Histological changes in the respiratory tract of calves exposed to aerosols of bovine herpesvirus 1 and Pasteurella haemolytica.

Histopathological changes in the respiratory tract of 19 calves exposed sequentially to aerosols of bovine herpesvirus 1 (infectious bovine rhinotracheitis virus) and Pasteurella haemolytica, either 4 or 5 days apart, are described. Tissues were examined 1.5 to 14 days following the bacterial aerosol. The salient features were hyperaemia of pharyngeal tonsils and lamina propria of air passages and alveoli; alveolitis associated with oedema and fibrin in lumen and septa; pneumocytes, mononuclear cells, granulocytes, oedema and fibrin in interlobular septa and pleura and lymphatic congestion and thrombosis. Initially, mononuclear eosinophilic granulocytes were numerous in turbinates, bronchi and lung in particular, but after day 3 polymorphonuclear leucocytes predominated. Bacterial colonies were seen in turbinate, tonsil, trachea and lung. Necrosis was present in the epithelium of nose, trachea and lung. Necropurulent changes due to viral-bacterial interaction were particularly evident in pharyngeal tonsils and alveoli. Blood vessels and walls of bronchioli and major air passages were generally free from inflammatory change. Healing was characterized by fibrosis of the alveolar and interlobular septa, and the lamina propria of the air passages and by proliferation of specialized septal cells surrounding micro-abscesses. It was concluded that the lesions were a feature of this experimentally produced synergistic viral-bacterial infection.