage

18

caused by the ova in the vessels they block and the resulting host
reaction leads also to the massive fibrotic changes seen in chronically
infected livers (Warren, 1972).

Further observations of the granulomatous response to the
eggs of S. mansoni in mice indicated that immunological sensitization
of the host to the eggs occurs shortly after the onset of egg
production. Eggs appearing in the tissue soon after ovoposition
begins do not elicit as intense a granulomatous reaction as those
appearing several weeks later in the course of infection (Warren,
1972). In an experimental procedure devised by von Lichtenberg
(1962) schistosome ova are injected via the tail vein of mice
and become trapped in the lungs where granuloma formation can
be observed. Under these conditions, more uniform and easily
controlled than those occurring during infections with adult
flukes, it has been shown (Warren and Domingo, 1970) that secondary
exposure to ova results in more rapid cellular infiltration and
granuloma formation. Bentonite particles injected by the same
method elicit a constant foreign body reaction despite previous
exposure. If bentonite particles are coated first with a soluble
egg antigen preparation (SEA) and then injected into mice previously
sensitized with an intraperitoneal injection of S. mansoni eggs,
the reaction elicited by the bentonite and SEA will be identical
to the granuloma forming around ova in the lungs (Boros and Warren,
1970).

Additional evidence of immunological sensitization has been
provided by the ability to significantly reduce the formation of

granulomas in the lung model with the use of immunosuppressive

19

drugs (Domingo et a1, 1967), thymectomy (Domingo and Warren, 1967),
and antilymphocyte serum (Domingo and Warren, 1968b). This
sensitization can be transferred with the spleen or lymph node
cells of infected animals (Warren et al, 1967). On the basis of
these observations, Warren has proposed that granuloma, or
pseudotubercle formation in response to schistosome ova is
consistent with an immunological reaction of the delayed type
hypersensitivity category, and that schistosomiasis is consequently
an immunopathological disease (Warren et al, 1967).

The substances which act as antigens in the delayed type
hypersensitivity response (DTH) have not yet been completely
identified. Eggs when laid are immature, development of the
miracidia occurring throughout the period of tissue migration,
and proteins released by the ova to facilitate passage through
the tissue may play an important part in the stimulation of
granuloma formation (Warren, 1976). Hang et a1 (1974) have shown
that immature eggs possess little ability to induce a host reaction.
Miracidia alone, however, are incapable of producing the pseudo-
tubercle, as are purified egg shells, indicating the importance
of the whole egg as an antigenic stimulus (von Lichtenberg and
Raslavicius, 1967). Some work has been done that indicates that
products of the schistosome eggs may stimulate other host reaction
pathways. Using the lung granuloma system of von Lichtenberg,
Smith et al (1971) have shown that SEA coupled to bentonite particles
is unable to sensitize mice to further injections of SEA unless
lysophosphatides of the type released by the egg are also present.

These workers suggest that the lysophosphatides released by the

20

ovum may have an adjuvant effect and facilitate uptake of egg
antigens by phagocytic cells through activation of inflammatory
pathways. Kellemeyer and Warren (1970) were able to cause partial
suppression of granuloma formation around eggs of S. mansoni in
mice by previous administration of ellagic acid, which consumes
kininogen, the parent molecule of bradykinin. They propose that
the inflammatory pathways may play a greater role in the overall
DTH reaction than is currently appreciated.

The sequence of events which produces the egg granuloma has
been followed histologically in an almost bewildering variety of
host-schistosome systems. The most extensively studied of these
have involved the human parasites in a number of laboratory rodent
hosts. However, histopathological studies have been performed
with S. bovis infections in cattle (Hussein, 1971, 1975) and
sheep (Hussein et al, 1976), with S. mattheei in cattle and sheep
(McCully and Kruger, 1969; Lawrence, 1978a, b); with S. incognitum
in sheep (Srivastava and Dutt, 1962); with S. douthitti in mice
(Kagan and Meranze, 1957, 1958) and with Heterobilharzia americana
in raccoons (Bartsch and Ward, 1976) amongst others.

Several classificatory schemes have been proposed (Hsu et al,
1972; von Lichtenberg et al, 1973) to characterize discrete stages
of host reaction to the ova. While such descriptive schemes are
perhaps useful in determining predominant cell types at different
periods during granuloma formation and resolution, they also tend
to obscure the nature of the granuloma as an actively evolving
pathological process. The establishment of these categories also

emphasizes differences between host-parasite systems which may be

21

irrelevant to a wider appreciation of the mechanism of host response
to the parasite.

Despite the variety of schistosome species and hosts employed
a generally uniform picture of host response can be drawn. The
arrival in the tissue of the newly deposited immature egg elicits
only a mild inflammatory reaction. As the miracidium within the
egg matures, polymorphonuclear cells, particularly eosinophils,
and monocytic cells accumulate. Macrophages become increasingly
prominent and an epithelioid granuloma, often containing giant
cells, surrounds the egg. Usually there is an accompanying
peripheral halo of lymphocytes, plasma cells and eosinophils.

With the death and destruction of the egg, fibroblasts and collagen
deposition predominate. Most commonly, all traces of the egg
disappear, but in some cases, particularly with ova of S. japonicum,
calcification may occur.

The development of the granulomatous response to the egg is
rapid. In naive mice, S. mansoni ova injected into the tail vein
produced granulomas in the lung which reached peak diameter in
16 days and thereafter declined in size, although some reaction
was still visible after 96 days (Warren and Domingo, 1970).

In sensitized animals the same peak intensity was reached in only
8 days. In the same study the rate of clearance of the ova from
the lungs of mice was examined. After 96 days only 15% of the
original inoculum of S. mansoni eggs remained. In sensitized
animals this rate of clearance was increased. Modulation of
granuloma formation occurs as the infection progresses. In mice

infected with S. mansoni for 16 weeks or more, granulomas forming

22

around newly arrived eggs are smaller than those which developed
earlier in the infection (Andrade and Warren, 1964; Domingo and
Warren,1968a). The activity of blocking antibody or suppressor
T cells has been suggested as the possible means by which such
modulation might be achieved (Ramalho-Pinto et al, 1976; Warren,
1976).

As in many other helminth infections, the eosinophil appears
to be a consistently important cell in the host response to the
schistosome eggs. It is frequently the dominant cell in early
granuloma formation (Cheever, 1965b; Hsu et al, 1973) and may
be found in the generalized inflammatory infiltration which develops
in the portal tracts of the liver and in the intestine (Hussein,
1972; Lawrence, 1978). Peripheral eosinophilia is encountered
in animals infected with schistosomes, usually reaching a peak
during the period of initial egg production (Hussein and Tartour,
1973; Mahmoud et al, 1975; Lawrence, 1977b). Despite its apparent
significance, the role of the eosinophil in the host reaction
is poorly understood. Several recent studies, however, have
indicated possible functions of the cell. Mahmoud et al (1975b)
have reported that the partial immunity of mice to secondary
infection with S. mansoni can be eliminated by treatment of
animals with anti—eosinophil serum. James and Colley (1976),
have proposed, based on in vitro studies, that the eosinophil
is an important agent of egg destruction. The involvement of the
eosinophil in the modulation of inflammation (Leid and Williams,
1978) may also prove to be of importance in the understanding

of its significance in schistosome and other helminth infections.

23

Despite the frequency with which the general pattern of
pseudotubercle formation occurs, variation exists not only between
host species, but within a species and even within individual
animals. In a comparative study of S. mansoni infections in
several laboratory rodents, Cheever (1965b) found that while
eosinophils and fibroblasts were the most frequent cell types
encountered in the hepatic granulomas of mice, macrophages were
more prominent in multimammate rats and mast cells were present
in unusually high numbers in the granulomas of gerbils. Von
Lichtenberg (1973) observed variation in the responses of individual
hamsters to eggs of all three human schistosome species, and
Bartsch and Ward (1976) in a study of H. americana found that
occasional ova in the liver of raccoons were surrounded by eosino-
philic microabscesses rather than the usual granuloma.

The importance of such differences in the reaction to the
ova is difficult to evaluate, but probably, in general, it is
not indicative of major differences in the response to the fluke
and its ova. However, Warren and his colleagues (Warren et al,
1975) have suggested within the past several years that the response
of laboratory animals to the eggs of S. japonicum may reflect
a qualitatively different reaction than is observed with the other,
thoroughly studied human schistosomes. Studies of normal infections
of S. japonicum in mice have shown that ova do not elicit a
granulomatous reaction as soon after their arrival in the tissue
as do eggs of S. mansoni and S. haematobium. When a response does
occur it frequently results in the production of large eosinophilic

necrotic granulomas. Normal infections with adult S. japonicum

24

or injection of eggs intraperitoneally do not sensitize mice
to subsequent intravenous injection of eggs. Although both these
methods are effective in sensitizing animals to S. mansoni or S.
haematobium, the authors point out that the observed difference
may only represent a requirement for higher concentrations of
antigen to successfully sensitize animals to S. japonicum (Warren
et al, 1975).

