Experimental Haemophilus somnus Pneumonia in Calves and Immunoperoxidase Localization of Bacteria

Pneumonia was produced in nine, conventionally reared calves by intrabronchial inoculation with Huemophifus somnus. Volumes of pneumonic lung were determined stereologically, following serial slicing of lungs fixed by vascular perfusion. Twenty-four hours after inoculation, consistent findings were: neutrophilic to fibrinoid vasculitis, degeneration of alveolar macrophages, necrotizing bronchiolitis, suppurative broncho- pneumonia, lobular necrosis, and dilation and thrombosis of lymphatics. Bacteria were identified histologically by an immunoperoxidase technique and were either free in alveoli or associated with degenerate alveolar macrophages. The latter suggests that macrophage degeneration may be a result of bacteridmacrophage inter- action. Immune complex deposition is unlikely to be the principal mechanism for the vasculitis because bacterial antigen was not generally found in necrotic vessel walls, and two colostrum-deprived, H. somnus antibody- negative calves also had neutrophilic vasculitis 12 to 24 hours after inoculation with the lowest dose of H. sornnus used in the above experiment. Haernophilus sornnus is an important cause of bo- vine pneum~nia'-~.'','~,'~.~~ and is associated with a wide variety of other diseases in cattle, including throm- boembolic meningoencephaliti~,'~,~',~~,~~, ~~ abor- ti~n,~J~J~ infertility,'6J8 and arthriti~.'~ Naturally oc- curring H. sornnus pneumonia is characterized grossly by grey to red consolidation of the cranioventral lung, involving from 5 to 80% of total lung volume.L Tra- cheobronchial and mediastinal lymph nodes are typ- ically edematous, with fibrinous pleuritis seen irregu- larly.',2 Histologically, there is neutrophilic, necrotizing bronchiolitis and bronchopneumonia with fibrinocel- lular thrombosis of septa1 lymphatics and multifocal hemorrhage.',2 The relatively few reported detailed studies of ex- perimental have concentrated on sub- acute to chronic phases of the disease, with mean times of death post-inoculation of 6.87 or 5.523 days and ranges of 1.5 to 147 and 0.3 to 15 days,Z3 and have used relatively large numbers of bacteria (1 08-1 O9 colony forming units (CFU)). In this study we experimentally produced acute H. sornnus pneumonia (at 12-24 hours) with smaller doses of H. sornnus (106-108 CFU), as well as measuring and grading lesions. These studies were done to examine early events in the pathogenesis of H. sornnus pneumonia and to provide a basis for future studies on prophylaxis. In addition, an immu- noperoxidase technique was developed to localize bac- teria in lesions, to characterize further the pathogenesis of acute H. sornnus pneumonia, and to determine the potential of this method in diagnosis.