Clinical, Pathological and Immunological Aspects of Transplacental PRRS Virus Infection: Results from Danish Experiments

The present paper describes Danish research activities on porcine reproductive and respiratory syndrome (PRRS) with emphasis on experimental infections in pregnant swine. The first case of PRRS was diagnosed in Denmark in 1992 and subsequently the disease spread to most other parts of the country. The first animal experiments elucidated the pathogenicity of Danish PRRS virus (PRRSV) isolates in pregnant sows together with the effects of infection at various stages of gestation. In 1996, the introduction of a vaccination program using an attenuated live PRRS vaccine led to an epidemic of American type PRRSV in the previously unaffected Danish pig population. Acute PRRS like disease was observed in non-vaccinated as well as in vaccinated herds, and it was demonstrated that the vaccine strain had reverted to virulence. By experimental infection of late term pregnant sows, we demonstrated that a field isolate of PRRS vaccine-derived virus (VDV) could cause disease in swine consistent with PRRS, thus confirming the etiological role of VDV. Since the complex pathology following in utero infection with PRRSV indicates impairment of the immune system of congenitally infected pigs, we studied various aspect of the host defence in piglets surviving transplacental infection with PRRSV. Leukocyte subpopulations in peripheral blood and bronchoalveolar fluid (BALF) were modulated, viability of lung macrophages was reduced, phagocytosis against Salmonella in blood monocytes as well as oxidative burst capacity of alveolar macrophages was inhibited, there was an over-expression of cytokine IL-10 in BALF cells, and ciliary disruption in the airways was observed. Altogether, our findings supported the hypothesis of the existence of immunosuppression in piglets congenitally infected with PRRSV.