Risk factors for the herd-level bacteriologic prevalence of Salmonella in Belgian slaughter pigs

Herd-level risk factors for salmonellosis in pigs were investigated in a cross-sectional study on 62 Belgian farrow-to-finish pig herds belonging to one slaughterhouse cooperative. Data concerning housing and ventilation, management, hygiene, biosecurity, production parameters, feeding, disease control and transport to the slaughterhouse were collected during a herd visit by means of a questionnaire. The percentage of positive animals in a slaughterhouse delivery, as determined by qualitative Salmonella isolation in the mesenteric lymph nodes taken from 30 slaughter pigs, was the outcome variable. All samples were taken in 4 different slaughterhouses. Variables first were submitted to a univariable analysis using a logistic mixed regression model, with herd as random effect. Variables which were related to the Salmonella prevalence (P < 0.05) were analysed further in a multivariable model. The clustering of Salmonella infection within a pen also was studied in a generalised mixed model with pen as random effect. Salmonella isolates were identified by serotype. In 57 (92%) of the herds, at least one sample was found positive for Salmonella. The median percentage of positive Salmonella samples per delivery was 64% (range: 0-100%). In the multivariable model, only type of floor was related significantly to the prevalence: 100% (95% CI 88-100) for herds with <50% slatted floors to 54% (36-70) for herds with fully slatted floors. The results from the analysis should be interpreted with care because only 62 herds were included in the study. Clustering between pigs from the same pen could not be demonstrated (variance +/- S.D.: 0.11 +/- 0.16). S. typhimurium (30%) and S. derby (20%) were most common among the 23 different serotypes that were found.