Lack of cross-immunity among Pasteurella multocida type A strains.

Active and passive protection studies in mice using sheep antisera revealed that the immunological relationship among Pasteurella multocida Type A strains could not be correlated with their serological relationship as determined by a haemagglutination or an agglutination test. Furthermore, strains possessing similar phenol extractable antigens or heat stable antigens did not provide complete cross-protection. The conclusion was reached that immunity to P. multocida Type A strains is induced by an antigen which is strain specific and not detectable by existing serological typing systems. The immunological relationship of strains can therefore not be predicted on the basis of their serological characteristics.