Cellular inflammatory response in the lungs of calves exposed to bovine viral diarrhea virus, Mycoplasma bovis, and Pasteurella haemolytica.

Three, 5, or 7 days after inoculation with bovine viral diarrhea (BVD) virus (n = 12) or Mycoplasma bovis (n = 12), groups of calves were exposed to aerosols of Pasteurella haemolytica and were euthanatized 4 hours later. Histologic lesions in the lungs and the ratios of neutrophils to alveolar macrophages, collected by bronchoalveolar lavage, were compared with those of clinically healthy calves (n = 8) and calves inoculated with BVD virus only (n = 4), M bovis only (n = 4), or P haemolytica only (n = 2). Inoculation with BVD virus or M bovis did not have a significant (P greater than 0.05) effect on the neutrophil/macrophage ratio in the bronchoalveolar lavage. Aerosol exposure to P haemolytica induced a marked and significant (P less than 0.01) change in the neutrophil/macrophage ratio (from less than 1:9 to greater than 9:1). The reversed neutrophil/macrophage ratio in calves exposed to P haemolytica correlated well with the histologic changes in which small bronchi and bronchioles were plugged with purulent exudate. Inoculation with BVD virus did not induce gross or microscopic lesions in the lungs. Inoculation with M bovis resulted in a severe peribronchial lymphoid hyperplasia with mild exudation of neutrophils and macrophages into the cranioventral parts of the lungs.