RATES OF T. CRUZI VECTORS IN CENTRAL AMERICA

(Cedillos 1987:47)

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 Carcavallo, 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).