Immune defence mechanism against blood-borne Pasteurella multocida in turkeys

Humoral and cellular immune defence factors involved in controlling blood-borne Pasteurella multocida were investigated in turkeys by the passive transfer of immune serum or by the treatment with macrophage-activating agents. The treated and untreated birds were intravenously inoculated with a virulent strain of P multocida, and the viable bacteria in the blood, liver and spleen were counted. In untreated birds, the bacteria were rapidly removed from the blood, and the majority were recovered from the liver and spleen 120 minutes after inoculation. Neither the transfer of immune serum nor the treatment with macrophage-activating agents significantly influenced the clearance rate of bacteria from the blood. The number of bacteria recovered from the liver 120 minutes after inoculation was slightly lower in the birds treated with macrophage-activating agents and significantly lower in those given immune serum than in the untreated birds. None of the treatments, however, significantly changed the number of bacteria recovered from the spleen 120 minutes after inoculation. The results suggest that the phagocytes in the liver, but not in the spleen, play a crucial role in the intravascular defence against P multocida in the presence of specific antibodies.