Home Economics American Trypanosomiasis Chagas Disease, Second Edition: One Hundred Years of Research
Who transmits the parasite?
At present, 140 species of the subfamily Triatominae have been described.22 These are grouped into 15 genera, which include species that are important vectors of Chagas disease or live in the human environment and therefore constitute a potential danger: Dipetalogaster (Usinger, 1939), Eratyrus (Stal, 1859), Meccus
(Stal, 1859), Panstrongylus (Berg, 1879), Rhodnius (Stal, 1859), and Triatoma (Laporte, 1832). The vector capacity of each species is difficult to determine. However, several indicators are usually used for laboratory colonies, and most often, several species are compared under the same experimental conditions.23 However, the vector capacity appears to be shaped by environmental conditions mainly related to food availability.
The vector capacity of a species depends on the ability of T. cruzi to complete its life cycle along the gut of the insect and to produce infective forms at the rectal gland (metacyclogenesis process) that are then deposited near skin abrasions or on mucous membranes. Metacyclogenesis is the transformation of epimastigote forms (noninfective) to trypomastigotes, which are metacyclic forms that are capable of infecting mammalian cells when released into the feces. The transitions to metacyclic forms seem to occur primarily in situ in the rectal gland, where both epimasti- gotes and trypomastigotes are attached by the flagellum.24,25 In the small intestine, transitional forms occur but are scarce. Several molecules play a role in the redox status of the triatomine gut microenvironment that influence metacyclogenesis,26 which appears to be vector-dependent.27-29 For example, during experimental comparisons of several species (R. prolixus, Rhodnius neglectus, Panstrongylus megis- tus, Triatoma sordida, T. infestans, Triatoma brasiliensis, Triatoma rubrovaria, Triatoma pseudomaculata, and T. dimidiata) reared in the laboratory, the authors observed a significant difference in the rate of metacyclic forms between species at the 120th day of infection, where metacyclic forms comprised 50% of the individuals in R. neglectus and 37% in its congener R. prolixus but were dramatically lower in the majority of Triatoma species (5% in T. sordida, 3% in T. brasiliensis, and 0% in T. pseudomaculata).27 Several studies have also shown that the rate of metacyclogenesis varies from one strain of T. cruzi to another. In T. infestans, a strain belonging to the T. cruzi discrete typing unit (DTU) TcI nearly always reached higher trypomastigote density in the rectum than another strain belonging to TcII DTUs.30 Another experimental work, comparing the percentage of infected insects, and the number of flagellates and metacyclic forms per insect, confirmed that parasites belonging to TcI are more efficiently transmitted in T. infestans than TcII and TcV.31 Several studies have examined the metacyclogenesis process in vitro. The incubation of epimastigote forms in an appropriate medium induced metacyclogenesis, and different strains exhibited highly heterogeneous differentiation rates, and in particular TcI strains exhibited the highest level of differentia- tion.32,33 Another factor that influences metacyclogenesis is the meal of the triatomine. Starvation reduces the total number of parasites and the number and percentage of trypomastigotes. Moreover, feeding the vector after 40 days of starvation induces the appearance of pure populations of trypomastigotes.34 These observations indicate that vector capacity depends on the availability of food sources.
Another important factor determining the vector capacity of triatomine species is their feeding behavior and defecation reflex resulting in the feces being deposited near the bite. Several indices were measured in laboratory colonies using the main vector species (T. infestans and R. prolixus) as references. Voracity (i.e., the delay before initiating a meal), the meal period, and especially the time to postfeed defecation were analyzed. It is impossible to formulate a comprehensive description of the feeding behavior of triatomines, but it is important to note that the main vector species are not the only species that exhibit eating and defecation behaviors that favor transmission of the parasite. Some species that are not currently recognized as vectors may be adapting to the human environment (e.g., Triatoma patagonica). For example, 69% and 58% of the nymphs of T. patagonica and T. infestans, respectively, produced their first defecation within 5 min after being fed, and the nymphs of T. patagonica were capable of defecating during or immediately after feeding.35 Similarly, several Mexican species exhibit a short time to defecation (<10min for Triatoma lecticularia, Triatoma protracta, and the youngest nymphs of Triatoma gerstaeckeri), which suggests that these three species may be important potential vectors of T. cruzi for human populations in areas of Mexico where these species are currently present.36 In general, the experimental studies on laboratory- reared triatomine species tend to evaluate a set of parameters to conclude whether the vector studied has good transmission competencies. The species T. protracta and Triatoma rubida are common triatomines in southwestern North America and were considered poor vectors.37 However, other studies have shown that of the three sympatric Mexican species, Triatoma recurva, T. protracta, and T. rubida, only the last one should be considered as an important potential vector.38,39 Triatoma williami, a species belonging to the T. oliveira complex that occurs in the Pantanal ecosystem (Mato Grosso, Brazil), also shows a short time to defecation, so this species found in intradomicile and peridomicile locations in rural and urban areas should be considered as a competent vector.40 This is also the case of Triatoma boliviana, a new species recently described in some valleys in the Department of La Paz in Bolivia.41 In the Gran Chaco region, Triatoma guasayana is a species commonly found in peridomiciles and exhibits a vectorial capacity very similar to that of T. infestans, the principal vector in the region.29
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