Submitted to: Journal of the American Veterinary Medical Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2002
Publication Date: 3/1/2002
Citation: Journal of the American Veterinary Medical Association. 2002. v. 220. p. 645-649.
Interpretive Summary: Bacteria in the genus Salmonella are frequent agents of infection and disease in both beef and dairy cattle. Salmonellosis is a costly disease to dairy producers because of treatment costs, increased culling rates, reduced feed efficiency, lowered weight gains, and decreased milk production. Inapparent (subclinical) infection with Salmonella is more common than clinical disease in cattle. Consequently, subclinically-infected cattle pose a risk to public health when they are culled and enter the food supply as beef products. In this study, we estimated the frequency (prevalence) of Salmonella in the feces of mature (i.e., >2 years of age) dairy cattle herds in 44 Ohio counties, and then used epidemiologic techniques to try to identify characteristics of dairy cattle herds that make them more or less likely to be infected with Salmonella. We found one or more Salmonella infected cows in 31% of 105 sampled herds, and 6% of 7,776 fecal samples were Salmonella-positive. Within-herd fecal prevalence (i.e., the proportion of infected cows within a herd) ranged from <1 to 97%. Larger herd size, use of free stalls, and use of straw bedding for non-lactating cows were associated with increased Salmonella fecal shedding based on simple statistical analysis. However, a more complex multivariate statistical model of the data showed larger herd size was the only significant risk factor for Salmonella-positive herds. There are more than 2,200 different Salmonella subtypes (i.e., serogroups and serotypes). We found a single Salmonella serogroup in 54% of the positive dairy herds, and serogroup C3 was the most frequently identified serogroup (15% of isolates). However, one herd had four different Salmonella serogroups present.
Technical Abstract: Objective - To estimate prevalence of Salmonella spp in Ohio dairy farms and to identify potential risk factors for fecal shedding of salmonellae. Design - Cross-sectional study. Sample Population - 105 Ohio dairy farms. Procedure - Individual fecal samples from all mature cows in study herds were tested for Salmonella spp by use of standard bacteriologic culture procedures. Herds were identified as infected if at least one cow was shedding Salmonella spp. Information regarding herd characteristics, management practices, and health history were collected. Potential risk factors for herd-level Salmonella infection were identified. Results - In 31% of the study herds (95% confidence interval, 22 to 40%), at least one cow was shedding Salmonella spp. Six percent of 7,776 fecal samples contained Salmonella organisms; prevalence within infected herds ranged from <1 to 97%. Herd size, use of free stalls for lactating and nonlactating cows, and use of straw bedding in nonlactating cows were significantly associated with fecal shedding of Salmonella spp, as determined by use of univariate analysis. By use of multivariate analysis, large herds were more likely to be infected than smaller herds; however, no other factors were associated with Salmonella infection after adjustment for herd size. Conclusions and Clinical Relevance - Subclinical shedding of Salmonella spp is common in Ohio dairy herds, although we could not identify specific interventions that may influence the prevalence of Salmonella spp on dairy farms. It appears that large herd size and intensive management may provide an environment conducive to Salmonella shedding and chronic dairy herd infection.