Predisposition of specific pathogen-free lambs to Pasteurella haemolytica pneumonia by Bordetella parapertussis infection.

Three groups of specific pathogen-free (SPF) lambs were inoculated intratracheally with an ovine isolate of Bordetella parapertussis (5.5 x 10(9) colony-forming units) or with B. parapertussis followed 2 or 5 days later with Pasteurella haemolytica serotype A2 (120-180 million colony-forming units). When P. haemolytica A2 was administered 2 days after infection with B. parapertussis all lambs became febrile for at least 72 h. At necropsy their lungs were discoloured, congested and showed large areas of collapse and consolidation which, in one case, covered the entire lung. Histopathological examination confirmed that the combined infection produced a severe acute bronchopneumonia in four of seven lambs. B. parapertussis and P. haemolytica were recovered from all of the lambs in this group. Seven lambs challenged with P. haemolytica 5 days after B. parapertussis and six lambs infected with B. parapertussis alone showed no clinical signs of disease other than mild pyrexia and only mild histopathological changes. B. parapertussis, but not P. haemolytica, was recovered from these lambs. The findings indicated that B. parapertussis predisposed the SPF lambs to P. haemolytica pneumonia. This effect appeared to be dependent upon the time interval between the administration of the two agents.