Immunological studies on a modified multiple emulsion (w/o/w) bivalent Haemmorhagic septicemia vaccine incorporated with Pasteurella multocida P₅₂ in the external aqueous phase

A modified double emulsion (w/o/w) vaccine against Haemmorhagic septicemia (HS) was prepared by incorporating a bovine isolate of Pasteurella multocida B:2 (P₅₂) in the external aqueous phase. Immune responses of the modified vaccine were compared with monovalent A:1 double emulsion vaccine in rabbit. The modified double emulsion vaccine provided 100% protection against the challenge of both A:1 and B:2 serotypes up to 21^st days post-infection (DPI), as observed by direct challenge test and passive mouse protection test (PMPT). However, on 90^th DPI, protection against the antigen in external aqueous phase was only partial. Though the monovalent vaccine also showed a low level of antibody titer and protection on 21^st DPI, the bivalent vaccine was found better in providing cross-protection against both the serotypes. Serum antibody titer revealed a quicker rise in antibody titer against the antigen incorporated in the external aqueous phase, however, it persisted for shorter duration of time. On the other hand, the antibody titer against the internal phase antigen showed a slow rise, which peaked around 30^th DPI and persisted up to 90^th DPI. This methodology can be used in areas of HS outbreak where multiple serotypes of P. multocida are involved.