Shiga toxin-producing Escherichia coli (STEC) is one of the most important food borne pathogens of humans globally, having caused numerous outbreaks in North America and worldwide. Severe clinical disease occurs primarily in children and immunocompromised adults and signs range from mild diarrhea to hemorrhagic colitis to Hemolytic Uremic Syndrome, which can result in kidney failure and mortality. Cattle are considered the main reservoir of STEC and food or water contaminated with cattle feces is considered to be a major source of human exposure. Common foods implicated in STEC outbreaks include ground beef, unpasteurized milk, leafy vegetables and apple cider. Other domesticated animals and wildlife can also shed STEC, but their importance as a source of human exposure is considered less significant. Human infections have also been reported following direct and indirect contact with animals at zoos, livestock exhibitions and petting farms. Identifying factors that influence STEC shedding and dynamics in cattle is important for the design and implementation of strategies to prevent STEC transmission. The studies describe in this dissertation are the results obtained from an epidemiological study performed in 11 herds in Mid-Michigan during 2011 and 2012. The primary aims of this project were to identify risk factors for STEC shedding and describe STEC dynamics in cattle. Of specific interest was the potential effect on STEC shedding of pre-existing chronic disease, specifically infection with Bovine Leukemia Virus (BLV) and Mycobacterium avium subsp. paratuberculosis (MAP), the causative agent of Johnes disease. We identified several variables including days in milk and number of lactations, to be important individual factors that influence the risk of STEC shedding in dairy cattle. Intervention strategies could be targeted towards these high risk cattle groups. We also confirmed the importance of seasonality, more specifically warm temperatures, on STEC shedding by cattle. No association was observed between STEC shedding and infection with BLV and MAP. We found a significant association between the independent variable herd and rate of new infections with STEC; also we found a significant association between herd and persistent STEC negative status, both in dairy herds. However, we were not able to identify specific management factors that influence the risk of STEC shedding over time. These finding highlight the complex and multifactorial nature of STEC epidemiology in cattle. Based on the results obtained in this dissertation, we conclude that first lactation cows and cows in their first 30 days of lactation have the highest risk of STEC shedding. As a consequence, these specific groups of cattle can be targeted for the design and implementation of intervention strategies at pre-harvest with the aim of reducing STEC infection in humans.
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