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Organisms resembling Histophilus ovis were isolated from sheep in Western Victoria in association with synovitis, septicaemia and/or abscessation in 2 to 8 week old lambs. Histopathological lesions in these lambs were indicative of bacterial embolism with thrombosis. Similar organisms were also isolated from sheep in 2 flocks with a high neonatal mortality rate. These isolations were from vaginal swabs from the ewes, placentas and from one lamb which had meningitis, ependymitis, choroid plexitis, hepatic necrosis and focal myocarditis. The morphology, cultural characteristics and biochemical activity of the organisms are described and it is suggested that the organisms may be better classified in the genus Actinobacillus.

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The serological relationships between 4 strains of Histophilus ovis, the neotype strain of Actinobacillus seminis and Brucella ovis were examined using a cross-absorption complement-fixation technique. It was found that the 4 strains of H. ovis were serologically homologous and that an incomplete relationship existed between these organisms and A. seminis. Anteriserums prepared against one strain of H. ovis and the strain of A. seminis gave a weak, apparently non-specific cross-reaction with Br. ovis antigen. The practical significance of these results is discussed.

Inoculation of lambs either intravenously or intranasally with Histophilus ovis resulted in septicaemia. There was bacterial thrombosis and vasculitis in the liver, lung, kidneys and heart. These lesions were similar to those in natural cases of the disease. With intravenous inoculation of a less virulent isolate there were abscesses in the subcutaneous tissues, muscle fascia and joint bursae.