ReviewThe revised Trypanosoma cruzi subspecific nomenclature: Rationale, epidemiological relevance and research applications
Highlights
► We review the population structure of Trypanosoma cruzi, causative agent of Chagas disease. ► We provide information on the eco-epidemiology of T. cruzi discrete typing units (DTUs). ► We outline the correlation of DTU with clinical manifestations. ► We summarize methods for DTU genotyping. ► Understanding of T. cruzi genetic diversity will provide new insights to future interventions.
Introduction
Infection with Trypanosoma cruzi is a complex zoonosis, transmitted by many hematophagous triatomine species and sustained by over 70 genera of mammalian reservoir hosts. T. cruzi has a broad endemic range that extends from the Southern United States to Argentinean Patagonia. The human infection, named Chagas disease in recognition of Carlos Chagas who first discovered American trypanosomiasis in 1909, is found mostly in South and Central America, primarily affects poor rural populations, and is considered to be the most important parasitic infection in Latin America with serious consequences for public health and national economies.
The spectrum of pathological outcomes associated with acute and chronic Chagas disease ranges from subclinical infection through the cardiac and digestive syndromes to death. Specific outcomes may be determined by a variety of non-exclusive factors including parasite genetics, host genetics, mixed infections, and cultural and geographical factors (Macedo et al., 2002, Macedo et al., 2004, Buscaglia and Di Noia, 2003, Campbell et al., 2004).
The diversity of the T. cruzi genome and multiplicity of its genotypes and phenotypes is well recognized (Dvorak et al., 1982, Barnabé et al., 2000, Brisse et al., 2000, Devera et al., 2003, Lewis et al., 2009a). Designation of ecologically and epidemiologically relevant groups for T. cruzi has oscillated between a few discrete groups (Miles and Cibulskis, 1986, Souto and Zingales, 1993, Souto et al., 1996, Zingales et al., 1999) and many (Tibayrenc and Ayala, 1988). Currently, six discrete typing units (DTUs) are assigned (Brisse et al., 2000). In 2009, these DTUs were renamed by consensus as TcI–TcVI (Zingales et al., 2009). Several reviews already describe how these DTUs correspond with former nomenclatures and with prospective biological and host associations (Campbell et al., 2004, Miles et al., 2009, Sturm and Campbell, 2010, Zingales et al., 2009).
The aim of this review is to explain further the rationale for naming TcI–TcVI, with reference to their known molecular genetics, eco-epidemiological features and pathogenicity. We also summarize methods for DTU genotyping, and discuss a possible seventh T. cruzi branch, provisionally named Tcbat. An understanding of the T. cruzi DTUs and their epidemiological implications will provide new insights to guide research and future interventions against this devastating infectious disease.
Section snippets
The concept of discrete typing unit
Since the late 1970s, T. cruzi has become one of the models for molecular epidemiologists and population geneticists, and consequently this protozoan parasite is a pathogenic agent for which evolution and population structure are among the best studied, although not necessarily the best understood. The emerging picture is that of a typical pattern of reticulate evolution, similar to that of many plant species (Avise, 2004).
The concepts of DTUs and clonal evolution have been designed within the
Two major models for the origin of hybrid DTUs
T. cruzi is predominantly diploid (El-Sayed et al., 2005) and the known cell replication method is binary fission, i.e. it is an asexual process. Under the clonal model (above) new DTUs evolve with the accumulation of discrete mutations, unaffected by rare events of genetic exchange. However, consistent with the caveat that some genetic exchange events may occur, evidence for T. cruzi heterozygosity in nature emerged through the study of individual genes (Chapman et al., 1984, Bogliolo et al.,
Phylogeography of the DTUs
Setting aside the theoretical origins of the DTUs, divergent geographical and biological characteristics are apparent, as is their relevance to understanding of the eco-epidemiology of Chagas disease (Fig. 2).
It is not surprising that, given the current level of sampling, the ecological history for all T. cruzi DTUs cannot yet be fully discerned. Relationships have been obscured by massive changes, from mammal migrations between the Americas, to climate induced retraction and expansion of
Standardizing genotyping for identification of the six T. cruzi DTUs
The standardized nomenclature for the six T. cruzi DTUs will improve scientific communication and guide future research on comparative epidemiology and pathology. To achieve this aim a straightforward and reproducible genotyping strategy is required for DTU identification, manageable in any laboratory and adopted by the T. cruzi research community.
Over the years, numerous approaches have been used to characterize the biochemical and genetic diversity of T. cruzi isolates. No single genetic
Comparative experimental pathology of the DTUs
Since the discovery of Chagas disease in 1909, heterogeneity of parasite strains has been considered one factor implicated in different clinical presentations of the disease. Andrade (1974) attempted to discriminate several distinct T. cruzi morphobiological and behavioral phenotypes in murine models using the criteria of virulence (capacity of multiplication in the host) and pathogenicity (ability to produce tissue lesions and immunological responses). These studies defined three main strain
Clinical presentations
T. cruzi is transmitted to humans mainly by triatomine insect vectors, blood transfusion, infected mothers during pregnancy, and oral infection by consumption of food contaminated with triatomines or their feces. Following infection, a short acute phase is recognized only in 1–2% of the infected individuals, characterized by an abundant parasitemia and mild symptoms that spontaneously decline after 4–8 weeks. The disease proceeds to a chronic phase with scarce parasitemia and an unpredictable
T. cruzi genomics
When the TriTryp genome projects published their initial findings in 2005, researchers were just coming to terms with the latest hitch in T. cruzi genetics. Contrary to prevailing expectations, T. cruzi DTUs TcV and TcVI are both largely heterozygous in their nuclear content (Westenberger et al., 2005). The strain CL Brener chosen to represent T. cruzi (Zingales et al., 1997, El-Sayed et al., 2005) is a member of DTU TcVI, derived from T. infestans. This complication resulted in several
Concluding remarks and perspectives
The revised subspecific nomenclature for T. cruzi (Zingales et al., 2009) recognized that T. cruzi strains should be assigned to one of six DTUs. The important change in the new nomenclature was that TcII was no longer divided into five subgroups (TcIIa-e) (Brisse et al., 2000) but each of those subgroups became independent DTUs (TcII–VI). The rationale for this change provides the underlying theme for the above review and is abundantly clear from several aspects.
The apparent affinities between
Acknowledgements
BZ thanks the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Ministério de Ciência e Tecnologia/Conselho Nacional de Desenvolvimento Científico e Tecnológico/Ministério da Saúde (MCT/CNPq/MS-SCTIE-DECIT-Edital de Doenças Negligenciadas) for financial support. MAM and MSL thank the Wellcome Trust (UK) and European Union Seventh Program Grant 223034 (ChagasEpiNet) for financial support. DAC and NRS are supported by NIH award AI056034. We thank Michael Lewis and Matthew Yeo for
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