Virulence and toxigenicity of capsular serogroup D Pasteurella multocida strains isolated from avian hosts.

Five capsular serogroup D strains of Pasteurella multocida isolated from avian hosts were examined for virulence and toxigenicity. Virulence was based on development of lethal infections or lesions following intramuscular exposure of turkey poults. The four strains isolated from turkeys varied from slightly to moderately virulent; the strain isolated from a chicken was avirulent. Poults exposed by intra-airsac inoculation with relatively few organisms of the more virulent of the strains had a high mortality rate; however, intranasal exposure of poults with this strain did not cause clinical disease or establish infections. All strains from turkeys were toxigenic, producing heat-labile toxins that killed poults when administered intraperitoneally and caused focal dermal lesions when administered intradermally. Using these criteria, the strain from a chicken was not toxigenic. The demonstration of virulence, particularly the high mortality in poults exposed via air sacs, indicates avian capsular serogroup D strains are a potential cause of fowl cholera.