Heterologous challenge with porcine reproductive and respiratory syndrome (PRRS) vaccine virus: no evidence of reactivation of previous European-typePRRS virus infection

In Denmark, a porcine reproductive and respiratory syndrome virus (PRRSV) c ontrol programme, comprising vaccination of seropositive herds with a live American type PRRSV vaccine, was started in 1996. In several of these herds , spread of vaccine virus from vaccinated 3-18 week old pigs to non-vaccina ted sows was demonstrated by the isolation of vaccine virus from fetuses an d stillborn piglets. Surprisingly, sows infected with the American type vac cine strain consistently exhibited significantly stronger serological respo nses towards European type PRRSV than American type PRRSV. Tn order to eluc idate whether the unexpectedly strong serological reaction towards European -type PRRSV in American type PRRSV infected sows was due to a booster react ion, or reactivation of an unrecognized, latent infection in the sows with European type PRRSV, a challenge study with the vaccine was carried out. In this study, the stronger serological response towards European type PRRSV than towards American type PRRSV was reproduced, and reactivation of the pr evious natural infection with European PRRSV could neither be demonstrated by virus isolation nor by RT-PCR. So, the increase in antibody titers towar ds European PRRSV in previously European PRRSV infected pigs after challeng e with the vaccine strain seems to be the result of a boosting effect on th e immune system, induced by the heterologous vaccine PRRSV strain. (C) 1999 Elsevier Science B.V. All rights reserved.