S. mansoni has been found naturally only in a few species
of rodents and primates besides man. S. haematobium appears to
be a successful parasite only of humans. However, S. japonicum
is capable of utilizing a number of species of wild and domestic
animals, as well as humans, as final hosts (Jordan and Webbe,
1969). This relative nonspecificity may reflect a less highly
evolved host-parasite relationship than exists for the other two
species. The host response to the parasite and its ova may be
correspondingly less efficient. Infection with S. japonicum
has always been considered the most severe of the human schisto-
somiases (Warren et al, 1975) because of the heavier egg output
by the female. Alternatively, this difference in severity of
infection may support the suggestion of Fine et a1 (1973) that
a highly evolved and specific DTH response is a largely advantageous
mechanism for destroying a pathogen of the nature of the schistosome
ovum. The granuloma provides a rapid and local means of destroying
eggs and egg antigens. Furthermore, under conditions in which
granuloma formation is inhibited by means of thymectomy, irradiation
and antilymphocyte serum eggs become surrounded by large areas

of coagulative necrosis with destruction of hepatic parenchyma.

25

Invasion by bacteria is common and the survival of animals is
significantly reduced (Buchanan et a1, 1973). Further investi-
gations may reveal that those individuals suffering most severely
from schistosome infections are those least capable of mounting
an efficient DTH response to the ova. If this should be the case,
the current understanding of schistosomiasis as an immunological
disease may require modification.

The role of DTH reactions in other helminth infections has
received little attention. Studies utilizing Capallaria hepatica
have been performed by Solomon and Soulsby (1973)and by Raybourne
et a1 (1974). This nematode parasite resides in the liver of
rats or man and migrates through the liver parenchyma depositing
ova which are retained in the liver and liberated only when the
liver is ingested by another animal. A granulomatous, anamnestic
DTH response develops to these eggs which appears strikingly
similar to the schistosome granuloma. Work with other helminths
may reveal a more important role for DTH in the immunological
response to parasitic infections than that presently attributed

to it.

Biology and Epidemiology of Non-Human Mammalian Schistosomes

 

Many aspects of the biology and life cycles of the schistosome
parasites of both wild and domestic animals have been adequately
described in the literature. However, while there are some reports
of the prevalence of infection in endemic areas, little attention
has been paid to the development and severity of the disease

in infected animals or to the possible role played by schistosomes

26

in the regulation of wild animal populations. There are several
factors which have contributed to the scarcity of information.

As in the human schistosomiases, the predominantly chronic nature
of the disease in animals makes it difficult to diagnose clinically.
Even the more clearly demarcated clinical syndrome associated

with acute schistosomiasis may be hard to differentiate from other
conditions. VanWyck et a1 (1974) have cautioned that rinderpest,
ngana, coccidiosis, arsenical poisoning and Johne's Disease all
share some clinical signs with schistosomiasis mattheei in South
Africa. Additionally, these mammalian flukes are primarily para-
sites of tropical regions where interest and money available

for research are directed to the human schistosomes. In these
areas, standards of animal husbandry may be low and veterinary
care inadequate. Consequently, the picture which can be assembled
of the biology and epidemiology and animal schistosomiasis is at

best fragmentary.

Schistosomes of'domestic animals

A variety of schistosome parasites of domestic animals is
found in domestic animals in the Far and Middle East, India,
Africa and Southern Europe. S. japonicum is shared by approx-
imately thirty other species of mammal in the Far East (Cheng, 1971).
However, its importance as a pathogen in its non-human hosts
has been entirely overshadowed by the human disease. Interest
in these animal infections has revolved around their roles as
reservoirs of human infection. The prevalence of infection may

be quite high, as indicated by a survey conducted by Pesigan et a1

27

(1958) in the Philippines. They found that 38% of the cattle and
22.7% of the rats sampled were infected with S. japonicum.
Unfortunately, further research into the nature of the disease
or its economic importance does not appear to have been performed,
although Cheng (1971) states that heavy losses of draft animals
have been reported from several Chinese provinces.

Two schistosomes of ruminants are common in livestock in
much of Africa. Schistosoma bovis, a parasite of the mesenteric
veins of cattle, sheep and goats, is found in an area covering
most of north and central Africa, extending as far south as
Zambia and Tanzania. Schistosoma mattheei is a parasite of
mammals in South Africa, its range extending northward into Tanzania
and Zambia (Hussein, 1973). Natural infections have been reported
from domestic ruminants, antelope, primates (including man) and
several species of rodents (Dinnik and Dinnik, 1965). S. mattheei
also parasitizes the mesenteric veins, although reports of flukes
in the venous drainage from the urogenital tract of cattle are
not infrequent (Hussein, 1973).

Surveys for the presence of these parasites in domestic
animals have revealed strikingly high rates of infection in
some areas. Pitchford (1961) found 70-90% of the cattle surveyed
in the Transvaal infected with S. mattheei. In certain regions
of Kenya 100% of the cattle from European farms were infected
with schistosomes (Dinnik and Dinnik, 1965). In the Sudan,
Makek (1969) found S. bovis in 37% of the cattle and 45% of the
sheep examined. It has been estimated by other authors that as

many as 90% of the cattle in the Sudan may be infected with

28

S. bovis (Hussein, 1973).

Despite the high rates of infection and descriptions of
typical schistosome lesions found in infected animals (McCully
and Kruger, 1969; Hussein et al, 1973), reports of accompanying
clinical disease are rare. That several of these surveys were
conducted at abbatoirs on animals considered healthy at the time
of slaughter is an indication of the absence of overt disease.
McKenzie (1970) found that 40% of slaughtered cattle were infected
with S. mattheei. A survey of 2900 Rhodesian slaughter cattle
for S. mattheei revealed an infection rate of 69% and led the authors
to conclude that infection was of little economic significance
(Lawrence and Condy, 1970). Malek (1969) described several cattle
in the Sudan which were parasitized by numerous S. bovis, but

showed few gross lesions and were apparently healthy.

Other schistosomes of domestic animals

 

The most common schistosome of domestic animals in the
Middle East is S. bovis. Its presence has been reported in
sheep, goats and cattle in Iraq, Iran and Israel. S. bovis
has also been described from several locations in southern Europe
(Hussein, 1973). Infections in these areas have not yet been
thoroughly investigated, but the high rates of infection in some
regions again point to the relatively low pathogenicity of the
parasite (Montgomery, 1906; Lengy, 1962; Arfaa et al, 1965; Ramajo and
Martin, 1972).

Although the Indian subcontinent probably contains more

species of schistosomes of importance to the livestock industry

29

than any other area of the world (Srivastava and Dutt, 1962),
information on these infections is singularly lacking. Morphological
descriptions of the parasites and histological examinations of the
lesions they induce are available (Datta, 1932; Rao, 1938; Kalapesi
and Purohit, 1954; Tewari et a1, 1966) but research on disease
processes has not been performed. Datta (1932) described a syndrome
of persistent debility accompanied by hepatic lesions in horses,
which he attributed to infection with S. indicum, but no conclusive
experimental studies have been carried out.

The importance of the host species in the pathogenesis of
schistosomiasis has been emphasized by observations on S. nasale
in Indian bovids. Adults of this unusual schistosome are found
in the nasal veins and eggs are released into the environment
in the nasal secretions. The granulomatous response which develops
in cattle frequently blocks the nasal cavities producing a marked
dyspnea commonly known as ”snoring disease" (Dutt, 1967). In
buffalo, however, the infection appears to be almost entirely
asymptomatic. Dutt and Srivastava (1968), in a survey of a herd
of 200 cattle and 60 buffalo, found that all animals were infected
with S. nasale. Seventy percent of the cattle showed definite
clinical signs of infection, while none of the buffalo appeared
to be clinically affected by the parasite. Dutt (1967) showed
that this difference is due entirely to the host response. Cattle
experimentally exposed to cercariae derived from buffalo flukes
developed clinical disease, while buffalo infected with cercariae
produced from the eggs of cattle schistosomes did not.

Clearly, these fluke infections of domestic animals in many

30

ways resemble the human schistosome infections as they are currently
understood. Distribution of the parasites is wide; in endemic

areas rates of infections are high, although many infections may

as asymptomatic. While the acute syndrome is more recognizable

it occurs only infrequently. The social and economic impact of
chronic schistosomiasis, the form of the disease most commonly
encountered, is difficult to accurately evaluate. However, as

a consequence of these similarities it is encouraging to believe
that the tools developed to control and eliminate infection will

be of value to both groups of schistosomiases.

Schistosomes of wild mammals

 

The importance of schistosome infections in wild mammals
lies not only in their effects on individual animals, but also
in their ability to influence host population levels. The latter
effect may be accomplished subtly through such mechanisms as a
reduction in the fertility of infected animals or, more obviously,
by the production of clinical schistosomiasis. From an epidemiologic
perspective these host-parasite systems offer an opportunity to
examine the parameters influencing transmission of the parasites
uncomplicated by the restrictions imposed by man on domestic animals.
Systematic investigation of a wild mammal schistosome has been
undertaken only in South Africa with S. mattheei (Basson et al,
1970; Pitchford et al, 1973; Pitchford et al, 1974). Although
the parasite is found in a number of wild animal species, the
African buffalo (Symmers caffer) appears to be the most important

host. In a survey of 100 culled buffalo the incidence of infection

31

was 62%. The prevalence and intensity of infection increased
with increasing age of the buffalo and were especially high in

older, solitary animals. These "reitbuffels," as they are commonly

 

known, are no longer herd members and are generally restricted
in their movements to riverbanks. They can frequently be observed
wallowing in the mud and water (Basson et al, 1970). This obser-
vation of higher worm burdens in older animals may be natural
evidence of the absence of resistance to reinfection seen in
laboratory infections of cattle with S. mattheei (Lawrence, 1973).
There was no indication of ill health in the infected buffalo,
although in some cases the schistosomal lesions were striking.

