The effect of vaccination with a parainfluenza type 3 virus on pneumonia resulting from infection with parainfluenza type 3 virus and Pasteurella haemolytica

Abstract

The effect of vaccination of specific pathogen-free lambs with inactivated vaccines of parainfluenza type 3 (PI3) virus on pneumonia resulting from infection with PI3 virus and Pasteurella haemolytica was investigated. Vaccines consisting of formalin-inactivated PI3 virus antigen alone and with double-stranded RNA (dsRNA) emulsified in oil were tested. Both vaccine preparations stimulated a serological response to PI3 virus as detected by haemagglutination inhibition and protected against challenge infection with PI3 virus. Vaccine containing dsRNA appeared more efficient in protecting against the effects of PI3 virus infection. Following infection with P. haemolytica fewer lambs in the groups vaccinated with inactivated PI3 virus died or were killed when showing respiratory illness than in the unvaccinated control group. Also at necropsy, the extent of pneumonic lesions was significantly less in vaccinated than in unvaccinated lambs. However, as regards the P. haemolytica component of the challenge, the vaccine containing inactivated PI3 virus alone in an oil emulsion effected greater protection. This result seems inconsistent with the finding that the vaccine containing dsRNA afforded greater protection against infection with PI3 virus. A possible explanation for this anomaly is suggested to be related to the transient clinical response observed following infection with PI3 virus in lambs previously given vaccines of inactivated PI3 virus. It is concluded that vaccination against PI3 virus resulted in protection against infection with the virus and reduced the severity of clinical disease and extent of pneumonic lesions associated with subsequent P. haemolytica infection.