Experimental Haemophilus somnus pneumonia in calves.

Purulent bronchopneumonia was induced in 12 young calves inoculated endotracheally with a total of 2 to 8 X 10(10) viable Haemophilus somnus organisms. Calves treated before inoculation of bacteria with dexamethasone had more extensive lung changes than nontreated calves. Two of six dexamethasone-pretreated calves died while none of the non-pretreated calves died in the 12-day experimental period. Cranial-ventral gray to red lung consolidations with multifocal abscesses were present in all calves. Histologic examination revealed purulent to fibrinopurulent bronchiolitis accompanied by alveolar filling with fibrin, neutrophils, and macrophages. In addition, peribronchiolar fibrosis and bronchiolitis obliterans were common as were interlobular fibrosis and thrombosis of interlobular and pleural lymphatics. Focal to diffuse ulceration of bronchiolar epithelium was present in five of 12 lungs. Abscessation and severe alveolar filling with fibrin, red blood cells, and streaming inflammatory cells were present in dexamethasone-treated calves. H. somnus was isolated from lungs of all 12 calves. Complement fixation and microagglutination antibody titers to H. somnus antigens increased in most calves following infection with H. somnus. This model of H. somnus pneumonia closely resembles the naturally occurring disease.