In two studies by Pitchford et a1 (1973, 1974) the extent
of infection with S. mattheei in the Kruger National Park was
thoroughly established. Fecal samples of a number of animal
species were collected and examined for schistosome ova. On the
basis of rate of infection, fecal characteristics, gregariousness,
range and water-entering behaviour, host species were assigned
to categories in relation to their ability to support the adult
parasite and successfully introduce the schistosome ova to water.
The distribution of S. mattheei in the area studied could be
seen as consisting of a number of foci of infection, each centered
around a water source. In each of these foci the parasite was
either independently maintained, or was the product of repeated
introduction. The buffalo, with its large range, waterloving
habits and high rate of infection was seen as the primary source
of infection.

The nature of the water supply in a given area could be

32

correlated with the prevalence of the infection. Where animals
relied on fast-flowing rivers with thick vegetation along their
banks, S. mattheei infections were infrequent. In other locations
where water was provided by man-made lakes or ponds,infection

rates were higher. The value of these studies lies in their de-
lineation of the relative importance of each factor which influences
transmission of the schistosome. Current work in South Africa

on the behaviour of S. mattheei in wild mammals and its eventual
comparison with infections of domestic animals may lead to an
increased understanding of the delicate relationship between

host and parasite.

Other schistosomes of'wild mammals

Several other schistosome parasites have been briefly described
from wild mammals (Price, 1931 Rao et al, 1932; LeRoux, 1933,
1955; Schwetz, 1953); one of these, Schistosomatium douthitti,
will be discussed in a later section. Another schistosome of wild
mammals which has received some attention experimentally is
Heterobilharzia americana, one of two schistosomes of mammals
found in North America. Adult flukes have been reported from
the portal and mesenteric venous systems of the raccoon, nutria
(Myocaster coypus), dog, opossum, bobcat, swamp rabbit, and
white—tailed deer in Florida, Louisiana, North and South Carolina
and Texas (Malek, 1961; Bartsch and Ward, 1976). Experimental
infections have also been produced in the mouse, hamster, rabbit,
guinea pig and cat, although all but the first two of these lab-

oratory hosts do not contain large numbers of eggs in the feces

33

(Lee, 1962). The raccoon has been suggested by Lee (1962) to be
the most important natural host in the areas studied. In two loca-
tions surveyed by Malek (1961) in Louisiana, 21 of 39 and 25 of

52 raccoons were found to be infected with H. americana. Two
naturally infected dogs in poor condition with diarrhea have been
described (Malek, 1961; Pierce, 1963), but in the absence of firm
experimental evidence of the pathologic effects of H. americana,

it is not possible to determine the role of the fluke in affecting
the condition of these animals.

An interesting but apparently anomalous schistosome has
been described by McCully et a1 (1967) from the vascular system
of the hippopotamus. Identified as S. hippopotami, these flukes
were distributed throughout the venous and arterial systems of
a large proportion of 97 culled hippopotami. Egg production by
the parasite was limited to worms in the veins of the adrenals.
These unusual characteristics of infection have led the authors
to suggest that the hippopotamus may be an abnormal host of this
fluke. Remarkably, although the schistosomes excited a profound
reaction in the vessel walls, all animals appeared clinically
healthy.

Infections of African rodents with S. mansoni have been
observed in a number of species since 1952 (Kuntz, 1952). These
infections are infrequent and it is now agreed that they are
probably unimportant as reservoirs for human infection, in contrast
to infections of several primate species (Martins, 1958; Pitchford
and Visser, 1962). In South America, however, since the introduction

of S. mansoni as a consequence of the slave trade, the parasite

34

has successfully adapted to a number of native rodent species
(Martins, 1958; Barbosa, 1972). Although viable ova have not
been found in the feces of all infected animals, Antunes et a1
(1973) have demonstrated experimentally the successful completion
of the parasite life cycle using infections of Nectomys squamipes
squamipes. The prevalence of S. mansoni in wild populations

of these aquatic rodents is as high as 57% in some rural areas.
While it is not yet clear that S. mansoni infections in Nectomys
are maintained independently of the human cycle, the impact of
the rodent infections on human schistosomiasis may be extensive.
Incidental findings of S. mansoni have been made in a variety of
other animals ranging from cattle (Barbosa et al, 1962) to the
giant anteater (Rijpstra and Swellengrebel, 1962), but these
infections are not known to be of significance in the distribution

of the parasite.

Schistosomatium douthitti

 

Natural Infection

 

Only two of the schistosome species of mammals found throughout
the world are native to North America. Heterobilharzia americana,
a parasite of the raccoon, bobcat, nutria, and occasionally the
dog, is restricted to the southeastern U.S. (Malek et al, 1961).
The rodent parasite, Schistosomatium douthitti, has been reported
from a more extensive area covering the north—eastern and central
United States, Alaska and Canada (Price, 1931; Penner, 1942;
Swartz, 1966; Choquette et al, 1973). Like most schistosomes,

adult S. douthitti which reach a length of about 1 cm, are found

35

in the mesenteric veins of the host. Since the first detailed
description of the fluke by Price (1931), S. douthitti has been
identified in the meadow vole, Mficrotus pennsylvanicus (Price,
1931); the muskrat, 0ndatra zibethica (Penner, 1938); the redbacked
vole, Clethrionomys rutilus (Swartz, 1966); and the porcupine,
Erithizon dorsatum (Choquette et a1, 1973).

Several species of Lymneid snails are known to be capable
of acting as intermediate hosts for S. douthitti. Naturally
infected Lymnea palustris and L. stagnalis and Physa gyrina
(Price, 1931) have been reported. L. catascopium (Kagan, 1954)
and Pseudosuccinea columnella have been successfully infected
in the laboratory (Malek, 1970).

Examination of snail populations for the presence of S.
douthitti has yielded uniformly low rates of infection. Bourns
(1961) examined 2474 L. stagnalis and L. palustris between 1957
and 1960 and found only 3.7% infected. Surveys by workers in
other areas revealed rates of infection ranging between .5% and
8.6% (Brackett, 1940a; Farley, 1962a). Bourns (1961) also observed
that a seasonal variation in the prevalence of infection occurs
when measured by the ability of snails to shed cercariae. Mon-
itoring of L. palustris and L. stagnalis between May and November
showed that the percentage of infected snails increased through
the early summer and reached a peak of about 6% in late summer and
fall. A similar observation has been made by Blankenspoor (personal
communication). This peak in infected snails coincides with the
period when the greatest numbers of mature snails are found

(Brackett, 1940a)

36

Penner (1942) stated that the muskrat (0ndatra zibethica)
was the most important natural definitive host of S. douthitti
and available information on prevalence of natural infections
stems almost exclusively from muskrat surveys. Like the snail
intermediate hosts, the muskrats examined have shown only low
rates of infection. Penner (1938) surveyed 330 muskrats in Wisconsin
and found only 32 infected animals. Between 1938 and 1942 he examined
a further 374 muskrats and found 11.2% infected (Penner, 1942).
A survey by Gilford (1954) of 250 muskrats in Illinois detected
only 1 infected animal. Beckett and Gallachio (1967) found 2 of
130 animals infected in Ohio, and of 12 muskrats inspected in central
and southern Michigan, 2 were infected (Ameel, 1942). TWO authors
have reported briefly on infection of Microtus pennsylvanicus
with S. douthitti. Price (1931) found that one of three animals
trapped in southeastern Michigan was infected, and Farley (1962a)
discovered one fluke in one of 13 voles captured.

Much of the interest in field infections of S. douthitti
results from the ability of the parasite to produce "swimmers'
itch”, the dermatitis associated with host sensitization to the
penetration of cercariae of non-human schistosome species (Cort,
1950). Although dermatitis can be produced experimentally with
S. douthitti (Herber, 1938; Olivier, 1949), instances in which
this species has been implicated in the production of clinical
cases of swimmers itch are rare (Cort, 1950). The preference
of the intermediate hosts for still, weedy water, and the release
of cercariae only in the evening hours are probably major factors

which limit the occurrence of S. douthitti-related dermatitis.

37

Laboratory Infections

 

Although field work with S. douthitti has not been extensive,
laboratory investigations have produced information about a number
of aspects of the development and behavior of the parasite.

Tanabe (1923) first described the fluke, calling it S. pathlopticum,
following observations of laboratory mice infected with cercariae
from naturally infected snails. (Price (1931) later published a
more detailed description of all stages of the life cycle of a
parasite which she named S. douthitti. Penner (1942) subsequently
established that only a single species was involved and assigned
the name used by Price.

