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.
Highlights
Trypanosoma evansi is an important disease-causing parasite of livestock in many African, Asian and South American countries
We found that all of the KETRI isolates amplified in the PCR test that is diagnostic for the ITS1 region of all African trypanosomes considered pathogenic: Members of the subgenera Nannomonas (T. congolense), Duttonella (T. vivax) and Trypanozoon (T. brucei, T. evansi, T. equiperdum) [22]
The low sample size of T. evansi in cluster “g” remains another possible reason for the non-significant p-value in from the non-significant differentiation (FST) estimates between “e” and “g”, and again highlight the need for further sampling of a greater diversity of T. evansi strains from non “e” clusters. These results indicate that the genetic diversity across all T. evansi isolates (“overall” in Tables 2C and S4C) represents a large amount of the genetic diversity found across T. brucei isolates (“overall” in Tables 2B and S4B)
Summary
Trypanosoma evansi is an important disease-causing parasite of livestock in many African, Asian and South American countries. T. evansi belongs to a group of five closely related named taxa of various ranks found in a wide diversity of mammalian hosts; Trypanosoma brucei brucei, T. b. Rhodesiense, T. evansi, and T. equiperdum [1,2,3,4,5] The other two members of this group (T. evansi and T. equiperdum) are found both inside and outside the African continent, use other means of transmission, and are responsible for surra in wild and domestic animals [8] and dourine in equines [9], respectively. Brucei strains than to other strains from the same named subspecies [3,13,14,15]. We further classify T. evansi based on their mitochondrial DNA (kinetoplast DNA or kDNA) configuration of type A or B [18,19,20] and their antigenic variant surface glycoprotein (VSG) Rode Trypanozoon antigenic type (RoTat) 1.2, used in serological and PCR-based diagnostic tests [11,21,22, 23,24]
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