Serological studies on the potential synergism of porcine reproductive and respiratory syndrome virus and influenza-, corona- and paramyxoviruses in the induction of respiratory symptoms in swine.

Sera from 265 finishing pigs belonging to 70 herds, in which severe respiratory disorders were observed, were examined for antibody prevalence to porcine reproductive and respiratory syndrome virus (PRRSV), influenza virus subtypes H3N2 and H1N1, porcine respiratory corona virus (PRCV) and a recently described porcine paramyxovirus (PPMV). By immunoperoxidase-monolayer assay 69.1% of these sera were positive for PRRSV. Hemagglutination inhibiting activity was found in 55.1% of the sera for influenza virus subtype H1N1 (strain A/swine/Arnsberg/1/81). in 51.3% for influenza virus subtype H3N2 (strain A/Hong Kong/1/68) and in 14.3% for PPMV. PRCV specific antibodies, as determined by differential competitive blocking enzyme-linked immunosorbent assay were demonstrated in 192 of 236 (81.3%) sera. In order to reveal associations interspecific coefficients were calculated for antibody prevalence between PRRSV and the other viruses. Positive associations to PRRSV titres were found to PRCV, PPMV and influenza virus subtype H1N1 titres. chi-square analysis showed the statistical significance of associations regarding PRCV and influenza virus subtype H1N1. Depletion of lung macrophages after PRRSV infection is discussed as a possible mechanism for the promotion of secondary infections.