Several species of mammalian hosts have been shown to be
susceptible to laboratory infections with S. douthitti. These
include the deer mouse, Peromyscus maniculatus (Price, 1931);
the laboratory mouse (Tanabe, 1923), the snow shoe hare, guinea
pig (Penner, 1939) and nutria (Malek, 1970). While the rat is
capable of maintaining adult worms, Price (1931) reported that all
eggs released by the worms were trapped by the host tissue reaction.
Experimental exposure of other animals, including the cat (Price,
1931) and bat (Dery, 1957) did not result in the development
of mature parasites.

Several authors have explored the possibility that the
parasite may develop to maturity in primates. Brackett (1940b)
was unable to recover either mature or immature flukes from the
liver of rhesus monkeys infected with cercariae. Penner (1941),
however, recovered schistosomula from the lungs 5 1/2 days following

infection of a rhesus monkey, and Kagan (1953) found sexually mature

38

worms in the portal system of rhesus monkeys 12-15 days after
infection. Worms in these animals were not as well developed
as mouse worms of a similar age, and infections were spontaneously

terminated after 2—3 weeks.

Life Cycle

 

The miracidia of S. douthitti are mature when the egg is
released in the feces, and the larvae emerge when the egg contacts
water. Farley (1962b) found that the half-life of the miracidium
at room temperature is about 10 hours. Miracidia are positively
phototaxic (Wright and Lavigne, 1972) and capable of responding to
chemical stimuli (Wright and Ronald, 1972). Formation of the
mother sporocyst in the snail is followed by the development of
several daughter sporocysts (Price, 1931). Cercariae are usually
first produced by the snail about six weeks following exposure
to infection, although lowered temperatures increase the incubation
period (Kagan et al, 1954). Darkness is the only stimulus required
for release of the cercariae (Olivier, 1951); Kagan et a1 (1954)
found that up to 5000 cercariae could be released by a single nat-
urally infected snail in one evening. Shortly after emerging,
the cercariae move to the surface of the water where they hang
suspended until a suitable host is encountered (Kagan et a1,

1954). Cercariae probably rarely live longer than 24 hours
(Cort, 1950).

Penetration of the skin of the definitive host is followed

by a migratory phase, with the schistosomula reaching the liver

of mice approximately 6 days after exposure to the cercariae

39

(Olivier, 1952). Worms spend a short time in the veins of the liver
and migrate into the mesenteric veins on about the tenth day of
infection (Short, 1952). Sexual development and mating of adult
worms have been followed by El-Gindy (1951), Short (1952), and
Armstrong (1965). Eggs can first be detected in the mesenteric
veins 22 days after infection, but ova are not recovered from the
feces for approximately another week (Price, 1931).

The interesting observation that female S. douthitti are
capable of parthenogenetic reproduction in the absence of males
(Short, 1947) has led to further studies detailing the genetic
constitution of the parasite (Short, 1955; Short and Menzel, 1955).
Parthenogenetically produced ova are fully infective for snails
(Short and Menzel, 1960) and ultimately produce normal adult worms

(Short, 1948).

Histological Studies

 

The histopathology of the host reaction to S. douthitti
has been followed in both the skin and internal organs of exper-
imentally infected mice. Batten (1956) examined the skin of mice
exposed to cercariae of S. douthitti and observed a histiocytic
and neutrophilic response to the parasite. Kagan and Meranze
(1955) also infected both naive and sensitized mice. The response
in both groups was composed primarily of polymorphonuclear cells,
but following sensitization the host reaction occurred more quickly
and was more highly localized around invading cercariae with a
higher proportion of monocytes and fibroblasts participating.

Kagan and Meranze (1957, 1958) also followed the response

40

to ova in the liver and other organs. Eggs in the liver were seen
to excite the formation of pseudotubercles with some areas of
inflammation and necrosis. Lesions ultimately became replaced
with connective tissue and eggs were destroyed entirely. The reaction
to ova in immune mice was identical to that in naive mice after a
similar period of time. Livers of mice whose infections had been
chemotherapeutically treated 57 days after infection were essentially
normal in appearance 270 days following removal of worms (Kagan
and Meranze, 1957).

In the intestine focal inflammatory reactions and pseudotubercle
formation were observed around the ova. Eggs in the spleen were
also associated with granuloma formation. In heavily infected
animals ova were found in the lungs, where they were surrounded
by inflammatory nodules. Mesenteric lymph nodes and Peyer's Patches
were enlarged and infiltrated with ova. Other organs, including

the bone marrow and kidneys, were unaffected.

Immunological Studies

 

The potential value of S. douthitti infection as a model
for the investigation of host reaction to schistosomes, with
particular reference to the immunological response, was recognized
by Kagan (1952). In a series of experiments, Kagan (Kagan, 1952;
Kagan and Lee, 1953; Levine and Kagan, 196 ) demonstrated that a
partial immunity to reinfection develops following primary infection.
Worm burdens may be reduced by as much as 50% in challenge infections,
and those worms which do develop are stunted. This partial

immunity developed about 35 days following primary exposure and

41

continued throughout the period of experimentation (123 days).
The 35 day period coincides approximately with the period elapsing
before large numbers of eggs are deposited by the female worm.

Additional evidence for the role of the ova in the generation
of the observed resistance has been provided by several experiments.
A primary infection of male worms alone was unable to induce
resistance, while single-sex infections of female worms, which
will produce eggs parthenogenetically, are as effective an immunizing
agent as normal infections (Kagan, 1952). Chemotherapeutic removal
of mature worms, followed by reinfection 8 to 52 days later resulted
in reduced establishment of flukes for about three weeks after
treatment. This three week period corresponds to the length of
time that tissue bound eggs would survive (Kagan and Lee, 1953).
Finally, Hunter and Crandall (1962) demonstrated a partial immunity
following a series of injections of ova or supernatants of egg
suspensions. The mechanism of this partial resistance is obscure.
Attempts by Levine and Kagan (1960) to immunize mice possibly
with injections of serum from infected hamsters were unsuccessful,
but no further investigations into the nature of the immunity have
been performed.

The possible value of S. douthitti in the production of vaccines
against human schistosomes has been explored in several studies.
Hunter, Weinmann and Hoffman (1961) found that while primary
infections of S. mansoni in mice were effective in reducing the
number of worms establishing from subsequent exposure to S.
douthitti, the reverse did not occur. Hsfi et a1 (1964) found

that infections of S. douthitti in rhesus monkeys led to reduced

42

susceptibility to challenge infections with S. japonicum. Non~
specific interactions between S. douthitti and other parasites
which reduce the numbers of adult flukes or the longevity of tissue
bound eggs have been observed with Ascaris suum (Crandall et al,
1966) and Fasciola hepatica (Maldonado-Moll, 1977).

During the past decade interest in S. douthitti has diminished
and currently little experimental work is being conducted. This
natural rodent parasite system, however, provides a practical and
valuable opportunity for the study of the relationship between
host and schistosomes andthe full extent of its usefullness has

yet to be determined.

Microtusgpennsylvanicus

 

Approximately 55 species of vole are found in North America
and northern Eurasia (Orr, 1971). In the Western Hemisphere
these rodents of the Family Cricetidae are found in almost every
region from the Arctic Circle to Guatemala (Cahalane, 1947).
Although none is a true desert dweller, voles have adapted to a
variety of habitats including woodland, fields and marshes.

The most common and widespread vole in North America is Microtus
pennsylvanicus, commonly known as the field mouse, meadow vole

or meadow mouse. The length of the mature meadow vole ranges

from 8.8 to 12 cm, and adult weights vary from 20 to 68 grams.

The pelage of the meadow vole is dark brown above with grey
underparts. Unlike mice, with which they are most commonly confused,
voles have short, hairy tails and the pinnae of the ears are

barely visible above the fur (Burt, 1975).

43

ML pennsylvanicus does not hibernate and is active throughout
the 24 hour day, with periods of least activity at noon and midnight.
The meadow vole is territorial, with a home range rarely exceeding
1/15 of an acre (Burt, 1975). Voles forage actively throughout
their territories, constructing elaborate systems of tunnels
in the undergrowth which lead to buried stores, nests and occasionally
to open water. Nests may be found either above or below the ground
(Hatt, 1930). While the preferred habitat is close to water, meadow
voles can be found in fields and grassland (Orr, 1971). Although
research on the tendency of M. pennsylvanicus to enter water does
not appear to have been performed, anecdotal evidence of the vole's
swimming ability and of the readiness with which it enters the
water is available from observers extending as far back as Audubon
(Hatt, 1930; Cahalane, 1947; Orr, 1971).

ML pennsylvanicus, like many other rodents, eats a remarkable
variety of foods. The diet is composed primarily of seeds, leaves,
flowers, stems and roots, but grains, alfalfa and clover, tubers
and bulbs will also be utilized. In winter voles may girdle
vines, trees and shrubs in order to feed on the cambium. They also
eat carrion of any kind, as well as insects, crayfish and snails
(Cahalane, 1947).

Meadow voles, in turn, are eaten by virtually all major
predators. They are preyed upon by hawks and owls, raccoons,
foxes and snakes, and this high rate of attrition is countered
only by their prolificacy. Breeding can occur throughout the
year, although it is rare in winter. A single female will produce

several litters annually, each containing one to nine young.

44

Gestation lasts about 21 days. Females are sexually mature at
approximately 4 weeks of age and males at about 6 weeks (Burt,
1975).

