Prevalence of Salmonella in Cull Cows in California

Adapted by Betsy Karle- UCCE Dairy Advisor, Northern Sacramento Valley

Cull dairy cows have long been identified as a critical food safety control point as they account for approximately 18% of the ground beef supply in the United States. Salmonella is one of several potential food safety concerns and previous studies have reported that between 9.6 and 93% of cull cows could be shedding Salmonella, depending on a wide range of factors. However, data specific to California and our unique management systems have been missing until now. Researchers from the University of California, Davis and the California Animal Health and Food Safety Laboratory set out to determine the prevalence of Salmonella incull dairy cows in California’s Central Valley and to understand what factors affect the likelihood of cows testing positive for the bacterial pathogen.

The researchers sampled 249 cows from four freestall and three drylot dairies, once per season from Spring 2014 through Winter 2015. Information about the management practices and reason for culling the cows was recorded. Overall, only 3.42% (9) of the cows were positive for any Salmonella species, lower than reported in previous studies. The prevalence ranged from a low of 1.97% up to 7.11% between the seven dairies, and all dairies had at least one positive sample throughout the study period. When the data set was analyzed, a statistical difference between the seasons was not apparent, but a closer look revealed some interesting trends. In four of the seven herds, the sampled cows only shed Salmonella during the summer sampling period. Though a statistically significant difference wasn’t clear, the data do suggest that this tendency for more positive Salmonella samples may be related to heat stress and a more conducive (warm and moist) environment for bacterial growth, given the increased use of water for cow cooling during hot Valley summers. Of the management practices that were evaluated, none rose to the top as clearly responsible for a higher or lower Salmonella prevalence.

While it may be tempting to rest on our laurels and proclaim that California doesn’t have a Salmonella problem in cull cows, it’s important to remember that the current study involved a small convenience sample of California dairies, and that further research is needed to tease out management practices significantly associated with Salmonella shedding. The study dairies implemented excellent animal health management practices and thorough treatment protocols, which likely contributed to the lower than previously reported prevalence. The primary reason for culling cows in this study was due to low milk production and reproductive issues, so the majority of the cows sent to market were healthy overall. Salmonella is much more prevalent in cows that are visibly sick, but it often manifests asymptomatically, especially in mature cows. Contamination of feed commodities, rodent and bird activity and seasonal management practices are all important factors to consider as Salmonella and a plethora of other pathogens are primarily transmitted via the fecal-oral route. Attention to the possibility of increased prevalence in Summer is warranted, making management practices during this season especially important.

The full text of this research is accessible at https://peerj.com/articles/2386/.

Aboud, O. A., Adaska, J. M., Williams, D. R., Rossitto, P. V., Champagne, J. D., Lehenbauer, T. W., Atwill, E. R., Li, X., Aly, S. S. (2016). Epidemiology of Salmonella sp. in California cull dairy cattle: prevalence of fecal shedding and diagnostic accuracy of pooled enriched broth culture of fecal samples. PeerJ, 4. doi:10.7717/peerj.2386.

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