Prevalence and serovars of Salmonella enterica isolated from ileocolic lymph nodes of market pigs reared in selected Midwest US swine herds

Abstract

Objectives: To describe the prevalence and serovars of Salmonella enterica in ileocolic lymph nodes of slaughtered swine in a sample of Midwest US herds and to assess as methods of study pooling and freezing of lymph-node samples prior to bacterial culture. Materials and methods: Ileocolic lymph nodes from 30 pigs from each of 146 herds were sampled at slaughter. Tissue from five pigs was pooled for one bacterial culture. Retained frozen tissues from the same pigs were cultured individually (n = 82 herds) from a subset of those with Salmonella-positive pools (n = 100 herds). A mathematical relationship was described to predict approximate individual prevalence based on number of positive pools. Isolates were serotyped. To test for effects of freezing on test sensitivity, lymph nodes from 100 pigs were cultured both fresh and after freezing. Results: Salmonellae were detected in 100 of 146 herds (68.5%). The mean number of positive pools per herd was 1.75, and the mean within-herd, individual-pig prevalence was 6.98% (95% CI, 4.88% – 9.07%). Freezing of samples did not result in decreased detected prevalence. Individual prevalence could be approximately predicted by pool results, although with low precision. Implications: Salmonellae were found in two-thirds of the herds studied. Culture of pooled samples with subsequent culture of retained frozen tissues from positive pools may be an effective way to test a larger number of herds on a given budget through laboratory-cost savings. However, pooling without culture of individuals from positive pools results in imprecise prevalence estimation.