Regional distribution of two dairy-associated Salmonella enterica serotypes

Salmonella enterica is a zoonotic pathogen that is often associated with dairy farms. The organism can cause disease in cows but is also frequently shed in large numbers by dairy cows that are asymptomatic. Long-term asymptomatic infections with serotypes Cerro and Kentucky were previously identified in cows on a 100-head dairy farm in Pennsylvania, United States (focal dairy). Milk filters were collected from farms within 30 miles of the focal dairy to determine whether the infections by Cerro and Kentucky were limited to the focal dairy or whether the infection might be more regional in nature. Analysis of milk filters showed that Cerro and Kentucky were widespread in the surrounding region with 16 of 39 farms (41%) positive for one or both serotypes. Pulsed-field gel electrophoresis showed that the milk filter Kentucky strains shared >90% similarity with strains from the focal dairy and from local streams. Although there was more variation between Cerro strains (>80% similarity), most milk filter Cerro isolates from most milk filters were highly similar (>90%) to strains isolated from the focal dairy and local streams. In this intensely dairy-farmed region, Salmonella infection of dairy cows appears to be regional in nature, a fact that will impact efforts to control these pathogens.