PREDISPOSITION OF SPECIFIC PATHOGEN-FREE LAMBS TO PASTEURELLA-HAEMOLYTICA PNEUMONIA BY BORDETELLA-PARAPERTUSSIS INFECTION

Three groups of specific pathogen-free (SPF) lambs were inoculated int ratracheally 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 sh owed large areas of collapse and consolidation which, in one case, cov ered the entire lung. Histopathological examination confirmed that the combined infection produced a severe acute bronchopneumonia in four o f seven lambs. B. parapertussis and P. haemolytica were recovered from all of the lambs in this group. Seven lambs challenged with P. haemol ytica 5 days after B. parapertussis and six lambs infected with B. par apertussis alone showed no clinical signs of disease other than mild p yrexia and only mild histopathological changes. B. parapertussis, but not P. haemolytica, was recovered from these lambs. The findings indic ated that B. parapertussis predisposed the SPF lambs to P. haemolytica pneumonia. This effect appeared to be dependent upon the time interva l between the administration of the two agents.