Doctor of Philosophy (PhD)


Plant, Environmental Management and Soil Sciences

Document Type



Shiga-toxigenic Escherichia coli (STEC) O157:H7 is an important human pathogen. Dairy cattle, beef cattle, and white-tailed deer (WTD) are involved in STEC O157:H7 transmission to humans. We conducted cross-sectional studies in Louisiana, using sensitive microbiological methods, in dairy cattle, beef cattle, and WTD to investigate the epidemiology of STEC O157:H7. Results of the dairy studies are as follows. In the point prevalence study, summer-time prevalence in herds was 38.5%, with a cow-level prevalence of 6.5%. Among positive herds, cow-level prevalence ranged from 3%-34.6%. Three of five herds sampled in the longitudinal study were positive. Cow-level prevalence increased during spring and summer. Adult dairy cattle during the warm season had increased odds of STEC O157:H7 fecal shedding. Lactating dairy cows had increased odds of STEC O157:H7 fecal shedding compared to dry cows. In the mouth, hide, and fecal study, cow-level prevalence estimates of STEC O157:H7 in the mouth, on the dorsal hide, and from feces were 0%, 0.7%, and 25.2%, respectively. We sampled weaned beef cattle at the beginning and end of a preconditioning program. Five of twenty-nine herds were shedding STEC O157:H7 or Escherichia coli (EC) O157:H7 initially. No cattle were shedding STEC O157:H7 or EC O157:H7 at the end. We found 0.7% of weaned beef cattle shedding STEC O157:H7. The animal-level prevalence of the O157:H7 serotype, including shiga toxin (stx)-deficient isolates, was 2.5%. We expected, but did not observe, increased shedding or the spread of STEC O157:H7 subtypes. Two WTD field studies were conducted. We collected 338 fecals from hunter-harvested WTD and found one positive sample. The isolate was stx-deficient and sorbitol positive. In the second field study, we isolated STEC O157:H7 in a captive WTD herd, but were unable to demonstrate seasonal trends in fecal shedding. Louisiana reports relatively few human STEC O157:H7 cases. We detected STEC O157:H7 in each of our studies. We demonstrated high fecal prevalence, seasonal shedding, and hide contamination in dairy cattle. Epidemiologic studies in ruminant populations should be revisited using sensitive methods. Studies investigating the human incidence of STEC O157:H7 in relation to presumed ruminant reservoirs are warranted.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Alex Thompson