Field experimentation with ML pennsylvanicus has been most
concerned with the striking cyclical nature of population increases
and declines. As with the more familiar lemming, meadow vole
populations fluctuate between several hundred and l or 2 animals
per acre over a 3 to 4 year period. Several explanations for
these dramatic reductions have been proposed including outbreaks
of disease or increased predator activity. More recent hypotheses
stress the importance of dispersal during periods of high population
density and genetic variation leading to reduced fitness of remaining
animals. For a more complete discussion of population changes,
see Krebs and Meyer, 1974.

Despite the extensive use of natural populations of M.
pennsylvanicus by animal behaviour and population biologists,
few workers have utilized the meadow vole in laboratory studies.
Consequently, only limited information is available related to
maintenance of the vole in the laboratory. Poiley, 1949; Lee and
Horvath, 1969; Shenk, 1976; and Dietrich and Preston, 1977, have
described basic laboratory management and maintenance of breeding
populations, and observations of litter size and frequency of
breeding have been reported by Colvin and Colvin, 1970; and
Morrison et al, 1976. Basic organ weights and hematologic values
for M. pennsylvanicus have also been published (Dietrich, 1973;

Dietrich et al, 1973).

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ARTICLE

PREVALENCE AND INTENSITY OF INFECTION OF THE MEADOW
VOLE (MICROTUS PENNSYLVANICUS) WITH

SCHISTOSOMATIUM DOUTHITTI (FAM. SCHISTOSOMATIDAE) IN MICHIGAN

Prevalence and intensity of infection of the meadow
vole (Microtus pennsylvanicus) with

Schistosomatium douthitti (Fam. Schistosomatidae) in Michigan

A. M. Zajac and J. F. Williams

Department of Microbiology and Public Health
Michigan State University

East Lansing, MI

Abstract

A survey was conducted for the presence of Schistosomatium
douthitti in a local population of meadow voles, Microtus
pennsylvanicus, in southern lower Michigan. Thirty-three of the
47 voles examined were infected and worm burdens ranged from 1
to 236. S. douthitti was also identified from the jumping mouse,

Zapus hudsonius and the white-footed mouse, Peromyscus leucopus.

59

INTRODUCTION

Schistosomatium douthitti (Fam. Schistosomatidae) is one
of two mammalian schistosomes indigenous to North America (Malek
et al, 1961). Infections have been reported from the northeastern
and north midwestern United States, Canada and Alaska (Penner,
1942; Swartz, 1966; Choquette et a1, 1973). Several species of
Lymneid snails are capable of acting as intermediate hosts of
the parasite, including L. stagnalis and L. palustris (Kagan et al,
1954). Adult flukes have been described from the mesenteric veins
of the muskrat, 0ndatra zibethica (Penner, 1938), the meadow vole,
Microtus pennsylvanicus (Price, 1931), the redbacked vole,
Clethrionomys rutilus (Swartz, 1966), and the porcupine, Erethizon
dbrsatum (Choquette et a1, 1973).

Penner (1942) suggested that the muskrat, 0. zibethica,
is the most important natural host and surveys for infection have
dealth entirely with this species. Although the mesenteric veins
of several hundred muskrats from various regions of the country
have been inspected in a series of surveys, rates of infection with
S. douthitti have not been found to exceed 11% (Ameel, 1942;
Penner, 1942; Gilford, 1954; Beckett and Gallicchio, 1967).
To determine whether these data from the muskrat are representative
of S. douthitti infection in other rodents, a survey of a local
population of the meadow vole, ML pennsylvanicus in Michigan,
was undertaken. The results indicate that a high prevalence
of infection occurs in this host, and that other small rodents

may also become infected.

60

61

MATERIALS AND METHODS

Live trapping of meadow voles was conducted at two permanent ponds
at the Rose Lake Wildlife Research Area in southern lower Michigan. The
ponds were shallow, but fairly extensive in area, the smaller about one
half acre in size, while the larger covered an area of approximately
two acres. In locations where voles were captured, the vegetation
was composed primarily of cattails, Typha latifolia.

The presence of S. douthitti in the area was initially established
by the collection of Lymneid snails, later identified as L. palustris.
Snails were taken to the laboratory where they were exposed to a
minimum of 12 hours of light followed by an hour of darkness which
stimulated the release of cercariae. After cercariae of S.
douthitti were identified, 60 Sherman live traps baited with
oatmeal or peanut butter and oatmeal were placed around the
perimeters of the ponds. Between July 21 and October 2, 1976,

47 M2 pennsylvanicus were trapped.

Captured animals were returned to the laboratory where they
were given intraperitoneal inoculations of a solution containing
a lethal dose of sodium pentobarbital and approximately 100 units
of ammonium heparin. Voles were sexed, weighed, and the portal
and mesenteric venous systems perfused with phosphate buffered
saline. The perfusate was filtered through 5 micron pore size
NucleporeR filters (25 mm diameter) (Nuclepore Corporation,

Pleasanton, CA 94566) which retained any worms present. The
filter was then transferred to a microscope slide and parasites

could be readily sexed and counted under the light microscope.

62

RESULTS

Thirty three (70%) of the 47 voles examined were found to
be infected with S. douthitti. The proportion of male and female
voles infected was approximately equal: 76% of 29 males and
67% of 18 females. The worm burdens in the infected animals
were found to range from 1 to 236 parasites with an average of
40 worms per infected vole. A11 male infections were found in
three animals, but the remaining 44 infected voles contained mature,
egg-bearing female flukes. All animals captured appeared healthy
and in every case but one there were no gross lesions directly
attributable to schistosome infection. The single exception was
a heavily infected (228 worms) female whose intestine contained
numerous plaques along its length resembling the schistosome ova
aggregates observed in mice experimentally infected with S.
douthitti.

In addition to captured voles, 13 jumping mice, Zapus hudsonius,
and 8 white-footed mice, Peromyscus leucopus were also examined.
Three (23%) of the Z. hudsonius and 2 (25%) of the P. leucopus
were also found to be infected with small numbers of S. douthitti,

ranging from 1 to 17 parasites.

DISCUSSION
Results obtained in this survey indicate that S. douthitti
occurs more frequently in some rodent populations than would be
expected from previous studies of muskrats. In fact, the high
prevalence of infection in M. pennsylvanicus suggests that the

meadow vole rather than the muskrat, 0. zibethica, may serve as

63

Table 1. Relationship between age of meadow vole and worm burden

 

Weight Class Juvenile Subadult Adult
<22g 22-33g >33g
Worm burden-range 1-11 6-72 2-236

Worm burden-average 4.75 23.81 76.58

 

64

the most important host in the maintenance and transmission of
this fluke. However, the difficulty of examining the mesenteric
veins of dead animals may have prevented the identification of
all muskrats infected with low numbers of parasites, and the
prevalence of infection may actually be greater in muskrats than
is currently appreciated.

It is of interest to examine the relationship between the
rate and intensity of infection and the age of infected voles.
While it was not possible to determine exactly the ages of trapped
animals, guidelines for estimating relative ages of voles based
on weight have been proposed by Krebs et a1 (1969). If the rates
of infection and ranges of worm burdens for each of these age groups
are determined (Table 1), it can be seen that the animals in the
subadult and adult classes not only have higher rates of infection,
but also carry heavier worm burdens than the juvenile animals.

The presence of large worm burdens only in older animals suggests
that reinfection might occur throughout the course of the animal's
life. Kagan (1952) demonstrated with experimental infections of

S. douthitti in laboratory mice that, although a partial resistance
to reinfection occurs, a proportion of challenge cercariae establish
as adult worms in the mesenteric veins. Continuous exposure to
cercariae in the natural situation might then result in increasing
worm burdens as the host ages.

However, these survey results may also represent the outcome
of seasonal variation in cercarial output by snails, and conse-
quently variation in the degree to which animals may be exposed

to the parasite. Observations of Bourns (1961) and Blankenspoor

65

(1977, personal communication), that shedding of cercariae reaches
a peak in late summer, when these voles were collected, militate
against this hypothesis. Further observations of the distribution
of infection over the course of the year will be necessary to
appreciate more fully the pattern of infection.

The role of other infected rodents in the transmission of
the parasite is unknown. Natural infections of S. douthitti
in both Z. hudsonius and P. leucopus have not been described
previously in the literature. The number of animals examined,
however, was insufficient to determine accurately the prevalence
of infection.

It is hoped that further examination of natural infections
of S. douthitti in its rodent hosts will provide some insights
into the dynamics of the host-schistosome relationship, which
may ultimately be valuable in the interpretation of epidemiologic

characteristics of infections of man and domestic animals.

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(Cort, 1914) Price, 1931, in Alaska in a new natural definitive

host, Clethrionomys rutilus (Pallas). Can J Zool 44: 729—730.

