The pulmonary clearance of Pasteurella haemolytica in calves infected with bovine parainfluenza 3 virus

The purpose of this study was to determine if parainfluenza-3 virus in calves interferred with normal pulmonary bacterial clearance. Three groups (A, B and C) of 4 calves each were exposed to an aerosol of parainfluenza-3 virus. Three days later the 4 h pulmonary clearance of P. haemolytica was determined on group A, 7 days later on group B and 11 days later on group C. Group A had a mean pulmonary bacterial retention of 3.6 .+-. 3.5%; group B was 83.1 .+-. 35.9% and group C was 41.2 .+-. 30.9%. Parainfluenza-3 virus interfered with the pulmonary bacterial clearance of P. haemolytica particularly on day 7 and also on day 11 but not on day 3. This inhibition of pulmonary clearance caused by the virus may be a key factor in the pathogenesis of pneumonic pasteurellosis. Histological examination of the lungs did not demonstrate a correlation between pulmonary retention of bacteria and the development of pathological changes.