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A colony of Glossina pallidipes which originated from Nguruman, Rift Valley Province, Kenya, was significantly more susceptible than a colony of the same species which originated from Shimba Hills, Coast Province, Kenya, to infection with a stock of Trypanosoma simiae CP 11 isolated from wild G. austeni in Coast Province, Kenya, irrespective of whether pigs or goats were used as infecting hosts. Male G.pallidipes from both the colonies were more susceptible than females to this T.simiae stock. Similarly, a G.pallidipes colony of Nguruman origin was significantly more susceptible than the colony of Shimba Hills origin to infection with another stock of T.simiae CP 813 isolated from wild G.pallidipes in Coast Province, Kenya, again irrespective of whether pigs or goats were used as infecting hosts. The susceptibility of the sexes of G.pallidipes from both the colonies to T.simiae CP 813 did not differ significantly when pigs were used as as infecting hosts, but male G.pallidipes from both the colonies were significantly more susceptible than female tsetse to this T.simiae stock when goats were used as infecting hosts. Nevertheless, if the observed differences in susceptibility of the two G.pallidipes colonies reflect transmission of trypanosomes by the two allopatric populations of tsetse in the field, then the epidemiology of simiae-trypanosomiasis probably differs between these two areas of Kenya.

The pathogenicity to mice of a stock of Trypanosoma evansi from Mauritania has been compared to that of stocks from Kenya, Niger, Chad and China, using doses of 5.10(5), 2.10(6) and 5.10(6) trypanosomes per mouse. The survival of mice inoculated with stocks from Kenya, Niger and China is short, on average 2.1 to 6.5 days. Mice inoculated with stocks from Mauritania and Chad survive longer, 12.5 to 22.7 days, depending on the dose.

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.