Vector Species of T. cruzi in the Americas

Vinchucas belong to the Triatominae subfamily of the Reduviidae family of the order Hemiptera, which contains over 4,000 species of large bloodsucking insects. Vinchucas are like squash bugs, another species of reduviid, but not a vector of T. cruzi, found on garden plants in the United States. Being hemipteran, vinchucas possess two pairs of wings and characteristic mouth parts adapted to piercing and sucking; they also undergo incomplete metamorphosis. There are 105 species of Triatominae in the Americas; they are clustered into tribes with thirteen genera, and over half of these species have been reported to be infected with T. cruzi (see WHO 1991:81‑86 for list and location).

The Triatominae species most responsible for Chagas’ disease in South America are, in order of importance, Triatoma infestans, Rhodnius prolixus, Panstrongylus megistus, Triatoma brasiliensis, Triatoma sordida, and Triatoma dimidiata with Rhodniuspallescens as the principal vector in Central America and Panama. The principal reason for their abundance is that these triatomines have adopted domiciliary habits.

The interaction between the different vectors, different strains of T. cruzi, and hosts is an important parameter affecting the susceptibility of a vector to infection (WHO 1991). The vectors’ ability to adapt to a domiciliary habitat and its susceptibility to harboring T. cruzi are principal determinants of its vectorial capacity. Factors influencing the vector’s susceptibility include genetic factors which regulate variances for each species concerning the susceptibility and intensity of infection, parasite density in the feces, and time of defecation. Local parasite strains more readily infect local vector species than do strains from other endemic areas, and vectors are capable of “selecting” subpopulations of T. cruzi from a natural heterogeneous population, which affects the parasite’s pathogenicity in human hosts. The morphological form of the trypomastigote ingested also influences the infection in that the “stout” forms seem to be more infective to the vector than are the “slender” forms, which seem to be more infective to the host. Feeding habits of vectors also influence vector infection.

Other important susceptibility factors include the stage and age of the insect vector, the climate, the blood‑meal size, the number of parasites ingested, the ability of the parasite to establish rectal gland infections in the vector, and parasite transformation in the insect’s digestive tract. After triatomines are infected, they remain carriers of T. cruzi for the rest of their lives, and they show no significant biological differences from uninfected triatomines. The insect apparently is not affected pathogenically (WHO 1991).

Within the triatomine subfamily there are fourteen genera and III species of blood‑sucking bugs. The vast majority are found in Central and South America, where thirteen genera and 103 species of triatomines have been reported and over half have been reported naturally infected with Trypanosoma cruzi: sixty‑eight species have been recorded as infected with Trypanosoma cruzi, twenty‑three with T rangeli, two with T. vespertilionis, one with T. conorrhini, and four with Blastocrithidia (Carcavallo 1987:15‑17; Molyneux and Ashford 1983:78‑84). Asian triatomines have not been reported as carrying T. cruzi, and the only known Asian vector of a trypanosome is Triatoma rubrofasciata, which is associated with the transmission of the rat trypanosome T. (Megatrypanum) conorrhini in Japan.

The three genera with the greatest epidemiological significance are Rhodnius, which is distinguished from other genera of Triatominae by its long head, with antennae inserted at the front (Lent and Wygodzinsky 1979); Triatoma, which have intermediate‑length heads, with antennae inserted midway between the eyes and the clypeus; and Panstrongylus, which have short, robust heads with antennae extending immediately in front of the eyes.

Triatoma infestans is the most widespread domestic species and the most important vector of Chagas’ disease in Argentina, Chile, Bolivia, Brazil, and Uruguay (also discussed in Appendix 4: Triatoma infestans). Generally adaptable, T. infestans has the unique advantage of being euribiotic (adaptable to many ecologic zones), eurithermic (adaptable to different temperatures), euriphagic (adaptable to a wide variety of hosts and feeding habits) as well as being relatively eurihydric (adaptable to varying humidity). It occupies the greatest climatic range of triatomines, from arid highlands in Andean countries and temperate plains in Argentina to the dry tropics in northeastern Brazil. It is the oldest‑domiciliated triatomine species, being now restricted to artificial human ecotopes in most of its distribution area but still thriving in sylvatic ecotopes.

Triatoma infestans’ long‑standing adaptation to human habitations and its ability to tolerate many ecotopes have enabled it to disperse with migrating human populations. Most common in poor adobe houses, it nests in the thatched or straw roofs, in cracks in the mud walls, and in areas between the roof and walls. It also colonizes cement‑block or brick houses, getting into crevices or breeding in household belongings. In Bolivia, blood meals commonly are taken from resident hosts (humans, chickens, dogs, cats, and guinea pigs) 84‑99 percent of the time. In Chile, 69 percent of T. infestans’ blood meals are from humans, who act as the main reservoir host of the parasite. In Argentina, 25‑49 percent of triatomine feeds are from dogs, which have become the main parasite reservoir for domestic transmission of Chagas’ disease there (WHO 1991).

