Effect of sequential porcine reproductive and respiratory syndrome and swine influenza on the growth and performance of finishing pigs.

Sows and gilts on a breeding and finishing unit seroconverted to porcine reproductive and respiratory syndrome (PRRS) virus between February and March 1992. The signs of the disease progressed through the breeding herd to the piglets, weaners, growers and finally the finishing pigs. Forty-eight finishing pigs, on a nutritional experiment in which the individual pigs' food intake was recorded daily, showed signs of PRRS and later of swine influenza virus infection. Blood samples collected at slaughter from 47 of the pigs were seropositive to PRRS virus but seronegative to swine influenza virus. However, an immunocytochemical examination of a representative sample of the lungs of the pigs showed that swine influenza virus was present, despite the negative serological results. The infection of the finishing pigs with PRRS virus was associated with recurrent periods of inappetence and a decline in growth rate and performance. The total financial loss from the sequential infection with the two viruses, in a herd without high health status, was estimated at approximately 7 pounds per pig.