Longitudinal study describing time to seroconversion in piglets on three farrow-to-finish farms.

Background: Pigs are frequently colonised with , and this constitutes a major risk for human salmonellosis. The infection can be assessed by the serological response of pigs to . A longitudinal study was undertaken on-farm to correctly describe this serological response and investigate factors associated with age at seroconversion.

Methods: Three pig farms and in each farm three successive batches were considered. Per batch, 40 piglets were selected at random from 10 sows (four piglets per sow). Blood was sampled from sows one week after farrowing and from piglets at weeks 1, 6, 10, 14, 18 and 22 and at the slaughterhouse. antibodies were detected in serum using a commercial ELISA test. Factors related to farm characteristics, batch management system, porcine reproductive and respiratory syndrome infection, and sows' serological status were recorded to assess their effect on age at seroconversion.

Results: At week 1 after farrowing, 96.5 per cent of the sows had antibodies against . The serological results of piglets at weeks 1 and 6 only were positively correlated with those of the sows. The average age at seroconversion was 137±2.2 days (confidence interval at 95 per cent). The first seroconversions occurred from weeks 10 to 14, but most of the pigs (54.6 per cent) were seropositive at the end of the fattening period, with variations between farms and batches (28.9-75.7 per cent). Herd/farm was significantly associated with age at seroconversion.

Conclusion: This longitudinal study allowed the authors to follow precisely the evolution of seroconversion from maternity to slaughterhouse and confirm the relationship between the seroconversion of sows and serology of their piglets. Moreover, factors related to farm practices and management as a whole are more influential than individual factors (at the pig level) on age at seroconversion.