Multiple evolutionary origins of Trypanosoma evansi in Kenya

Abstract

Trypanosoma evansi is the parasite causing surra, a form of trypanosomiasis in camels and other livestock, and a serious economic burden in Kenya and many other parts of the world. Trypanosoma evansi transmission can be sustained mechanically by tabanid and Stomoxys biting flies, whereas the closely related African trypanosomes T. brucei brucei and T. b. rhodesiense require cyclical development in tsetse flies (genus Glossina) for transmission. In this study, we investigated the evolutionary origins of T. evansi. We used 15 polymorphic microsatellites to quantify levels and patterns of genetic diversity among 41 T. evansi isolates and 66 isolates of T. b. brucei (n = 51) and T. b. rhodesiense (n = 15), including many from Kenya, a region where T. evansi may have evolved from T. brucei. We found that T. evansi strains belong to at least two distinct T. brucei genetic units and contain genetic diversity that is similar to that in T. brucei strains. Results indicated that the 41 T. evansi isolates originated from multiple T. brucei strains from different genetic backgrounds, implying independent origins of T. evansi from T. brucei strains. This surprising finding further suggested that the acquisition of the ability of T. evansi to be transmitted mechanically, and thus the ability to escape the obligate link with the African tsetse fly vector, has occurred repeatedly. These findings, if confirmed, have epidemiological implications, as T. brucei strains from different genetic backgrounds can become either causative agents of a dangerous, cosmopolitan livestock disease or of a lethal human disease, like for T. b. rhodesiense.

Fig 1. Map of Africa showing in black location of Kenya (https://commons.wikimedia/wiki/Atlas_of_the_world).

The insert to the right shows the location of the Trypanosoma evansi (Tev) and T. brucei brucei (Tbb) isolates genotyped for this study (small black circles). Sample details are listed in Table 1.

Fig 2

Plot of assignment scores of all isolates using STRUCTURE v2.3.4 [51] with K = 7 of (A) all isolates, and (B) the close up of Trypanosoma evansi isolates (Tev) with labels added showing isolate ID and kDNA type in parentheses (based on literature, where available, or predicted from the A281del PCR assay). Each vertical bar represents an isolate’s probability of assignment to one of seven genetic clusters "a" through "g" shown in orange, purple, blue, green, yellow, grey and red, as presented in the legend to the right. T. brucei brucei is indicated with a diamond, T. b. rhodesiense is indicated with a bullet point, and T. evansi is indicated by a plus "+" if RoTat 1.2 positive and minus "-" if RoTat 1.2 negative. The high virulence isolate is marked with a double asterix "**", and the low virulence isolate is marked by a single asterix "*". Note that Tev isolates in panel B are ordered according to Table 1 and not strictly according to cluster assignment.

Fig 3. Evaluation of the genetic differentiation between isolates of Trypanosoma brucei brucei and T. b. rhodesiense (Tb) and T. evansi (Tev) genetic clusters using principal components analysis (PCA) of microsatellite data.

PCA was performed in R using the package “adegenet” [86]. Points representing individual genotypes are marked by color of their STRUCTURE assignment following the key and connected by a line to the centroid of an ellipse, which circumscribes a region encompassing 95% of the variance observed within each subgroup identified. Black arrows point out the Tev isolates.