Experimental bovine respiratory syncytial virus infection in conventional calves: light microscopic lesions, microbiology, and studies on lavaged lung cells.

Conventionally raised male Holstein calves, 1 month of age, were infected by intranasal and intratracheal inoculation with bovine respiratory syncytial virus. Viral antigen was identified by fluorescence microscopy most commonly in the cytoplasm of tracheal and bronchial epithelial cells 3 to 5 days after inoculation. Cytoplasmic viral antigen was identified also in nasal, nasopharyngeal, bronchiolar, and alveolar epithelial cells and in alveolar macrophages. Bronchitis and tracheitis, characterized in part by epithelial necrosis, formation of syncytial epithelial cells and epithelial hyperplasia, were the most common lesions observed histologically. Rhinitis, bronchiolitis, and interstitial pneumonia were observed less frequently. Alterations were not detected in the numbers of cells recovered by bronchoalveolar lavage after inoculation. An increase in the phagocytic rate of latex beads occurred in macrophages 5 days after inoculation. Viral-induced lesions were resolved by 30 days after inoculation. The results indicated that bovine respiratory syncytial virus inoculation of calves results in reversible alterations in airway epithelial structure and in the phagocytic function of alveolar macrophages.