Porcine reproductive and respiratory syndrome: NEB-1 PRRSV infection did not potentiate bacterial pathogens

A 2-phase study was conducted to evaluate the ability of the NEB-1 strain of porcine reproductive and respiratory syndrome virus (PRRSV) to potentiate common bacterial pathogens of swine. In phase I, 25 of 50 4-5-week-old specific-pathogen-free (SPF) pigs were exposed to NEB-1 PRRSV (day 0). Seven days after virus inoculation, 8 groups received 1 of 4 bacterial pathogens: Haemophihs parasuis, Streptococcus suis, Salmonella cholerasuis, and Pasteurella multocida. The ability of NEB-1 PRRSV to produce clinical disease, viremia, neutralizing antibody, gross and microscopic lesions and to potentiate bacterial pathogens was assessed. Response to NEB-1 PRRSV was similar among inoculated pigs; prolonged hyperthermia, lethargy, mild to moderate dyspnea, and cutaneous erythema were consistent clinical signs. No clinical differences were observed in groups after bacterial challenge. Virus was isolated from serum at weekly intervals through the end of the study, and all PRRSV-inoculated pigs had seroconverted by study termination. Two of 5 pigs died in non-PRRSV-inoculated groups challenged with H. parasuis and Streptococcus suis. Mortality in PRRSV-infected pigs was limited to 1 of 5 pigs from the Salmonella cholerasuis-challenged group. Gross lesions were seen in pigs dying after inoculation in H. parasuis- and Streptococcus suis-inoculated groups, in Salmonella cholerasuis-and P. multocida-challenged pigs, and in 1 non-PRRSV-inoculated control pig. Microscopic lesions consisted of mild to moderate proliferative interstitial pneumonia, nonsuppurative myocarditis, lymphoid hyperplasia, and nonsuppurative encephalitis in PRRSV-inoculated pigs. Findings in phase I indicated that NEB-1 PRRSV does not potentiate bacterial disease while inducing consistent clinical signs, viremia, seroconversion, and microscopic lesions. Based upon initial findings in phase I, in phase II, 18 of 36 4-6-week-old SPF pigs were exposed to NEB-1 PRRSV (day 0). Two days after virus inoculation, 2 of 4 groups were exposed to either Streptococcus suis or P. multocida. Clinical findings were similar to those of phase I. One of 6 pigs died in each of the Streptococcus suis-challenged groups. Animals succumbed 3 and 5 days after bacterial challenge in the non-PRRSV-inoculated and PRRSV-inoculated groups, respectively. Mild gross and microscopic pulmonary lesions were observed. Phase II findings support the conclusions of phase I that NEB-1 PRRSV does not potentiate certain bacterial infections of swine.