Time-course of antibody and cell-mediated immune responses to Porcine Reproductive and Respiratory Syndrome virus under field conditions.

Major discrepancies are observed between experimental trials of PRRS-virus (PRRSV) infection in isolation facilities and observations made in the field on farm. Owing to the above, a cohort study was carried out in a farrow-to-finish, PRRSV-infected pig farm to characterize the time-course of the virus-specific immune response in two groups of replacement gilts. Despite the occurrence of three and two distinct waves of infection in groups 1 and 2, respectively, the large majority of animals showed little if any PRRSV-specific response in an interferon-gamma release assay on whole blood, whereas non-specific responses were consistently observed. To rule out any possible bias of our test procedure, this was used along with an ELISPOT assay for interferon-gamma-secreting cells with the same reagents on a group of PRRS-virus infected pigs in isolation facilities. A very good agreement was shown between the two sets of results. Also, as opposed to the PRRS model, plenty of Pseudorabies virus-vaccinated pigs under field conditions scored positive in another experiment in the interferon-gamma release assay, ad hoc modified for the Pseudorabies virus. Our results indicate that under field conditions poor or no development rather than delayed development of the PRRS virus-specific interferon-gamma response could be the rule for a long time in non-adult pigs after PRRS virus infection. Housing and hygiene conditions, as well as heavy exposure to environmental microbial payloads in intensive pig farms could adversely affect the host's immune response to PRRS virus and partly account for the discrepancies between experimental and field studies.