Clinical effects of experimental dual infections with porcine reproductive and respiratory syndrome virus followed by swine influenza virus in conventional and colostrum-deprived pigs.

Previous studies demonstrated that experimental dual infections of pigs with porcine reproductive and respiratory syndrome virus (PRRSV) followed by H1N1 influenza virus cause more severe disease and growth retardation than the respective single virus infections. Here three experiments were undertaken to better define the clinical impact of combined PRRSV-H1N1 infections in conventional and caesarean-derived colostrum-deprived (CDCD) pigs. Groups of pigs were inoculated by aerosol with PRRSV followed by H1N1 at 3-, 7- or 14-day intervals. During the post-H1N1 period, mean body temperatures, respiratory signs and mean weight gains in the PRRSV-H1N1 inoculated groups were recorded and compared with those in uninoculated controls (experiments 1 and 2) or in singly virus-inoculated pigs (experiment 3). In a first experiment with conventional pigs, the PRRSV-3d-H1N1 and PRRSV-7d-H1N1 infections induced mean body temperatures > 40.5 degrees C during 8 days (peaks 41.1 and 41.6 degrees C, respectively) and mean growth reductions of 3.4 and 4.8 kg, respectively, during the 2 weeks after H1N1, along with marked depression and respiratory disease. The PRRSV-14d-H1N1 infection, on the contrary, was largely subclinical. In a second experiment with conventional pigs, PRRSV-3d-H1N1 and PRRSV-7d-H1N1 infections were clinically milder, with smaller increases in mean body temperatures (peak 40.5 degrees C in both groups) and growth reductions (1.4 and 1.6 kg, respectively). In both groups, only one pig showed prominent general and respiratory signs. In a final experiment with CDCD pigs, PRRSV-7d-H1N1 infection had minimal effects on mean clinical performances and growth and, except for one pig that was severely affected, differences with the single virus inoculations were negligible. Thus, both the time interval between infections and the sanitary status of pigs can affect the clinical outcome of dual PRRSV-H1N1 infections. However, factors so far unknown seem to cause large variations in the clinical response between individual pigs.