Evaluation of land usage impacts on waterborne Salmonella of public health importance

Salmonellosis continues to be a significant cause of human gastrointestinal disease, and a recent rise in the incidence of infection with resistant organisms has been associated with increased morbidity and mortality. There is increasing evidence that drinking water may be an under-recognized cause of human illness. This study examined the potential for surface water to be a source of Salmonella of human clinical importance in Eastern North Carolina. Surface waters adjacent to human land usages and swine confined animal feeding operations (CAFOs) were studied monthly and during significant precipitation events for 18 months. Study swine CAFO wastewaters and samples from regional human wastewater treatment plants (WWTPs) were also examined. The density of Salmonella spp. in all samples was estimated, and environmental factors were analyzed in univariate and bivariate analyses to ascertain variables associated with observed Salmonella densities. Negative binomial regression models were used to evaluate significantly associated land use and environmental variables in multivariable analysis. In addition, over 400 isolates were selected for serotyping and antimicrobial resistance testing. Wastewater and surface water serotypes were matched temporally and by antimicrobial resistance profiles; and pulsed field gel electrophoresis was done to evaluate molecular clonality. Human and swine CAFO land usages were significantly associated with elevated Salmonella densities in impacted surface waters in the final regression model. Other factors associated with increased Salmonella counts included precipitation of at least 0.5 inches, total suspended solids, and summer months. Salmonella was more frequently isolated and measured densities were significantly higher during precipitation event sampling. In addition, Salmonella serotypes found in surface waters were consistent with those commonly isolated from human clinical disease. Seven isolates obtained from swine CAFO wastes and adjacent surface waters were clonally related, and antimicrobial residues in surface waters were associated with resistance among waterborne Salmonella isolates. Swine CAFOs were also associated with an increased prevalence of antimicrobial residues and Salmonella resistance in surface waters. The findings of this study suggest that waterborne Salmonella may be a source of human infection, and exposure to water impacted by swine CAFOs may be a pathway of human exposure to zoonotic pathogens.