Serum resistance as an indicator of virulence of Pasteurella multocida for turkeys

Wildlife isolates of Pasteurella multocida, whose virulence for turkeys had previously been determined by intravenous inoculation, were characterized regarding their ability to survive incubation in fresh non-immune turkey serum. The relative virulence of the isolates was significantly associated with their ability to resist the bactericidal power of the serum as determined by standard plate counts following incubation. Organisms with a high survival value were more virulent; those with a low survival value were less virulent. A statistical model was specified and was successfully used to predict relative virulence of the P. multocida isolates. This method of assaying serum resistance was rapid, repeatable, and practical and could be performed with minimal laboratory equipment. Also studied was the serum resistance of seven serotype 3, 4 isolates obtained from the lungs of M9-vaccinated turkeys from seven flocks experiencing increased mortality due to fowl cholera. These isolates were shown to be identical to the M9 vaccine by restriction endonuclease analysis of chromosomal DNA. Six of the seven isolates had higher serum survival values than the original M9 vaccine.