Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/2/2011
Publication Date: 4/1/2012
Citation: Van Kessel, J.S., Karns, J.S., Wolfgang, D., Hovingh, E., Schukken, Y. 2012. Dynamics of Salmonella serotype shifts in an endemically infected dairy herd. Foodborne Pathogens and Disease. 9:319-324. Interpretive Summary: Salmonella is a leading cause of foodborne illness in the United States. Although, it is found in many species of food animals, contamination of foodstuffs by strains of Salmonella found on dairy farms is considered an important source of human exposure. Dairy cattle are known reservoirs of this pathogen; therefore, the products leaving a dairy farm can be contaminated and the environment around the farm can also become contaminated with Salmonella. There are many different serotypes of Salmonella and, although cows can become sick when infected, many serotypes of Salmonella have no impact on the animals. In this case it is difficult for the farmer to know that the animals are harboring a potential human pathogen. In this study we monitored one herd of dairy cows over almost 6 years. We sampled their feces and areas around the farm. We also sampled the bulk milk before it left the farm. The cows on this farm were infected with Salmonella throughout the course of the study. The percentage of infected cows ranged from 8% to 97% and the cows never showed any signs of illness associated with this infection. These bacteria were living commensally or in symbiosis within the animals' digestive systems. During the study period the Salmonella that were infecting the animals shifted from one serotype (Cerro) to a different serotype (Kentucky) and then to a mix of the two serotypes. Understanding how these two serotypes compete with each other in colonizing cows may help us to more fully understand the mechanisms by which Salmonella establishes infection in the gut of dairy cows and may lead to the development of measures to prevent or limit Salmonella infection in cows. Continued efforts are needed to reduce the potential for products leaving the farm to be contaminated with pathogens and thus to lower the risk of public exposure. This information will be useful for both the dairy industry and public health agencies.
Technical Abstract: Salmonella is a leading cause of foodborne illness in the United States. A zoonotic pathogen, it is found in many species of food animals and contamination of foodstuffs by strains of Salmonella found on farms is considered a primary source of human exposure. Here we describe a long term (2004 - 2010) study of Salmonella colonization on a typical dairy farm in the Northeastern United States. The fecal shedding prevalence in the herd ranged from 8% to 97% and greater than 50% of the herd was shedding Salmonella for more than two thirds of the study period. Salmonella enterica serotype Cerro was first detected in September, 2004 after a small and very short-lived outbreak of S. Kentucky. Cerro persisted within the herd for over 3 years, with no clinical signs of salmonellosis in the animals. In the winter of 2006 Kentucky was again detected within the herd and over a 2 year period Kentucky gradually supplanted Cerro. Kentucky was the only serotype detected from March 2008 until September 2009 when Cerro was again detected in 15% of the cows on the farm. Since September 2009 Kentucky and Cerro have coexisted within the herd which continues to harbor these serotypes at high prevalence. PFGE could not discern differences in any of the Cerro strains isolated during this study but it did suggest that the strain of Kentucky that seemed to behave as a commensal in these dairy cows is distinct from the transient strain isolated in 2004. Understanding the dynamics of competition between these two serotypes that seem to behave as commensal colonizers of dairy cows may provide insights into the mechanisms by which Salmonella establishes infection in the lower gut of dairy cows and may lead to the development of measures to prevent or limit Salmonella colonization of dairy cows.