Anti-trypanosome specific immune responses in bovids of differing susceptibility to African trypanosomiasis.

A clone of Trypanosoma brucei brucei (DiTat 1.1) was injected into 32 bovids of various breeds, 11 animals being kept as controls. Five animals, Simmental-Ndama F1 crosses, were extremely sensitive. They showed overt symptoms and one died on day 18 of infection despite treatment with a trypanocidal drug. Seven other animals became ill but recovered progressively and cleared the infection. Twenty animals, of breeds generally considered to be trypanosensitive as well as ones from trypanotolerant breeds, did not show symptoms apart from anaemia and cleared the infection. Putative protective antibody, i.e. directed against exposed determinants on the coat variant-specific glycoprotein, was detected by agglutination, complement-mediated lysis and inhibition of infectivity. All animals showed a primary immune response consisting of IgM whose kinetics and amplitude were indistinguishable between animals of differing sensitivity. The response was long-lasting, whether the animals had been treated or not with a trypanocidal drug 3 weeks after infection, and antibody of IgG1 and IgG2 types were detected in certain sensitive as well as resistant animals after 2 months. Some animals were rechallenged with DiTat 1.1 either 1 year after the primary infection or 6 months after inoculation of irradiated trypanosomes. Peak titres of antibody were lower than was the case following primary infection but higher levels of mercaptoenthanol-resistant antibodies were seen. In no case was there any difference in the response of sensitive or tolerant animals. Our results do not support the idea that resistance of certain bovids to African trypanosomiasis is due to a better protective antibody response.

Trypanotolerance. an individual not a breed character.

Ten animals from a reputedly trypanosensitive bovine breed, the Zebu, and ten from a reputedly trypanotolerant breed, the Baoulé, were exposed to natural trypanosome challenge in an area of high Glossina density. The ten Zebus and five of the Baoulés died within 6 to 11 weeks with heavy parasitaemia and severe anaemia. The five other Baoulés showed little or no parasitaemia, no anaemia and were all in good condition after weeks of exposure: Seven animals of a herd of Baoulé/Ndama crosses indigenous to the same area were followed concurrently and showed very rare or no patent parasitaemia. Clearly, only some individuals within a "trypanotolerant breed" are able to withstand natural fly challenge. Selection of trypanotolerant animals has to be done on an individual basis and not on the biometrical characteristics of the breed.