Longitudinal study of Salmonella enterica serotype typhimurium infection in three Danish farrow-to-finish swine herds

A longitudinal study of the infection dynamics of Salmonella enterica was carried out with three Danish farrow-to-finish swine herds. To account for variations in Salmonella shedding over time, litters from each herd were divided into two cohorts. Each cohort consisted of 30 pigs, for a total of 180 pigs. Pigs were individually monitored by monthly bacteriologic and serologic examinations from weaning to slaughter. At weaning, individual sows were examined bacteriologically and serologically. At slaughter, cecal contents, ileocecal lymph nodes, and carcass swab samples were obtained from 131 pigs. A total of 88 pigs were found to be shedding Salmonella on one or more occasions. Only the Salmonella serotype Typhimurium was detected during the study period. At weaning, no sows or piglets were found to be shedding, but a serological reaction was detected in 11 sows. The prevalence in culture peaked in the nursery and subsequently declined to undetectable levels before slaughter. The seroprevalence peaked approximately 60 days after the peak prevalence in culture. Salmonella was detected in individual fecal samples at least once in 53% of the pigs, and 62% of the pigs were seropositive more than once. Only 3.7% of all pigs were found to be culture positive on more than one occasion. Piglets from seroreacting sows had a significantly (P = 0.0339) lower probability of shedding in the nursery. Under the assumption that shedding lasted at least 1 or 2 weeks, the average shedding time was estimated to have been 18 or 26 days. An association between serology, on-farm bacteriology, and Salmonella prevalence in culture at slaughter was shown. Marked differences in prevalence in sera and prevalence in culture between cohorts and within herds were observed. These differences emphasize the need for caution when using point estimates in on-farm interventions and surveillance in subclinically infected swine herds.