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.
Table 2
RATES OF T. CRUZI VECTORS IN CENTRAL AMERICA
(Cedillos 1987:47)
Countries Triatomine Sp. Number % pos. Source
Costa Rica T. dimidiata 3276 30.9 Zeledón et al. 1975.
El Salvador R. prolixus 2068 13.6 Pefialver et al. 1965.
T. dimidiata 1767 30.8 Pefialver et al. 1965.
Guatemala T. dimidiata 5747 23.4 Pefialver, Fajardo Aguilar 1953.
Honduras R. prolixus 3238 34.7 Ponce and Zeledón 1973.
T. dimidiata 791 32.2 Ponce and Zeledón 1973.
Nicaragua R. prolixus 282 9.6 Urroz 1972.
T. dimidiata 18 39.0 Urroz 1972.
Panama R. pallescens 3283 32.7 Pipkin 1968.
The parasite T. rangeli is also found along with T. cruzi (but in lesser percentages) in triatomine species R. prolixus and R. pallescens in El Salvador, Honduras, and Panama (Sousa and Johnson 1971, 1973). T. rangeli is nonpathogenic in humans, and its precise identification is important to determine the true occurrence of T. cruzi. In El Salvador, one study indicates a higher percentage of triatomines (R. prolixus) infected with T. rangeli (21.4 percent) than with T. cruzi (0.5 percent), which suggests the possible displacement of T. cruzi by T. rangeli when both trypanosomes occur in the same insect (Cedillos 1975, 1987). Another possibility, especially in earlier studies, is that the identification of T. cruzi from T. rangeli was incorrect because it was primarily made by direct microscopic examination of an insect’s intestinal content, and Giemsa stain was seldom used (Cedillos 1987:49).
The severity of T. cruzi infections is only moderate in Central America, where the parasite does not cause the significant myocardial damage, megaesophagus, and megacolon that it does in Bolivia and central parts of Brazil (Cedillos 1987:54). Although this geographical variation could be related to genetic and nutritional conditions, it is most probably a function of the prevalence of different infective T. cruzi strains in the two regions. One virulent strain, T. cruzi Zymodeme 2 (Z2), is common in central Brazil and is associated with severe cases of megasymptoms. The Z2 strain has not been found in El Salvador and Panama, where Zymodeme 1 (Z1) and Zymodeme 1 and 3 (Z1, Z3), respectively, are identified (Miles, Provoa, Prata, et al. 1981:1338, Kreutzer and Souza 1981:30).
In Bolivia, thirteen species of triatomines have been found (Valencia 1990b: 9). Within the subfamily Triatominae, three of its five tribes are reported: Tribe Rhodniini Pinto, 1926, with genera Rhodnius Stal, 1859, and Psammolestes Bergroth, 1911; Tribe Triatomini Jeannel, 1919, with genera Triatoma Laporte, 1832, Panstrongylus Berg, 1879, and Eratyrus Stal, 1859; and Tribe Bolborderini Usinger, 1944, with genera Microtriatoma Prosen and Martinez, 1952. Not recorded in Bolivia are Tribe Alberproseniini Martinez and Carca
vallo, 1977, and Tribe Cavernicolini Usinger, 1944.
Triatomine vectors of Trypanosoma cruzi in Bolivia are principally those reduviids that have adapted to living in peridomicile and domestic environments and to sucking blood from humans and domestic animals (see Figure 13). These species are Triatoma infestans, Rhodnius prolixus, and Panstrongylus megistus, which show a long relationship with humans as hosts. Approximately eight other species of triatomines are infected with T. cruzi; they are in the process of changing biotopes from sylvatic to domestic environments as their preferred hosts, smaller wild animals, are depleted and forests are cut down. As human groups migrate, they spread triatomines to other biotopesfor example, bringing T. infestans to higher altitudes in Bolivia. Also, as nomadic groups become sedentary, they provide sylvatic triatomines time to colonize their communities. The vector, host, and parasite relationship is changing daily in Bolivia and no rigid classification can be maintained. Important variable factors include the degree of adaptability of a species, the passive transport of bugs by humans, destruction of a species habitat and hosts, and personal and household hygiene. (See Appendix 5.)
