Induction of protective immunity in cattle by tsetse-transmitted cloned isolates ofTrypanosoma congolense

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Abstract
Cattle infected with cyclically (tsetse)-transmitted clones of Trypanosoma congolense were treated with the trypanocidal drug diminazene aceturate (‘Berenil’) at various intervals during development of local skin reactions (chancres), in order to investigate the role played by trypanosomes in the skin in the induction of protective immunity. Cattle, treated on or before Day 12 after infection and prior to detection of trypansomes in the blood, showed a range of susceptibility to cyclically-transmitted homologous challenge three weeks later. An animal treated on Day 5, when the skin reaction was first detected, was completely susceptible, whereas of the animals treated on Days 10 to 12 at the peak of the skin reaction, some were immune, others exhibited partial immunity, while the rest were susceptible. The susceptible animals, however, showed evidence of sensitization to trypanosomes, as demonstrated by development of Arthus-type reactions following homologous challenge with tsetse-transmitted parasites or intradermal inoculation with lysed or irradiated homologous trypanosomes. Cattle treated 15 or more days after cyclically-transmitted infection, when chancre reaction was subsiding and trypanosomes were present in the blood, were completely immune to challenge. Immunity was achieved by treating cattle on Day 5 and Day 10 after infection with cyclically-transmitted trypansomes but only if this procedure was repeated on several occasions, indicating that bloodstream forms of trypanosomes were not essential for the induction of protective immunity. Protective immunity appeared to be effective against the metacyclic trypanosomes at the level of the skin, as animals which were immune to homologous challenge with cyclically-transmitted parasites did not develop detectable skin reactions at the site of the tsetse bite. Furthermore, the time of the appearance in the serum of antibody which completely neutralized the metacyclic population (Day 15) coincided with the time at which cattle had developed immunity against homologous challenge. The immunity produced lasted for five months.