Initial lung lesions in two calves experimentally infected with Haemophilus somnus

The initial lung lesions in two calves intrabronchially inoculated with Hae mophilus somnus are described. The animals were euthanized wit hin 7 h afte r challenge. The in situ location of H. somnus and accompanying lesions wer e examined by light microscopy, immunohistochemistry and transmission elect ron microscopy (TEM). Inoculation with H. somnus resulted in the development of acute pulmonary l esions within 3.5 h. H. somnus antigen was demonstrated only within the lum inal spaces of the airways and in one area of bronchio-associated lymphoid tissue (BALT). As observed by TEM, the bacteria were phagocytized by both n eutrophils and alveolar macrophages. Antigen was never demonstrated in the pulmonary intravascular macrophages.