ARTICLE

PATHOLOGIC CHARACTERISTICS OF INFECTION
WITH SCHISTOSOMATIUM DOUTHITTI
(FAM: SCHISTOSOMATIDAE) IN THE

LABORATORY MOUSE AND THE MEADOW VOLE,

MICROTUS PENNSYLVANICUS

Pathologic Characteristics of Infection
with Schistosomatium douthitti
(Fam: Schistosomatidae) in the

Laboratory Mouse and the Meadow Vole,

MHcrotus pennsylvanicus

A. M. Zajac and J. F. Williams

Department of Microbiology and Public Health
Michigan State University

East Lansing, Michigan

ABSTRACT

Laboratory mice and meadow voles (Microtus pennsylvanicus)
were infected with Schistosomatium douthitti, a natural rodent
schistosome. Mice developed marked and often fatal clinical
schistosomiasis at levels of infection which produced no signs
of disease in voles. Examination of tissues of infected animals
showed that despite wide individual variation, each host species
developed a characteristic granulomatous response to the
schistosome ova. Naturally infected meadow voles also showed
no sign of clinical schistosomiasis even when maintaining a worm
burden of several hundred adult S. douthitti. Histologic response
of these wild voles to the parasite ova resembled that of the

experimentally infected ML pennsylvanicus.

67

INTRODUCTION

The proposal that chronic schistosomiasis is an immunopath-
ological phenomenon (Warren et al, 1967) has led many investigators
to examine the histological course of events in the development of
the schistosome egg granuloma. Results derived from experimental
infection of laboratory rodents with S. mansoni have provided
the greater part of the current knowledge of the cellular reaction
to the parasite ova. However, rodent host species differ markedly
in their responses to S. mansoni ova (Cheever, 1965b) and unqualified
extrapolation of conclusions drawn from studies in rodents to
the human may be misleading.

Consequently, experimentation with natural host-schistosome
systems may prove to be extremely valuable in the understanding
of disease processes and the pathogenesis of schistosomiasis.
Schistosomatium douthitti is one of a small number of these natural
schistosome systems which lends itself to laboratory investigation.
A North American fluke, S. douthitti is found primarily in the
meadow vole, M. pennsylvanicus and the muskrat, 0ndatra zibethica
(Kagan et al, 1954). The value of this parasite as a model for
the study of schistosomiasis was recognized by Kagan and Meranze
some two decades ago (1957, 1958). They followed the course of
infection in experimentally infected mice and observed some
similarities in the histopathologic reactions in mice infected
with S. mansoni. Also like S. mansoni (Warren, 1972), S. douthitti
in large numbers is highly pathogenic in mice.

The syndrome of clinical schistosomiasis in mice infected
with S. douthitti is in sharp contrast to that seen in the vole.

68

69

Naturally infected meadow voles have been found carrying worm
burdens many times greater than those which would prove fatal

to mice (Zajac and Williams, 1978). This striking difference

in the pathogenicity of the parasite in two rodent hosts is com-
parable to a situation seen with several mammalian schistosomes

of domesticated ruminants. S. mattheei, for example, a common
parasite of sheep and cattle in southern Africa, has been noted

by several authors (Strydom, 1963; McCully and Kruger, 1969;
Lawrence, 1974) to produce clinical schistosomiasis more frequently
in sheep than cattle. The clinically important nasal granulomas
formed in response to the presence of S. nasale in Indian cattle

do not develop in similar infections of buffalo (Dutt and Srivastava,
1968).

S. douthitti infections in mice and voles provide a convenient
and easily manipulated schistosome system in which to explore the
comparative pathogenesis of schistosomiasis in two closely related
host species. Furthermore, the identification of those factors
determining the development of disease may be highly relevant to
a clearer understanding of disease processes in human infections.
In view of these consideratons, a study was undertaken to compare
the cellular reaction to infections of S. douthitti in mice and
voles. Both similarities and differences were observed and their

possible significance is discussed.

70

MATERIALS AND METHODS

Histopathological examinations were carried out on tissues
from mice and voles experimentally infected with S. douthitti
and from wild voles naturally infected with the parasite. Six
week old female mice (Spartan Research, Haslett, Michigan)and
both male and female juvenile and adult meadow voles from a
locally derived laboratory strain were infected with cercariae
of S. douthitti. The parasite was maintained in the laboratory
in infected Lymnea catascopium and was derived from the University
of Michigan (Ann Arbor) strain originally established from locally
infected snails.

Cercariae were administered via an intraperitoneal inoculation
and adult worm burdens were determined by perfusion of the mesenteric
and portal venous systems. The perfusate was filtered through
a 5 micron pore size NucleporeR filter (Nuclepore Corporation,
Pleasanton, California) which retained any schistosomes present.
Worms were then counted on the filter under the light microscope.
Animals whose tissues were examined contained on postmortem exam-
ination, worm burdens ranging from 2 to 29 S. douthitti.

A minimum of 3 animals of each species was sacrificed on Days
15, 30, 45, 60 and 90 after infection (DAI). The clinical condition
of infected animals was observed and a gross pathologic examination
made of the internal organs after death. The liver, spleen, small
intestine, cecum, lungs, mesenteric lumph node and kidneys were
removed and placed in 10% buffered formalin. Control tissues
were similarly removed from uninfected animals. Tissues were

routinely sectioned and stained with hematoxylin and eosin and

71

with Giemsa stain. Determination of maturity of the schistosome
embryo within the egg in tissue sections was based on the class-
ification of von Lichtenberg et a1 (1973).

In addition, tissues of wild ML pennsylvanicus naturally
infected with S. douthitti were examined. Voles were live trapped
as described previously (Zajac and Williams, 1978) and returned
to the laboratory where they were sacrificed and the portal and
mesenteric systems perfused. Tissues were preserved, sectioned

and stained in the same manner as for experimentally infected animals.

RESULTS

Clinical Schistosomiasis and Gross Pathologic Changes

Voles experimentally infected with S. douthitti did not show
clinical signs at any stage of infection. No weight loss or
behavioural changes were observed. Hair coat and consistency
of feces remained normal. All internal organs appeared normal
on postmortem examination. Occasionally, pinpoint white spots
were seen on the surface of the liver, but no alteration was seen
in its color or consistency. Wild voles naturally infected with
S. douthitti were all apparently healthy at the time of capture.
Hair coat, behaviour and fecal pellets appeared normal. There
were no internal changes directly attributable to schistosome
infection with the exception of one animal which had raised areas,
about 1 mm in diameter, along the mesenteric border of the serosal
surface of the cecum and small intestine.

Mice with adult worm burdens exceeding approximately 15

S. douthitti first developed signs of clinical schistosomiasis

72

about 6 weeks following infection. Progressive unthriftiness with
staring and unkempt hair coat, weakness and emaciation were
observed, accompanied by mild hemorrhagic diarrhea. Examination
of the internal organs revealed greatly enlarged mesenteric

lymph nodes and spleens. The liver usually contained pinpoint
white spots on its surface and blood was observed in the lumen

of the cecum.

Mice maintaining a mild chronic infection only infrequently
showed pronounced unthriftiness. In some cases, fecal pellets
were hard and dry and contained visible traces of blood. The
spleen and mesenteric lymph nodes were enlarged. The liver
also appeared enlarged and was darker in color than normal with
numerous white spots on its surface. The cecum occasionally
contained some blood. Along the length of the intestine were
small raised areas, about 1 mm in diameter, which, when examined
with a dissecting microscope, proved to be aggregates of schistosome

eggs surrounded by connective tissue.

Histopathologic Observations
Voles

Although the changes associated with infection with S. douthitti
and ML pennsylvanicus were uniform enough to permit some descriptive
generalizations, great variation in the appearance of lesions
was detected throughout the period of observation both within
and amongst individuals. However, the relative frequency of
each type of reaction in the overall response of the vole varied

at different periods during infection.

73

Fifteen DAI changes associated with infection were visible
only in the liver. A periportal infiltrate consisting primarily
of mononuclear cells and eosinophils was observed in some portal
areas (Fig. 1). The most striking change was the presence of
schistosome derived pigment in hepatic phagocytic cells. In some
cases these cells were multinucleated and contained up to 25 nuclei.
Thirty DAI schistosome ova measuring 65-75 H in length were
visible in the liver and intestinal tissue. In both organs ova
were primarily surrounded by an unstructured mixture of eosinophils,
and mononuclear cells with some macrophages and lymphocytes (Fig. 2).
The degree to which each cell type occurred in these reactions
varied remarkably from egg to egg, although the eosinophil appeared
to play a consistently important role and in some cases was the
only cell type observed around an ovum. These eosinophilic reactions
at times covered an area up to 20 times greater than that of the egg
(Fig. 3). Giant cells also participated in these reactions and
could frequently be seen directly apposed to the egg. In the intestine
another type of granulomatous response was observed in which the
giant cell was the primary component. Ova, either singly or
in clumps ranging from 2 to 20 eggs were circumscribed by giant
cells. In egg clumps, the giant cells around the eggs formed a
continuous matrix (Fig. 4). On the periphery of the giant cell
reaction highly variable numbers of eosinophils and lymphocytes
were present. The intestinal mucosa appeared normal and there
was no indication of hemorrhage into the intestinal lumen.
Schistosome ova were present in all stages of development

from those containing an undifferentiated mass of cells to those

 

Fig. 2.

Vole.

\, , » ""."‘..'..l - 3.5 ‘1“

Typical mononuclear cell periportal infiltrate in

the liver. x 433.

Vole.

 

Diffuse inflammatory reaction surrounding ova in

the liver. x 200.

 

7'41.
.

Fig. 3. Vole. Intense eosinophil response surrounding ova in
the liver. x 176.

 

.» -,~ ~-
1““, - 32% V
.Et- wt. 33'

Fig. 4. Vole. Hepatic giant cell granuloma. x 186.