Rhodnius prolixus is the next most important vector of Chagas’ disease in much of tropical America. Like T. infestans, it has evolved and adapted to domiciliary habitats. R. prolixus is a native of northern South America, where it occupies many sylvatic arboreal habitats associated with mammals and birds that nest in palm trees or bromeliads (WHO 1991). However, it is present exclusively inside houses in a number of Central American countries and in parts of Mexico. In its domiciliary habitat, it feeds mainly on the blood of humans and chickens, and to a lesser extent on cats and dogs. In its sylvatic habitat, it feeds mainly on opossums and rodents. Because of its preference for nesting in palm trees, it also likes to nest in palm‑thatched roofs as well as in cracks in walls and in household goods.

Panstrongylus megistus is a distant third to Triatoma infestans and Rhodnius prolixus as a carrier of T. cruzi, being limited to Central America, northern South America, and the forests of coastal Brazil. It is a stenohydric species, endemic in sylvatic ecotopes and sometimes in peridomestic structures. In northeastern Brazil, humans have deforested its natural ecotopes, thus helping make it an important domiciliary species. One indication of this is that in southern and central Brazil blood meals from humans account for only 14‑30 percent of the total feeds of bugs collected in domestic and peridomestic sites (birds and rodents were more important blood sources); while toward the northeast intense deforestation has destroyed the natural habitats of P. megistus and it has become an important domiciliary species, taking an increasing number of blood meals from humans (WHO 1991).

Also in northeastern Brazil, Triatoma brasiliensis is the most important vector of T. cruzi, being highly susceptible to infection. It is found in sylvatic and peridomestic habitats, especially rocky areas and cattle shelters, and it has infected rodents and goats. Birds are its principal blood source, followed by humans.

T. sordida is another species found throughout southern Bolivia, Paraguay, northern Argentina, and central and southern Brazil that has extended its range northwards and southwards because of intense deforestation of its original home. Originally a sylvatic and peridomestic species that fed mainly on birds, it is becoming increasingly domiciliated in southeastern and central Brazil, taking 16‑32 percent of its blood meals from humans (WHO 1991).

Triatoma dimidiata is an important vector in Central America and parts of Mexico. It is a domiciliary species found in wooden houses, woodpiles, and earthen floors. It prefers to feed upon human blood, but it also feeds upon rodents, dogs, chickens, and opossums.

Rhodnius pallescens has become increasingly domiciliated, invading houses from its breeding places in palm trees. In Panama, 59 percent of its blood feeds were from humans, followed by preferred feeding on opossums and poultry. Chagas’ disease has been more on the increase in central Panama than in western areas of that country because of the association of R. pallescens with the opossum (Didelphis marsupialis ). In western Panama, the principal vector of Chagas’ disease is Triatoma dimidiata.

In the United States, triatomines thus far have not adapted to household ecotopes (Ryckman 1986). Rhodnius prolixus and Triatoma dimidiata are the most important vector species from Mexico to northern South America. In Ecuador, Triatoma dimidiata is the primary domiciliary vector species. Rhodnius prolixus is the main vector in Colombia, French Guiana, Guyana, Suriname, and Venezuela. Triatoma venosa and T. maculata are found in homes in Colombia but have only secondary importance. In Peru, major vectors are Panstrongylus lignarius in the north and T. infestans in the south. P. megistus is found in limited areas in Bolivia and Paraguay. T. sordida covers the eastern part of Bolivia, a broad band in southern Brazil, and areas in Argentina, Paraguay, and Uruguay (WHO 1991).

Several species of triatomines are naturally infected with T. cruzi in Brazil. As in Bolivia, T. infestans is the most important, and it has dispersed northwards, reaching the northeastern states of Pernambuco and Parába. Second in importance, P. megistus has a wide geographical distribution and high rates of natural infection. It is domiciliary in parts of northeastern and eastern Brazil. In northeastern Brazil, T. brasiliensis is the main vector, with T. sordida and T. pseudomaculata generally replacing the main domiciliary species after fumigation. T. sordida and T. pseudomaculata have low rates of natural infection with T. cruzi.

Percentages of infected triatomines are smaller in countries of Central America than in South America, especially Bolivia and Brazil, where averages are around 50 percent. Table 2 shows the infection rate of triatomine bugs with T. cruzi parasites in Central America.

T. dimidiata and R. prolixus frequently harbor the Chagas’ parasite in Central America. T. dimidiata has a much higher rate of infection than R. prolixus in El Salvador and Nicaragua; but, in Honduras, the rates of infection of both species are similar, 34.7 percent and 32.2 percent, respectively, for R. prolixus and T. dimidiata. In Costa Rica, T. dimidiata is the principal vector, with an infection rate of 30.9 percent; and, in Panama, R. pallescens is the principal vector, with an infection rate of 32.7 percent. Within the Panama Canal zone and the provinces of Panama, Colon, Chiriquí, and Bocas del Toro, percentages of infected R. pallescens ranged from 68.7 to 84.1 percent (Sousa and Johnson 197I). In one house alone, approximately 100,000 triatomines were found.