This study explored occurrence of the environmentally transmissible forms of Cryptosporidium spp. and Giardia spp. and disease incidence patterns in rural and urban land cover types. The risks of infection and illness caused by Cryptosporidium spp. and Giardia spp. in Combined Sewer Overflow (CSO) discharge receiving waters were also examined. Significant differences in patterns of giardiasis and cryptosporidiosis were found between urban and rural zip codes. The correlation of population with both diseases suggests that population levels are important factors of transmission in the study area. Although differences in the pattern of disease occurrence between urban and rural areas were found, no evidence of a difference was found in post-hoc testing of the occurrence of the environmentally transmissible forms of the parasites between the surface waters of the Grand River and River Raisin watersheds.Health risks due to Cryptosporidium spp. and Giardia spp. in the Grand River at the point of CSO discharge and at recreational sites downstream were assessed. A hypothetical swimming scenario at the discharge point represents moderate daily health risk to a swimmer while multiple day recreation at the discharge point over the recreation season represents a high health risk. Despite CSO discharges, recreational use of the Grand River is predicted to meet the recommended freshwater recreational criteria of 8 illnesses per 1000 swimmers at least 92.4% of the time for both cryptosporidiosis and giardiasis at the three recreational sites examined.
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