Prevalence and Relatedness of Salmonella in the Environments of Livestock Markets Handling Surplus Dairy Calves

Livestock markets are critical hubs within animal trade networks that influence pathogen dissemination at the regional and national scale. Indeed, a 2016 Salmonella serovar Heidelberg outbreak, initially linked to surplus dairy calves at livestock markets, sickened 63 people across 17 states. The objectives of this repeated cross-sectional study were to (i) assess the prevalence and relatedness of Salmonella serovars of public health relevance in Ohio and Wisconsin livestock markets; and (ii) determine if S. Heidelberg was circulating in Midwestern livestock markets.

Twenty-four livestock markets (14 from WI and 10 from OH) were enrolled in the study from April to November 2019. Market environments were sampled twice 3 months apart. State inspectors used boot swabs to sample the loading docks, main livestock thruway, and two pens used to hold surplus calves at each market. Swabs were shipped overnight to the Wisconsin Veterinary Diagnostic Laboratory for sample culture and serotyping. Whole genome sequencing was conducted at the Ohio Department of Agriculture.

Sample-level Salmonella prevalence was 99.1% (111/112) in WI markets and 94.9% (75/79) in OH markets with all negative OH samples collected from the same market. Salmonella ser. Heidelberg was not recovered from any market; however, other serovars of public health relevance were identified including Newport, Agona and Typhimurium. Sequencing data revealed closely related strains across markets. For instance, a group of closely related Salmonella ser. Panama isolates was recovered from three WI markets, and the isolates clustered closely with isolates from retail meat, other livestock species, and human diagnostic labs.

These results suggest livestock markets play an important and under-researched role in the dissemination of pathogens between livestock populations.