 

1|. Iii}..l .l

76

with well developed embryos. The type and composition of the
reaction associated with the egg could not be correlated with

the reaction to the parasite ova and was generally highly localized
around eggs. In the intestine all eggs and accompanying reactions
were limited to the lamina propria. No generalized cellular
infiltration was apparent in the intestine. In the liver, dis-
tribution of the ova did not seem to follow a consistent pattern.
Periportal cellular infiltrates were of the same magnitude and
composition as those observed at 15 DAI but with the added appearance
of occasional plasma cells. Small areas, containing about several
dozen eosinophils apparently unrelated to ova were seen in the
liver parenchyma in some cases. Mast cells were seen both within
and on the periphery of some granulomas and the number of mast
cells in the portal areas had increased.

Little change in the overall pattern of response at 30 DAI
was detectable 45 DAI. However, the giant cell type of granuloma
was more frequent in intestinal tissue and had begun to appear in
the liver. In many instances, these granulomas were highly
organized with only small numbers of plasma cells, eosinophils
or lymphocytes on their periphery (Fig. 5 ). Other, more diffuse
reactions with some of the characteristics of classic epithelioid
granulomas, contained eosinophils, macrophages, mononuclear
cells, lymphocytes and fibroblasts. Frequently, eosinophils
and lymphocytes formed a halo around a central cellular mass of
macrophages and giant cells surrounding the egg. Plasma cells
seemed to have increased in both liver and intestine. The

periportal infiltrates contained more plasma cells and as previously

77

noted, plasma cells were often encountered on the periphery
of the giant cell granulomas. Pigment continued to be present
in hepatic phagocytic cells. In most reactions to ova an
accumulation of pigment was visible.

The frequency of the giant cell granuloma increased at 60
and 90 DAI. By the end of this period of observation this type
of reaction was the most typical encountered in the intestine
and was also present in the liver along with the eosinophilic
reactions and more diffuse epithelioid granulomas. Some of
these hepatic reactions were highly complex, including within
a single response confluent areas of macrophages, giant cell,
eosinophils and plasma cells (Fig. 7, Fig. 8). In the intestine
eggs continued to be seen only in the lamina propria. Mature ova
could be found in both liver and intestine, where they were sometimes
encountered close to the epithelium (Fig. 6).

Although some fibroblasts were observed in the egg granulomas,
collagen deposition was not marked. Periportal fibrosis was not
a feature of the response in the vole. Nor did extensive connective
tissue replace dead eggs. While the eosinophil continued to
participate in many reactions, plasma cells had become increasingly
more numerous. In one animal, by 90 DAI plasma cells outnumbered
all other cells in the periportal infiltrate. Plasma cells were
found closely associated with giant cell granulomas. Calcification
of ova and central necrosis with granulomas were seen extremely
rarely. Sixty DAI schistosome pigment was readily recognizable
in the spleen, but neither ova or pigment were seen in other

organs examined, nor were changes resulting from infection seen

"i‘5li"':
o .'
.- I”
r I
'1
£9
'
I
O
f' 3‘
o O 10>.
.c.
"';'.I
I '....’

 

Fig. 5. Vole. Typical intestinal giant cell granuloma. Darkly
staining plasma cells are present on the periphery. x 440.

 

.
.k a.-\‘.

Fig. 6. Vole. Mature schistosome egg in cecal lamina propria
partially associated with giant cell reaction. x 338.

 

Fig. 7. Vole. Complex hepatic reaction to schistosome ova.
x 160.

 

Fig. 8. Vole. Fig. 9 detail. x 433.

80

in other organs.

Naturally Infected Voles

The tissues of 14 wild ML pennsylvanicus naturally infected
with S. douthitti were examined. These animals carried worm
burdens ranging between 8 and 236 parasites. Although it was
not possible to determine how long fluke infections had been
established, the microscopic appearance of their tissues most
closely resembled that of laboratory voles infected for 60 and
90 days. Reaction to the ova of the parasite was generally not
very marked. Often ova in both the liver and intestine were
surrounded only by a giant cell with a few eosinophils or plasma
cells at the periphery (Fig. 11). Plasma cells were prominent
both in periportal areas and surrounding the giant cell granulomas.

The overall response of some animals was more extensive than
those containing only giant cell granulomas. Ova in these voles
were surrounded by epithelioid granulomas or by an unstructured
accumulation of eosinophils or eosinophils and mononuclear cells.
However, the giant cell granuloma was the most common response
seen. Extensive fibrosis was not visible in naturally infected
animals. Fibroblasts were not evident in giant cell granulomas.

In one heavily infected vole (228 parasites) clumps of eggs
formed plaques along the length of the intestine. The ova were
usually within a matrix of giant cells often bordered by plasma
cells (Fig. 9). Ova were present at all stages of development.
In the small intestine mature eggs were observed in the lamina

propria of the villi unassociated with any reaction (Fig. 10).

a
‘ c: o '
\..

f

.9
0‘1 I
a
ll

‘Q(J.'\

7‘ .
\“9.

-\.
p‘ n
»f .C‘

3’
/f

 

Fig. 9. Wild vole. Adult schistosomes and ova in cecum of a
heavily infected animal. x 40.

 

Fig. 10. Wild vole. Mature schistosome ova in the small intestine
unaccompanied by host response. x 64.

82

 

‘\h.

Fig. 11. Wild vole. Typical giant cell granuloma in the liver.
x 360.

83

Few ova were visible in the liver, but there the reaction was

very highly circumscribed and of the giant cell type and plasma
cells were prominent. Adult schistosomes were also present in the
blood vessels of the intestine, where they appeared to elicit

no reaction in the wall of the parasitized vein.

Mice

In mice, as in voles, the cellular composition of the reaction
to schistosome ova was variable and could not be correlated with
the stage of maturity of the ova. Also, as has been observed
with ML pennsylvanicus eggs at all stages of development could
be found unassociated with significant tissue reaction (Fig. 13,
Fig. 18).

Mice infected for 15 days had no schistosome lesions in their
tissues. No adult schistosome pigment was present in the liver,
however, a cellular infiltrate was observed in some portal areas
composed of mononuclear cells, eosinophils and some lymphocytes
and neutrophils (Fig. 12).

Thirty DAI schistosome pigment was visible in hepatic phago-
cytic cells. Ova in both the liver and intestine were generally
surrounded by an unstructured collection of eosinophils, macro-
phages, mononuclear cells and lymphocytes similar to those seen
in voles (Fig. 15). In some instances, an extensive eosinophilic
response was observed. In the intestinal tissue ova were observed
not only in the lamina propria, but also in the submucosal muscle
and connective tissue. Submucosal muscle showed an increased

number of mast cells in areas near ova and their accompanying reactions.

84

 

Fig. 12. Mouse. Mononuclear cell periportal infiltrate in the
liver. x 421.

 

Fig. 13. Mouse. Undifferentiated ova in the intestine. x 400.

85

 

Mouse. Extensive inflammatory reaction in the liver.
Eosinophils are prominent. x 200.

Fig. 14.

 

Numerous

Unstructured host response to ova.

15. Mouse.

Fig.

x 205.

cell types are present.

86

Mast cells were also seen with some frequency within the granulomas
themselves.

Reaction to parasite ova in tissue from animals infected
for 45 days was largely similar to that of 30 day animals. Dev-
eloping granulomatous responses to eggs were in many cases diffuse
and covered an extensive area in relation to the size of the
ovum (Fig. 14), while other ova were enclosed in more organized
epitheliod granulomas (Fig. 16). Typically, a central area of
macrophages, fibroblasts and giant cells was surrounded by a periperal
border of eosinophils or eosinophils and lymphocytes. No reaction
comparable to the giant cell granuloma of voles was observed.

In the intestine ova were seen in all layers, where they
frequently elicited an active reaction (Fig. 19). Only infrequently,
however, were calcification of eggs and central necrosis of the
granulomasobserved. The intestinal muscoa appeared intact and
there was little evidence of hemorrhage into the intestinal
lumen.

By 60 and 90 days post infection massive involvement of the
intestinal tract was present in some individuals (Fig.20, Fig.2fl).
Extensive inflammatory reactions similar in character to those
described at earlier stages involved all parts of the intestinal
tissue. In the liver, fibrosis was observed in the periportal
areas and within established granulomas (Fig. 17). Plasma cells
appeared more frequently in the hepatic portal infiltrates, as they
did in the intestine, but were not a prominent part of the reaction
in either location. As in the voles, schistosome pigment was

visible in the spleen at 60 DAI, but fUrther involvement of other

organs was not visible.

87

 

Fig. 16. Mouse. Epithelioid granuloma with lymphocytes and
eosinophils present on the periphery. x 400.

J.” .

:>-. ' 't'_ _ ‘_ '
_ . 'J '- .
. '. _ . . ‘
- - Lilia 1. ..
‘ o f ' ' ‘

4“ ‘,.~'_' 314‘...’

 

Fig. 17. Mouse. Granuloma showing prominent deposition of
connective tissue. x 400.

 

Fig. 18. Mouse. Mature ova in the lamina propria of the small
intestine. x 200.

'g,fi iii;wr ;;
‘ ‘3"?132

D'.

