Is Trypanosoma vivax genetically diverse?

In their recent review on diversity of African tsetse flytransmitted trypanosomes, Tait et al. [1] suggest that genetic diversity of Trypanosoma vivax, a major pathogen of African and South American cattle, is limited. This is primarily based on DNA microsatellite analysis of 31 T. vivax isolates from a single area in The Gambia [2]. However, this assessment ignores evidence from several studies that have used isoenzymes [3], DNA microsatellites [4,5] and more recently sequence data [6–9] that have revealed high levels of genetic diversity within this species, particularly in East Africa. A partial T. vivax 18S rDNA sequence, obtained from an infected tsetse fly in Tanzania had greatest similarity to that of a West African T. vivax isolate, yet diverged by 14% [6]; two novel genotypes were discovered in wild antelope from Mozambique, one of which caused severe disease in a goat [7], and three diverse genotypes were discovered in tsetse flies from Tanzania [8]. Indeed, analysis of glycosomal glyceraldehyde phosphate dehydrogenase (gGAPDH) gene sequences suggests that T. vivax diversity is similar to that of Trypanosoma congolense [8], a species where the different strains (forest, savannah and kilifi) arguably represent different species. The limited genetic diversity found in The Gambia [2] reflects the results of a study which found isolates with identical gGAPDH gene sequences from across West Africa (The Gambia, Nigeria and Cameroon) and South America [8]. By contrast, considerable polymorphism (11 distinct alleles) was identified in 31 T. vivax infections in fieldcollected tsetse flies from Cameroon, using primers targeting a region that contains a microsatellite sequence [4]. The results from these studies are not directly comparable, as they used different genotyping techniques; DNA microsatellites evolve quickly, whereas the gGAPDH gene has a slow evolutionary rate and is generally used for resolving species-level relationships (e.g. [10]). It is therefore difficult to assess the true genetic diversity of T. vivax in West Africa, particularly as T. vivax has not been extensively sampled from wild mammals, tsetse flies and other biting flies in the region, which could potentially harbor greater diversity. Additionally, the primer sets used for initial identification [11] in the study of Duffy et al. [2] lack the ability to detect divergent T. vivax genotypes [4], and this may also be the case for the primer sets used for microsatellite genotyping that were designed from genomic sequences from a West African strain [2]. By contrast,