Differential serum bactericidal and neutrophil phagocytic responses in cattle and buffalo to Pasteurella multocida (B:2) responsible for haemorrhagic septicaemia (HS)

This study evaluated comparatively serum bactericidal activity and neutrophil phagocytic responses to P. multocida in cattle and buffalo to elucidate species-specific innate immune differences contributing to haemorrhagic septicaemia (HS) susceptibility. Serum bactericidal and neutrophil phagocytic assays were performed using immune and non-immune sera from cattle and buffalo, with Escherichia coli used as a control organism. Normal (non-immune) sera from both species failed to reduce the viability of P. multocida. Immune serum from adult cattle completely inhibited bacterial growth, whereas immune buffalo serum exhibited only partial bactericidal activity. Heat inactivation of immune sera abolished bactericidal effects, indicating complement dependency. Neutrophils from both species were ineffective in phagocytosing P. multocida in the absence of immune serum; however, immune serum markedly enhanced opsonophagocytic activity, with significantly higher phagocytosis observed in cattle compared to buffalo. These findings demonstrate that antibody-dependent complement activation and neutrophil-mediated clearance of P. multocida were more effective in cattle than buffalo, providing a functional immunological basis for the greater resistance of cattle to HS.