Trypanosoma congolense: susceptibility of cattle to cyclical challenge.

Cattle primed by cyclical infection with Glossina morsitans morsitans infected with cloned derivatives of Trypanosoma congolense and treated with the trypanocidal drug Berenil after 3 or 4 weeks were immune to cyclical challenge with homologous clones 3 to 5 weeks later. In these animals, localized skin reactions (chancres) and parasitemia did not develop. The same results were obtained in cattle given a homologous superinfection without prior treatment. On the other hand, cattle subjected to a cyclical challenge with heterologous clones were completely susceptible as demonstrated by the development of chancres. Immunity to homologous challenge was achieved irrespective of the bloodstream variable antigenic types used to infect the tsetse. It was concluded that for a given serodeme the variable antigen composition of the metacyclic population which develops in the tsetse is constant and characteristic. Immunity to cyclical challenge was also obtained with uncloned stocks, providing the same stock was used for challenge. On the other hand, cattle immune to homologous cyclical challenge with cloned material were not always immune to cyclical challenge with parent stock, indicating that certain stocks consist of more than one serodeme. On the basis of these findings, it may be possible to use the chancre as a marker for serodeme analysis.