Identifying Active Salmonella Infections in Swine Nurseries Using Serology and Bacterial Culture and Evaluating Associated Risk Factors

Abstract

The objectives of this study were: to identify nursery cohorts with an active Salmonella infection using combined serological and bacteriological methods, and to try to identify risk factors associated with swine nurseries with active Salmonella spread. Twenty pigs from each of 50 cohorts of weaned pigs from 44 different nursery barns were sampled about the time of weaning and near the end of the nursery stage. Information regarding farm management and biosecurity practices were collected using a questionnaire. Blood samples were obtained at both visits, while rectal swabs were collected at the second visit. An enzyme-linked immunosorbent assay (ELISA) was used to test sera for Salmonella antibodies and rectal samples were cultured for Salmonella. A nursery cohort was identified as having an active Salmonella infection if Salmonella was cultured from one or more of the 20 pigs or if serological evidence suggested exposure to Salmonella. The association between farm-level management covariates and active Salmonella infection was assessed in 46 cohorts using a logistic regression model. Nine of 46 (20%) cohorts produced Salmonella-free pigs. The remaining 37 (80%) cohorts were classified as having an active infection. Examination of risk factors failed to identify how negative and positive nurseries differed.

Keywords: Salmonella; epidemiology; nursery pigs; risk factors; swine.

Figure 1

The change in Salmonella antibody titres in nursery pigs (n = 930, from 50 swine cohorts) from weaning, at visit 1 (V1), to the end of the nursery, at visit 2 (V2), is characterized into eight possible Salmonella antibody response patterns (1–8) ^a. Salmonella seropositivity status (seropositive (Sp) if antibody titre level (based on S/P ratio) ≥0.3, seronegative (Sn) if antibody titre level <0.3).^a Salmonella antibody response patterns (1 to 8) 1—Sp at V1, increases in antibody titre 2—remains Sp (no change) 3—Sp at V1, decreases in antibody titre but remains Sp 4—Sp at V1, Sn at V2 5—Sn at V1, Sp at V2 6—Sn at V1, increases in antibody titre but remains Sn 7—remains Sn (no change) 8—Sn at V1, decrease in antibody titre.