Triatoma infestans has become such a major transmitter of T. cruzi in Bolivia because it cohabits with humans and can live at altitudes between 1,000 and 11,000 feet (Valencia 1990b:9); it has been found at sea level and as high as 12,500 feet in Lallagua, Bolivia (Carcavallo 1987:17). Entomologists captured 10,070 triatomines in Bolivia from domiciliary and peridomiciliary areas within environments ranging from 1,000 to 11,000 feet; 98.5 percent were T. infestans, 1.35 percent were T. sordida, and 0.2 percent were Eratyrus mucronatus (Valencia 1990a:39). As already mentioned, the adaptive features of T. infestans enable it to live within the various ecologic zones of Bolivia, a country noted for its many different life zones, from the Amazon basin to the crested Andes, including every ecotope. Although T. infestans thrives best within temperatures ranging from 60° to 80°F and altitudes from 1,200 to 6,000 feet, it has followed migrating human populations to most parts of Bolivia. At higher temperatures and altitude, however, these bugs reproduce at a slower rate. (See Appendix 4.)
Although only a few species of Triatominae are principally responsible for the transmission of T. cruzi, a variety of other insects, including ticks, bedbugs, and mosquitoes, can incubate T. cruzi in the laboratory (Brumpt 1912); however, their role in the transmission of the parasite to humans appears negligible.
Nevertheless, further research needs to be done concerning the possibility that after insecticide application bedbugs may proliferate and transmission might be possible if infected insects were crushed on the skin (Marsden 1983:257). Trypanosoma cruzi remains infective for months in dead triatomines kept refrigerated (Soares and Marsden 1980).
The ability of these bugs to adapt to artificial ecotopes illustrates a dynamic process (Zeledón 1983:327). Certain species with higher adaptability invade new territories, while other, less-adaptable, species are passively transported by humans to other regions where they begin their process of adaptation. Some species have adapted very well to peridomestic and domestic niches. This phenomenon is occurring in South and North America and, more recently, in some areas of Asia. The adaptive skills of trypanosome-carrying assassin bugs make it certain that Chagas’ disease will spread to areas where it was not known before.
In summary: Triatoma infestans is by far the most common vector of Chagas’ disease in Bolivia and Brazil, with R. prolixus and P. megistus running a distant second; however, T. infestans does not appear to be a principal vector in Central America. Also, T. infestans carries more virulent strains of T. cruzi associated with megasyndromes than the strains found in Central America. Although this leads to the impression that the severity of T. cruzi infections is only moderate in Central America, no studies have been done in Central America to ascertain what proportion of those infected develop the chronic phase of the disease (Cedillos 1987:54). Other factors relating to geographical variation in pathology include genetic and nutritional conditions, incidence of infection, socioeconomic conditions, climatic factors, and reservoir hosts.
Ecological factors, such as climatic factors and reservoir hosts, also affect vectorial transmission of Trypanosoma cruzi (WHO 1991). Climatic factors, mainly temperature, affect the rate of increase of triatomine populations. Where maximum and minimum annual temperatures show little variation in central Brazil (Gois), T. infestans populations produce two generations a year (WHO 1991). The rate of female fecundity, development rates of nymphs, and adult emergence rates are highest during the summer (December and January), followed by a minor peak in the winter (June and July). Adults and fourth- and fifth-instar nymphs mainly compose the winter population. Molting and reproduction are resumed at the beginning of spring.
The proportion of infected vectors has been found to be higher at the beginning of the hot season (WHO 1991). Seasonal changes in age structure and density of populations produce changes in the proportion of infected vectors. In temperate regions, transmission of Trypanosoma cruzi is concentrated in the warm season; in warm climates, transmission occurs throughout the year, with the highest level in the summer. The frequency of acute cases of Chagas’ disease markedly increases during the summer months. These features of infection rates and of population dynamics should be considered when agencies program control operations and primary health care measures.
