The diversity of the Chagas parasite, Trypanosoma cruzi, infecting the main Central American vector, Triatoma dimidiata, from Mexico to Colombia

Abstract

Little is known about the strains of Trypanosoma cruzi circulating in Central America and specifically in the most important vector in this region, Triatoma dimidiata. Approximately six million people are infected with T. cruzi, the causative agent of Chagas disease, which has the greatest negative economic impact and is responsible for ~12,000 deaths annually in Latin America. By international consensus, strains of T. cruzi are divided into six monophyletic clades called discrete typing units (DTUs TcI-VI) and a seventh DTU first identified in bats called TcBat. TcI shows the greatest geographic range and diversity. Identifying strains present and diversity within these strains is important as different strains and their genotypes may cause different pathologies and may circulate in different localities and transmission cycles, thus impacting control efforts, treatment and vaccine development. To determine parasite strains present in T. dimidiata across its geographic range from Mexico to Colombia, we isolated abdominal DNA from T. dimidiata and determined which specimens were infected with T. cruzi by PCR. Strains from infected insects were determined by comparing the sequence of the 18S rDNA and the spliced-leader intergenic region to typed strains in GenBank. Two DTUs were found: 94% of infected T. dimidiata contained TcI and 6% contained TcIV. TcI exhibited high genetic diversity. Geographic structure of TcI haplotypes was evident by Principal Component and Median-Joining Network analyses as well as a significant result in the Mantel test, indicating isolation by distance. There was little evidence of association with TcI haplotypes and host/vector or ecotope. This study provides new information about the strains circulating in the most important Chagas vector in Central America and reveals considerable variability within TcI as well as geographic structuring at this large geographic scale. The lack of association with particular vectors/hosts or ecotopes suggests the parasites are moving among vectors/hosts and ecotopes therefore a comprehensive approach, such as the Ecohealth approach that makes houses refractory to the vectors will be needed to successfully halt transmission of Chagas disease.

Fig 1. Trypanosoma cruzi strains TcI and TcIV identified in Triatoma dimidiata from Mexico, Central America and Colombia as determined by 18S rDNA and SL-IR (spliced leader intergenic region) sequences.

Size of the circles is proportional to the numbers of T. dimidiata specimens with a particular T. cruzi strain (TcI -dark gray or TcIV—light gray) in different countries. Political map was modified from: https://commons.wikimedia.org/wiki/Atlas_of_the_world#/media/File:BlankMap-World6.svg under public domain.

Fig 2. Principle component plots of variation in TcI isolates for the two markers by geographic region (A: 18S, B: SL-IR) and ecotope (C: 18S, D: SL-IR).

Ellipses cover 50% of the variables. For geographic regions, both 18S and SL-IR are significantly different among regions. For ecotopes, the 18S sylvatic is significantly different from domestic, but for SL-IR, the differences are not statistically significant.

Fig 3. Median-Joining Network for 18S rDNA haplotypes.

Networks were constructed with 39—18S haplotypes and the size of each node proportional to the frequency of the haplotype. Small red circles (median vectors) represent hypothetical intermediate nodes. TcII and TcBat are the outgroups. The number of mutational steps ≥3 are shown. Clustering is examined by: (A) geographic origin, (B) vector or host, and (C) ecotope.

Fig 4. Median-Joining Network for the single nucleotide polymorphism region of the intergenic region of the spliced leader gene (SL-IR).

Networks were constructed with 89—SL-IR haplotypes and the size of each node proportional to the frequency of the haplotype. Small red circles (median vectors) represent hypothetical intermediate nodes. TcBat is the outgroup. The number of mutational steps ≥3 are shown. Clustering is examined by: (A) geographic origin, (B) vector or host, and (C) ecotope.