1p. ‘3' ~44
;' . ‘15;

4 ’3‘.~" ,Y b. ‘ . ,_ ' _
H. gay,” r, ‘15,. _

 

Fig. 19. Mouse. Intestinal granuloma containing mature ova.
x 150.

90

 

Fig. 20. Mouse. Schistosome ova and extensive host reaction in
the small intestine. x 45.

     

l

L

Fig. 21. Diffuse host reaction in the submucosal tissue of the
small intestine. x 64.

91

DISCUSSION

Results of these studies permit several general conclusions
to be drawn about the clinicopathologic course of S. douthitti
infections. Experimental infections in the mouse are clearly
capable of producing a severe and often fatal disease which has
no parallel in equivalent infections in the meadow vole, M.
pennsylvanicus, although schistosome eggs and associated lesions
are readily demonstrable in both species. Wild voles naturally
infected with the parasite are evidently capable of maintaining
massive worm burdens with no appearance of ill health or loss
of condition. Comparison of the histologic reaction to schistosome
ova shows that marked variation occurs between individuals of each
host species. Within the tissue of individual animals variation
also occurs in the types of reactions which develop, but maturity
of the schistosome embryo does not appear to be related to the stage
of the granuloma associated with it. The types of cells participating
in mouse and vole reactions are generally similar, and differences
between the two hosts are manifested primarily as variation in
the degree to which particular cell types are present at any given
time. In the vole fibrosis does not seem to be a characteristic
of the reaction, whereas both giant cells and plasma cells are
prominent; on the other hand, in the mouse these two cell types
are not abundant, but fibroblastic activity and deposition of
connective tissue is extensive. However, in view of the dramatic
difference in the degree of clinical disease seen in the two hosts,
the nature of the responses to ova were not as dissimilar as might

be expected. Mice and voles both showed an absence of response

92

to adult worms, so that like S. mansoni (Smith and von Lichtenberg,
1974), S. douthitti adults appear to stimulatelittle host reaction.
A generalized vasculitis comparable to that seen in the vessels

of cattle infected with S. bovis (Hussein, 1971) and S. mattheei
(Lawrence, 1978b) is certainly not a feature of this infection.

The sequence of events observed in mice confirms the results
described by Kagan and Meranze (1957, 1958) who demonstrated
that the development of schistosome lesions in laboratory mice
experimentally infected with S. douthitti shares a number of
characteristics with the response described by other authors
for murine S. mansoni infection. However, it is clear that while
basic similarities occur in the host response in mice and voles,
differences between the two species are present and future research
will attempt to identify the underlying reasons for these differ-
ences. Nonimmunological factors probably contribute to the
observed variation, but it also seems likely that species var-
iation in the manifestation of delayed type hypersensitivity to
schistosome ova could play a large part in influencing the patho-
genicity of the infection in the two rodent species.

Several factors increase the difficulty of quantifying the
differences between the response of the two hosts. For example,
accurate description and detailed sequencing of events leading
to the development of the mature granuloma is hampered by the wide
variation existing between and even within individuals at any
given time after infection. Likewise, the continuous arrival
of ova in the tissue makes it impossible fortfluaobserver to relate

the type of reaction with the length of time an egg has been

 

93

present in a particular organ. Von Lichtenberg et a1 (1973)
and Hsu et a1 (1972) attempted to correlate the stage of development
of the schistosome embryo with the maturity of the granuloma,
but the former workers concluded that both inflammatory and
granulomatous reactions could be seen surrounding eggs at all stages
of development.

In both species the eosinophil was one of the most striking
constituents of the response to the parasite throughout infection.

Its prominence, both in the peripheral blood and in granuloma

 

formation has been recorded by several authors (Cheever, 1965b;
Hussein and Tartour, 1973; Mahmoud et a1, 1975; Bartsch and Ward,
1976; Lawrence 1978a). In view of these observations it is
surprising that Kagan and Meranze (1957, 1958) did not identify
the eosinophil to be an important component of response to S.
douthitti. The function of this cell in the host response to
schistosome ova has not yet been elucidated. James and Colley
(1976) have postulated that the eosinophil plays an important
part in the destruction of the egg, but their observations have
been based on experiments performed in an in vitro system.
Currently, there is no evidence to suggest that similar events
occur in vivo. The participation of the eosinophil in the mod-
ulation of inflammation has recently been described (Leid and
Williams, 1978) and its function in this capacity should be
investigated particularly in light of the presence of mast cells
in the schistosome granulomas.

The prominent giant cells and plasma cells and the absence

of fibrosis in the response of the vole to ova of S. douthitti

94

may all be factors in influencing the clinical course of infection.
The compact giant cell granuloma seen in voles may more effectively
contain the metabolic antigens of the schistosome embryo which
could otherwise stimulate a more extensive inflammatory reaction.
Giant cell granulomas intrude less on normal tissue than the more
diffuse epithelioid granuloma seen in mice. This consideration

is likely to be particularly important in view of the finding of
Cheever (1965a) who demonstrated a direct relationship between
granuloma size and the development of increased portal pressure.
The extensive fibrotic response seen in chronically infected mice
prolongs the period of tissue damage and extends its area, and

this too probably contributes to an increase in portal pressure.
Plasma cell activity has been suggested as another means of
modulating the intensity of the reaction to ova (Warren, 1976),

by the production of blocking antibodies which may enhance the
capture and sequestration of egg antigens, thereby preventing

the stimulation of a more elaborate and destructive inflammatory
reaction. The prominence of plasma cells in the reaction of the
vole may therefore also form a part of the mechanism whereby ex-
tensive tissue damage is avoided in this species.

Some indication of the importance of these differences between
mice and voles is provided by the observations on wild vole tissue.
Wild ML pennsylvanicus, at times carrying massive worm burdens,
were found to be apparently healthy. The tissue reaction to schis-
tosome ova in these infected animals was dominated by the giant
cell granulomas. In the light of the intensity with which voles

are preyed upon by most predators in the area (Burt, 1975), any

95

reduction in the fitness of infected animals would be expected to
result in their almost immediate elimination from the population.
It seems reasonable to suppose therefore that infection and the
associated tissue reaction did not affect the performance of the
voles adversely.

It is important to recognize, however, that while the form
of the granuloma and plasma cell activity may contribute to the
inhibition of chronic disease in voles, these factors do not

necessarily account for the remarkable absence of clinical disease

 

early in infection when mice suffer a high rate of mortality.

At this stage, about six weeks following exposure to cercariae,

there was little difference in the appearance of the tissue reaction

to ova in the two host species. It seems likely that additional

factors, as yet unidentified, also contribute to the differential

mortality seen in the two hosts. One possible mechanism has been

suggested by Lawrence (1973) in work with S. mattheei. He found

that cattle are able to inhibit egg production of S. mattheei

adults shortly after patency is reached, thereby directly elimin-

ating the stimulus leading to clinical schistosomiasis. Sheep,

which are far more susceptible to the disease, do not show a sim-

ilar decrease in egg production (Lawrence, 1974). Whether com-

parable differences occur in mice and voles infected with S.

douthitti is not known, but the importance of the quantity of

eggs produced by the parasite should be pursued in future studies.
The most thoroughly studied mechanism of affecting the

severity of disease in schistosomiasis is the modulation of granuloma

formation developing over the course of infection, first described

96

by Andrade and Warren (1964). The mechanism of this modulation

has not been conclusively established, but both suppressor T cells

and the activity of blocking antibodies have been implicated

so far (Lewis and Colley, 1976; Warren, 1976). No evidence of

modulation of response in mice was visible in this study, but

more extensive experimentation may demonstrate its presence in

S. douthitti infections of mice. +-
It is currently impossible to select one factor which is

responsible for the absence of disease in voles infected with i

 

S. douthitti. A more realistic assessment may be that several
factors, each of which would be ineffective alone, contribute to the
reduction in pathogenicity of the parasite. Some of these factors
may be characteristic of the vole's response to a wide range of
pathogens. Although little information is available on histo-
pathologic responses of the vole, Robb-Smith (1946) has reported
that fibrosis is limited in the reaction of the European meadow
vole, Microtus agrestis, to Mycobacterium infection. While giant
cells and plasma cells were present in the response to the bacteria,
they were not abundant, and the prominence of these cells in

S. douthitti infections of the vole may reflect more specific
immunologically mediated adaptations to the parasite. In any

event, the pecularities of the nature of the vole reactivity
justify further interest in this system. Understanding the com-
ponents of the response of the vole may, in the future, provide
important and useful insights into potential means of attenuation

of immunologic disease in man.

10.

11.

12.

97

LITERATURE CITED

Andrade, Z. A. and Warren, K. S. 1964. Mild prolonged
schistosomiasis in mice: Alterations in host response with
time and the development of portal fibrosis. Trans. Roy.
Soc. Trop. Med. Hyg. 58:53-57.

Bartsch,R. C. and Ward, B. C. 1976. Visceral lesions in
raccoons naturally infected with Heterobilharzia americana.
Vet. Pathol. 13:241-249.

Burt, W. H. 1975. Mammals of the great lakes region.
The University of Michigan Press, Ann Arbor.

Cheever, A. W. 1965a. A comparative study of Schistosoma
mansoni infection in mice, gerbils, multimammate rats and L:
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