The use of quantitative Real-Time PCR to estimate Salmonella shed in fecal samples from naturally infected finishing pigs

Abstract

The objective of this study was to describe the shedding pattern of Salmonella in finishing pigs, as well to quantify the Salmonella load. A longitudinal study was conducted in 12 cohorts of pigs in a multi-site farrow-to-finish production system. At the beginning of each cohort, 50 pigs (10 ± 2 weeks old) were randomly selected and individually identified. Individual pig fecal samples were collected and cultured every 2 weeks for 16 weeks (8 collections). Further, quantitative real-time PCR (q-PCR) targeting the invA gene was performed in a subset of the culture negative samples (555) and in all available culture positive samples. At the time of submission, Salmonella was cultured from 397 (8.74%) of 4540 individual fecal samples. Overall incidence of Salmonella was 24.8% (149/600 pigs). The proportion of positive samples decreased over the finishing period from 16.75% (10 weeks old) to 4.30% (24-26 weeks old). At the present, all Salmonella culture negative samples subjected to q-PCR were PCR negative. Of culture positive samples, 16% were detected by q-PCR, and only 3 % of the samples were within the quantifiable range of detection (>103 colony forming units per gram of feces). Of those samples within the quantifiable range, the bacterial concentration ranged from 1.05x103 to 1.73x106 invA gene copies/g feces. The results suggest that point estimates of Salmonella prevalence may not accurately describe the Salmonella status of finishing pigs. The majority of pigs shed Salmonella at low concentrations. These preliminary data can contribute to quantitative risk assessments of the association between concentrations of Salmonella shed by pigs during the finishing phase and contributions to carcass contamination at slaughter.