Use of fiberoptic bronchoscopy in experimental production of bovine respiratory tract disease.

Fourteen 6-month-old calves were infected with Pasteurella haemolytica or infectious bovine rhinotracheitis (IBR) virus. Four calves were inoculated sequentially with IBR virus followed by P haemolytica at a 5-day interval. Calves were inoculated by allowing them to inhale an aerosol of the organism or by placing an inoculum in the right lung, using fiberoptic bronchoscopy. Clinical signs of infection were recorded, and the calves, if they survived, were killed and necropsied 3 days after inoculation with P haemolytica (or 8 days after inoculation with IBR virus). The extent of pulmonary lesions was determined, and the lower respiratory tract (lungs and lower trachea) was examined for both organisms. Inoculation of the calves by aerosolization with IBR virus alone resulted in mild respiratory tract disease. Mild-to-moderately severe respiratory tract disease developed as a result of sequential inoculations by aerosolization with IBR virus and P haemolytica. However, respiratory tract disease did not develop in calves exposed by aerosol to P haemolytica alone. Large numbers of these organisms were recovered from the lower respiratory tract of the dually inoculated calves, only indicating delayed pulmonary clearance. Mild clinical signs of disease but substantial, though localized, pneumonic lesions developed in calves inoculated with P haemolytica by fiberoptic bronchoscopy. Calves inoculated with IBR virus by the latter procedure developed moderately severe respiratory tract disease involving 25% to 30% of the total lung volume. Lesions occurred mainly in the right lung, but the left lung also had marked lesions in these calves.(ABSTRACT TRUNCATED AT 250 WORDS)