Concurrent infection of lambs with parainfluenza virus type 3 and Pasieureiia haemolytica

When conventionally reared lambs were infected with Pasteurella haemolytica 6 days after exposure to parainfluenza virus type 3, 79% of the animals developed severe pneumonic lesions. Uninoculated lambs or those receiving virus or bacteria alone had significantly lower levels of pneumonia (25%, 21% and 12% respectively). Because 25% of the uninoculated lambs had severe pneumonic lesions it could not be determined whether the combined infection had actually caused the pneumonia or merely aggravated existing lesions. However, it was apparent that neither agent alone was capable of increasing the prevalence or severity of pneumonia in the flock.