Home Economics American Trypanosomiasis Chagas Disease, Second Edition: One Hundred Years of Research
Parasite phylogeography and ecology
T. cruzi is genetically diverse and is classified into a series of strains or subtypes. This genetic diversity was initially discovered using a panel of isoenzyme markers to investigate differences between parasites involved in the domestic and sylvatic cycles in Bahia state of Brazil.28,29 This study was a breakthrough, revealing that in Bahia there were substantial genetic differences between the parasites in sympatric sylvatic and domestic transmission cycles. These described variants were designated zymodemes I and II (ZI and ZII). It was soon after revealed that the widespread strain associated with the sylvatic cycle in Brazil (ZI) was the predominant cause of human disease in Venezuela.29 These groundbreaking findings opened the door to investigating the etiology of Chagas disease, allowing host—vector—parasite associations and comparative geographical distributions to be explored, as reviewed by Miles et al.30 In the following two decades, various authors proceeded to characterize strains, applying other molecular methods as they became available. As a result, further diversity within the original zymodemes was discovered. However, designations of subtypes in the literature started to become confusing. To standardize the nomenclature, it was decided by the scientific community to formally recognize two main groups that corresponded with the original zymodemes; these were called TcI and TcII.31 Most recently, the scientific community has redesignated the six genotypes as TcI to VI discrete typing units (DTUs).32
The ordering of the current nomenclature better indicates the perceived phylogenetic relationships between the strains: TcI and TcII are the most anciently divergent.33 TcIII and TcIV show similarities. Some argue that they represent the result of ancient hybridization events between TcI and TcII,34 alternatively they could represent a separate lineage.35 Finally, TcV and TcIV are generally accepted to be hybrids derived from parental stocks of TcII and TcIII.33 TcI has remained a constant grouping in the nomenclature since first described.
However, in recent years at least four subgroupings within TcI have been identified, with further characterization with microsatellite markers. Both sequence variation37 and microsatellite markers demonstrate that genotypes/hap- lotypes involved in human infections in Venezuela and Colombia are distinct from sympatric sylvatic variants. This is intriguing as it was recently demonstrated that the main vector in the region, R. prolixus, demonstrates panmixia between domestic and sylvatic environments across the western endemic region in Venezuela.40
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