Longitudinal Study ofSalmonella entericaSerotype Typhimurium Infection in Three Danish Farrow-to-Finish Swine Herds

Abstract

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.Salmonella enterica does not normally cause clinical disease in pigs, but subclinical Salmonella infections constitute an important food safety problem throughout the world. From a consumer viewpoint, continuing efforts are needed to reduce the occurrence of Salmonella in pork. In order to achieve this, information about the dynamics of Salmonella infections in swine herds over time (e.g., duration of infection and disease transmission patterns) can be a useful tool. However, so far only limited information in this field is available.Results from a comprehensive longitudinal study in two multiple-site pig production systems in the United States revealed considerable temporal variability in Salmonella prevalence between cohorts of pigs (11). In a Danish study, the Salmonella status of sow herds was measured based upon (i) the prevalence of serotype Typhimurium bacteria among weaners and (ii) the seroprevalence among sows. Both factors were shown to constitute important risk factors for Salmonella infection in finisher herds, as measured by the high seroprevalence found by examining meat juice samples at slaughter (15)....

Salmonella in sows: a longitudinal study in farrow-to-finish pig herds

“…Comparable low prevalences were seen in other studies [12,17,28]. Despite the hormonal changes like the rise in plasma cortisol [16], and the decreased immunity in the periparturient sow [33], a change in the shedding pattern of Salmonella in sows could not be demonstrated during these periods.…”

ModellingSalmonellaspread within a farrow-to-finish pig herd

“…This increase is related to the health states and transitions modelled, and has been shown in the literature [3,17]. Our model differs from other models, which, probably because they considered a recovery state [13], simulate a seroprevalence fade-out at the end of the finishing period.…”

“…Infected animals shed intermittently Salmonella in their faeces [17,22] and, hence, contaminate their room. The model represents the indirect faecal-oral transmission via free-living Salmonella in the room (in the faeces, on the room floor, pen separations and pig bodies).…”

Experimental airborne transmission ofSalmonellaAgona andSalmonellaTyphimurium in weaned pigs

“…In fact, seroconversion has been shown to vary substantially among inoculated pigs (6-37 days) and seroconversion usually peaks around 30 DPI [33]. In naturally infected pigs, this lag-time between infection and seroconversion may be even longer [34].…”