Fowl cholera: induction of cross-protection in turkeys with bacterins prepared from host-passaged Pasteurella multocida.

Modified fowl cholera bacterins prepared by inoculating agar medium with infected liver tissue from birds which died of acute fowl cholera induced 70% cross-protection in turkeys, i.e., protection against a different immunologic type of Pasteurella multocida. Standard bacterins prepared from cultures which had been lyophilized and stored showed variable cross-protection (0--40%). Repeated subculturing of the standard inoculum on agar reduced cross-protection. The protection with either the modified or standard bacterins was comparable (80--100%) when immunity was challenged with the homologous strain. With lyophilization of P. multocida and subculturing on agar, it appears that antigens capable of inducing cross-immunity may be lost more readily than antigens capable of inducing homologous immunity.