Potential value of localized skin reactions (chancres) induced by Trypanosoma congolense transmitted by Glossina morsitans centralis for the analysis of metacyclic trypanosome populations.

Goats infected with Trypanosoma congolense transmitted by Glossina morsitans centralis and then treated with the trypanocidal drug diminazene aceturate are immune to tsetse-transmitted infection with a homologous, but not heterologous trypanosome clone. Immune animals fail to develop localized skin reactions (chancres) and do not become infected, whereas on heterologous challenge chancres appear followed by parasitaemia. In this study, the feasibility of using chancre reactions to characterize several different metacyclic populations of T. congolense was evaluated. Provided goats were immunized, it was found that the chancre reaction could be used to distinguish different populations of T. congolense. However, problems were encountered when attempts were made to induce immunity against more than one trypanosome population. When goats were infected by tsetse flies with four antigenically unrelated metacyclic populations of T. congolense, either simultaneously or at 4 day intervals, adequate immunological priming was not always achieved. In fact, goats exposed to superinfection 8 to 12 days after a primary infection did not develop chancres or an immune response to the second infection (although they were immune to the organism used for primary infection). Following trypanocidal treatment these animals were subsequently completely susceptible to homologous infection with the metacyclic population used for superinfection, indicating that some form of interference phenomenon effective at the level of the skin appears to prevent or delay the development of the second trypanosome population following superinfection of infected animals.