Experimental infection of lambs with an adenovirus followed by pasteurella haemolytica

Groups of caesarian-derived, colostrum-deprived lambs aged 10 to 11 weeks were inoculated with the adenovirus WV419/75 followed 4, 6 or 8 days later by Pasteurella haemolyttica type Al (P.h.). Other groups of lambs were inoculated with the virus or P.h. alone.The majority of animals inoculated with P.h. 6 or 8 days after the virus developed an acute necrotizing bronchopneumonia and pleurisy, whereas, the bacteria were cleared rapidly from the lungs of the majority of animals inoculated with P.h. alone, and pneumonic lesions only rarely developed. Between groups, the development of bacterial superinfection and thus bronchopneumonia could not be related to virus titres in either the lung or nasal secretions. However, within individual animals that developed pneumonia, the distribution of virus paralleled the distribution of lesions.In animals inoculated with the virus alone (or the virus plus P.h. where bacterial superinfection did not develop) areas of epithelial cell proliferation and consolidation with basophilic intranuclear inclusion bodies in swollen epithelial cells were present. The consolidation and inclusion bodies had disappeared by 10 days after inoculation, together with the virus, but the proliferation of airway epithelium was still present 20 days after inoculation. On the other hand, when bacterial superinfection was present, areas of acute necrotizing bronchopneumonia were the predominant feature. These lesions were surrounded by a zone of consolidation with a massive neutrophil infiltration or, where bacterial numbers were high (usually ? 10^3.0/g), a band where oat-cells predominated although, in animals killed on day 14, oat-cells were still present but bacterial numbers were lower.