Fortunately for humans, many species of triatomines are sylvatic and feed on animals and birds; consequently, relatively few triatomine species feed on humans. Most of the species of triatomine bugs are prevalent in tropical and subtropical areas at altitudes between 200 and 1,500 meters (600-4,500 feet) above sea level, although eighteen species are found in near arctic biomes (T. patagonica is found in the Patagonian region of Argentina), twenty-three species in xerophytic forests, sixteen in desertic and semidesertic plains and plateaus (most notably the Altiplano, where T. infestans is found), and eleven in temperate foothills and valleys (Carcavallo 1987).
APPENDIX 8
Hosts for T. cruzi
Although triatomines are opportunistic blood feeders, the seven most important vector species (see Appendix 7: Vector species of T. cruzi in the Americas) show a preference for blood meals, with humans being the most desirable, then chickens and pigeons, and dogs and cats to a lesser degree. The nocturnal activity of cats saves them to a degree from these nocturnal predators. Guinea pigs are a delicacy to vinchucas in Bolivia and Peru, where Andeans traditionally raise them inside the kitchen. Guinea pigs have also been a factor for the spread of Chagas’ disease throughout Andean countries. Rats and mice play a lesser role in providing blood meals and a major role as predators of triatomine bugs, as are chickens and cats, thus somewhat suppressing triatomine populations.
An important ecological factor influencing transmission of Chagas’ disease is the association of triatomines with synanthropic animals (WHO 1991). Synanthropic animals are those animals that live around humans. They range from pets, livestock, and rodents to opossums, raccoons, foxes, deer, and other animals that, in part because of deforestation and encroachment upon forests, live close to humans. Because these animals serve as blood sources, they contribute considerably to maintaining or increasing population densities of domiciliary and peridomiciliary vectors. Animals also serve as vehicles to disperse triatomines to other parts of the world. The migratory wood stork (Mycteria americana), as one known example, carried Rhodnius prolixus from the north of South America to Central America and Mexico.
Epidemiologically, sylvatic and synanthropic animals serve as reservoir hosts for Trypanosoma cruzi. (Humans have become the principal hosts.) After Carlos Chagas (1909) found Trypanosoma cruzi in house-dwelling triatomines, Panstrongylus megistus, he discovered that the two important mammalian hosts in the domestic environment in the transmission cycles were humans and cats. Three years later, Chagas discovered infections in armadillos and recognized Panstrongylus geniculatus as the vector in this purely sylvatic cycle in armadillos. Subsequently,
throughout the countries of Latin America, a wide variety of mammals and triatomine vectors have been identified as involved in the transmission cycle of T. cruzi and related flagellates. More than 150 species of wild and domestic mammals have been found to be reservoirs of T. cruzi (see WHO 1991: Annex 4 for list).
Certain animals are better reservoir hosts than others. Dogs, cats, and rodents are the prime reservoir hosts within the peridomestic arena, and opossums (Didelphis species) and armadillos within the sylvatic arena. T. cruzi infections in dogs have been reported from fifteen countries and infections in cats from seven, with great variability in infection rates (from 4.5 percent to 100 percent in dogs and from o.5 percent to 60.9 percent in cats). Dogs are important reservoir hosts due to their close contact with humans during the night, the age-independent persistence of parasitemia in dogs, and the possibility of congenital or lactogenic infection of dogs, as has been indicated by a study in Argentina. Guinea pigs are bred indoors in Bolivia and Peru, where high rates of infection have been reported (Bolivia, 10.5-61.1 percent; Peru, 19.2 percent [Gürtler et al. 1990; WHO 1991]).
Other domestic animalscattle, goats, pigs, donkeys, and horseshave rarely been found infected. They are not considered to play an important role as reservoirs because of their low population density, their less-close contact with humans, and their low rates of parasitemia (WHO 1991:25). Some species, such as goats and certain rats, appear to be able to eliminate the infection. Although they serve as blood meals, chickens, turkeys, ducks, and pigeons are not susceptible to T. cruzi infection (see Appendix 10: Immune Response). This is also true for all other birds as well as reptiles. Chickens are used in laboratories to blood feed sterile vinchucas that are used for xenodiagnosis. Veterinary researchers and animal environmentalists need to assess the considerable impact Chagas’ disease has upon domestic and sylvatic animals.
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