Numerical taxonomy was used to review isoenzyme variation in isolates of Trypanosoma brucei obtained from cattle, tsetse, humans and wildlife from the Lambwe Valley, Kenya. From isoenzyme information alone, it was possible to classify isolates as to source through the use of linear discriminant functions analysis, with an error rate of only 2% in humans, and 14% over all groups. Differentiation was mostly dependent on patterns in the enzymes ASAT, PEP1, and ICD. Parasites from non-human sources, especially tsetse, were characterized by high isoenzyme diversity, and many unique zymodemes. Observed frequencies of genotypes for ICD, ALAT, and ASAT did not agree with expected frequencies based on random mating of a diploid organism. Deviations were particularly large for tsetse isolates, and were mostly due to a deficiency of one homozygote. Cluster analysis revealed complex relationships among isolates, with patterns evolving through time. Major human zymodemes from the 1970s clustered together with most wildlife isolates from East Africa. This chronic human-wildlife transmission cycle was characterized by ASAT pattern I. Other, minor human zymodemes were associated with a human-cattle transmission cycle characterized by ASAT pattern VII. These original chronic transmission cycles appeared to change in 1980 with the appearance of two new zymodemes in humans. These zymodemes involved changes in ALAT and/or PGM to patterns typical of tsetse and cattle isolates. A resultant epidemic was halted with repeated aerial spraying of endosulfan in 1981. Since then, a variety of new zymodemes of unknown human infectivity have appeared. The origins of these changes are discussed in terms of genetic exchange in tsetse, adaptation to human and cattle transmission cycles, and selection resulting from chronic use of insecticides.

An in vitro system for studying the transformation of bloodstream forms of Trypanosoma brucei brucei into procylic (midgut) forms is described. In this system, transformation of the parasites was stimulated by Glossina morsitans morsitans midgut homogenates at 27 degrees C but not at 4 degrees C. The transformation-stimulating capacity was irreversibly destroyed by heating the midgut homogenates at 60 degrees C for 1 h. A correlation was established between the transformation activity of the midgut homogenates and trypsin activity. The protease inhibitors (soybean trypsin inhibitor and N-p-tosyl-L-lysine-chloromethyl-ketone) inhibited trypsin activity and completely blocked the transformation of the parasites. Furthermore, the midgut homogenates could induce transformation only in the presence of blood. These results provide evidence for the involvement of trypsin or trypsin-like enzymes within the tsetse midgut in stimulation of the transformation of bloodstream trypanosomes.

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