Virological, immunological and epidemiological features of porcine reproductive and respiratory syndrome virus (PRRSV) in adult populations.

摘要

Since it was first described in the U.S., the virus inducing porcine reproductive and respiratory syndrome (PRRS), has caused an important productive and economical impact on the global swine industry (Polson et al. 1990). Porcine reproductive and respiratory syndrome virus (PRRSV) produces devastating effects on the breeding herd performance (Polson et al. 1990) and also affects health and growing performance in the growing pig (Keffaber et al. 1992). Therefore, the aim of this thesis was to have a better understanding of PRRSV persistence, epidemiology and genetic variation, and the virological and immunological features following homologous and heterologous challenge of PRRSV in adult populations. First, patterns of seroconversion in seronegative or seropositive replacement gilts following inoculation with the homologous strain of PRRSV as a method of acclimatization, were studied. Following, the dynamics of PRRSV persistence was characterized from 0--135 days pi., to determine if PRRSV persisted for 90 days or more in a population of gilts and if experimentally infected gilts shed virus to naive sentinel gilts beyond 90 days pi. Viremia was detected for up to day 30 pi. and persistence in tissues was confirmed from days 30 to 135. Finally, the kinetics of the humoral and cellular immune response and the virological and clinical features of the primary infection and also the response to secondary infection, following homologous and heterologous PRRSV challenge, were described. Results to primary infection indicated the presence of serum neutralizing antibodies and IFN-gamma on 14 day pi., IFN-gamma peaked at day 50 pi. and declined thereafter until day 120 pi. These results suggest that serum neutralizing antibodies might be involved in the clearance of the virus from blood but not from tissues. A robust homologous protection was detected since the homologous challenge group presented a high and rapid CMI response. In contrast, the groups receiving the heterologous challenge presented an intermediate or null response. However, viremia was only present in all the heterologous groups on day 3 pi. and clinical signs were very mild. It was clear that none of the parameters measured, either alone or in combination, are solely responsible for viral clearance.