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

Previous studies demonstrated that experimental dual infections of pigs wit h porcine reproductive and respiratory syndrome virus (PRRSV) followed by H 1N1 influenza virus cause more severe disease and growth retardation than t he respective single virus infections. Here three experiments were undertak en lu better define the clinical impact of combined PRRSV-H1N1 infections i n conventional and caesarean-derived colostrum-deprived (CDCD) pigs. Groups of pigs were inoculated by aerosol with PRRSV followed by H1N1 at 3-, 7- o r 14-day intervals. During the post-H1N1 period, mean body temperatures, re spiratory signs and mean weight gains in the PRRSV-H1N1 inoculated groups w ere recorded and compared with those in uninoculated controls (experiments 1 and 2) or in singly virus-inoculated pigs (experiment 3). In a first expe riment with conventional pigs, the PRRSV-3d-H1N1 and PRRSV-7d-H1N1 infectio ns induced mean body temperatures > 40.5 degreesC during 8 days (peaks 41.1 and 41.6 degreesC, respectively) and mean growth reductions of 3.4 and 4.8 kg, respectively, during the 3 weeks after H1N1, along with marked depress ion anti respiratory disease. The PPRSV-14d-H1N1 infection, on the contrary , was largely subclinical. In a second experiment with conventional pigs, P RRSV-3d-H1N1 and PRRSV-7d-H1N1 infections were clinically milder, with smal ler increases in mean body temperatures (peak 40.5 degreesC in both groups) and growth reductions (1.4 and 1.6 I;g, respectively). In both groups, onl y one pig showed prominent general and respiratory signs. In a final experi ment with CDCD pigs, PRRSV-7d-H1N1 infection had minimal effects on mean cl inical performances and growth and, except for one pig that was severely af fected, differences with the single virus inoculations were negligible. Thu s, both tile time interval between infections and the sanitary status of pi gs can affect the critical outcome of dual PRRSV-H1N1 infections. However, factors so far unknown seem to cause large variations in the clinical respo nse between individual pigs.