Experimental Dual Infection of Specific Pathogen‐Free Pigs with Porcine Reproductive and Respiratory Syndrome Virus and Pseudorabies Virus

Summary Twenty 6‐week‐old specific pathogen‐free pigs were divided into four groups. On day 0 of the experiment, PRRSV–PRV ( n  = 6) and PRRSV ( n  = 4) groups were intranasally inoculated with porcine reproductive and respiratory syndrome virus (PRRSV) (10 5.6 TCID 50 ). On day 7, the PRRSV–PRV and PRV ( n  = 6) groups were intranasally inoculated with pseudorabies virus (PRV) (10 3.6 TCID 50 ). Control pigs ( n  = 4) were kept as uninoculated negative controls. Half of the pigs in each group were euthanized and necropsied on day 14 or 21. Clinical signs such as depression and anorexia were observed in the PRRSV–PRV and PRV groups after inoculation with PRV. Although febrile response was observed after virus inoculations, the duration of that response was prolonged in the PRRSV–PRV group compared with the other groups. The lungs in the PRRSV–PRV group failed to collapse and were mottled or diffusely tan and red, whereas the lungs of the pigs in the other groups were grossly normal. Histopathologically, interstitial pneumonia was present in all PRRSV‐inoculated pigs, but the pneumonic lesions were more severe in the PRRSV–PRV group. Mean PRRSV titres of tonsil and lung in the PRRSV–PRV group were significantly ( P  < 0.05) higher than that in the PRRSV group on day 21. These results indicate that dual infection with PRRSV and PRV increased clinical signs and pneumonic lesions in pigs infected with both viruses, as compared to pigs infected with PRRSV or PRV only, at least in the present experimental conditions.