Anaplasma marginale: failure of sera from immune cattle to confer protection in passive-transfer experiments

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Abstract
High levels of immunity toAnaplasma marginale were induced in cattle either by vaccination using sonically disruptedA. marginale-infected erythrocytes or by repeated infection with different strains of the rickettsia. In both instances, high levels of anti-A. marginale antibody were detected in the sera of the immune cattle by immunoblotting. Serum from one animal that had been made immune by repeated infection was transferred intravenously toA. marginale-susceptible calves (three non-splenectomised and two splenectomised) undergoing initialA. marginale infection at serum doses of 2–10 ml/kg. Neither the course nor the outcome of infection as indicated by the parasite levels attained or the level of anaemia induced was altered in the calves that received the immune serum relative to the course or outcome of infection in control calves (two non-splenectomised and two splenectomised) that received serum from anA. marginale-naive donor animal. In a similar experiment, a pool of sera from four steers that had been vaccinated with sonically disruptedA. marginale initial bodies was transfused into two intactA. marginale-susceptible calves during the early stage ofA. marginale infection at a dose of 10 ml/kg. No difference was observed in the course or outcome of infection in these calves relative to the course or outcome of infection in the two non-splenectomised calves that were transfused with non-immune serum. The failure of serum obtained from animals that had been made immune toA. marginale either by infection or by vaccination with disrupted initial bodies to confer any protection against infection following its transfer into splenectomised or non-splenectomised naive calves indicates that antibody per se is not a significant factor in immunity to this parasite.