A national epidemiological study by Valencia (1990a) found that rates of infestation varied greatly according to the type of housing and density of population throughout Bolivia. This suggests that the presently high rates of Chagas’ disease are as much a function of improper housing, crowding, and poverty as they are of climatic zones. It has long been commonly believed in Bolivia that Chagas’ disease was limited to lower regions in Bolivia. To evaluate housing and population variables, the following classifications were made (see Table 1): category “x” were superior houses, with improved floors, stuccoed walls, and ceilings with metal roofs; category “y” were houses with improved floors, walls plastered with earth, a ceiling of wood or cloth, and a metal roof; category “z” were houses with dirt floors and straw or palm roofs without ceilings. Densities of population were classified into strata. Stratum “A” (urban population) consisted of communities with 5,000 or more inhabitants. Stratum “B” (populated centers) were communities of varying sizes, with clusters of houses (some urbanization, a plaza, church, school, etc.). Stratum “B” was divided into “B’,” with populations of from 1,000 to 4,999 inhabitants, and “B2,” with populations from 200 to 999 inhabitants. Stratum “C” consisted of dispersed populations of less than 199 inhabitants.
In general, Table 1 indicates that 28 percent of the total houses investigated in Bolivia were infested by triatomine bugs, with an accumulation index often (9.97) triatomines per infested household. The accumulation index varies little between the different strata, indicating that triatomines populate to a carrying capacity (the optimal population size that a particular environment can support). Urban centers, Stratum A, have the lowest infestation rate (10.81 percent), accumulation index (7.6), and density index (0.8). These lower figures are also attributable to the fact that investigators disproportionately studied category x housing (1,045 houses), compared to category y (129 houses) and category z (219 houses) found within urban centers. A counter objection, however, is that urban centers have fewer houses in category y and z; however, this is unlikely in Bolivia with its massive urban migration and its accompanying poverty. Peri-urban areas of Cochabamba, Santa Cruz, and La Paz (El Alto) indicate rising rates of vinchuca infestation and Chagas’ disease.
Table 1
INDEX OF COMMUNITY AND HOUSE INFESTATION IN BOLIVIA
(adapted from Valencia 1990a:36)
- STRATA
TOTAL A B1 B2 C
TYPE OF HOUSING 3,286 1,393 358 309 1,226
Category x 1,454 1,045 139 58 212
Category y 638 129 84 82 343
Category z 1,194 219 135 169 671
Infested Houses 930 150 99 138 542
Entomology Indices:
Infestation (percent) 28.3 10.81 27.71 45.51 45.41
Density (number) 3.4 0.8 2.5 5.2 5.4
Accumulation (”) 9.97 7.6 9.0 11.4 11.9
Within Stratum B1, communities from 1,000 to 4,999 inhabitants, the infestation rate was 27.71 percent, which was close to the national average. These communities are market, provincial, and regional centers scattered throughout the plains, valleys, and mountains of Bolivia. Some communities have pre-Columbian origins, while others were founded during the colonial period as reducciones and pueblos to administer the colonies. Spanish conquistadores attempted to control Andeans by clustering them into religious indoctrination centers (reducciones) and municipal centers (pueblos). These houses are constructed of adobe, with thatched roofs and dirt or brick floors, and many stand as architectural monuments to another generation. They have become centers for triatomines. Rarely does one find that the vecinos (villagers from pueblos, as distinct from campesinos, or farmers) replace these antiquated buildings with new ones; rather, they build alongside the old ones, thus providing areas for triatomines. This helps explain why 139 families are living in category x housing and an almost equal number, 135 families, are living in category z housing. It also explains why the density index (which is derived from the number of bugs captured divided by the houses investigated) is high (2.5). The fact that housing in B1 is fairly divided between categories x, y, and z places all residents at risk, because triatomines will travel over 1,000 meters in search of blood meals.
Similar explanations more readily apply to stratum B2, which has 45.51 percent of the houses infested, a density rate (5.2) double that of B1, and an accumulation index of 11.4. Within B2, 55 percent of the homes fall within category z and 27 percent within category y, which increases the probability for infestation.
The peasants in stratum C suffer the same risk as those in stratum B2, with similar indices for infestation (45.51 percent), density (5.2), and accumulation (11.9); also, 55 percent of households fall within category z. The major difference is that the sample (1,226) in stratum C is much larger than the sample (309) in B2. An obvious conclusion is that Bolivians living in small communities (200 to 1,000 inhabitants), smaller hamlets, and farms suffer from high rates of triatomine infestation due to their living conditions and large number of domestic animals. More than half the houses within these strata have dirt floors, thatched roofs, cloth ceilings, and adobe walls, usually unplastered. Also, 17,588 animals were counted (25 percent dogs, 13 percent cats, 27 percent pigs, and 35 percent guinea pigs). The census did not include poultry such as chickens, ducks, and geese that attract and provide blood meals for triatomines but are not subject to, or hosts of, Chagas’ disease. Bird nests also provide favored hiding places for triatomines.
Even though lower-class strata have a higher incidence of infestation than other strata, within urban centers and peri-urban peripheral areas triatomines are spreading because of massive migration, both permanent and seasonal, from rural areas in Bolivia to industrialized areas. Housing can be even worse in these crowded areas, with scores of people living in rapidly constructed dwellings.
The percentage of vectors positive for T. cruzi in parts of Bolivia was indicated by a study done in 1991 by the SOH/CCH Chagas Control Pilot Program (1994). It conducted a baseline study within the departments of Chuquisaca, Cochabamba, and Tarija of 1,037 houses (93.6 percent coverage of the areas’ 1,108 houses); examined 4,128 blood samples from persons living in the houses (64 percent were seropositive for Chagas’ disease); and collected 13,000 peri- and intradomiciliary vectors. My analysis of these results indicates that Cochabamba had the lowest percentage of houses with basic services (only 3 percent had latrines, and none had potable water and electricity) and the lowest percentage (38.6) of houses infested by vectors positive for T. cruzi, with 46 percent of the insects carrying the parasite. The availability of latrines, potable water, and electricity does not appear to correlate with infection rates. Tarija has the highest percentage of electricity, 55.6 percent, and the highest percentage50 percentof intradomiciliary vectors positive for T. cruzi. This calls into question the notion that lighting would keep photosensitive triatomines outside of the house.
Environmental and climatic factors play a key role in the epidemiology of Chagas’ disease. The Department of Tarija is a sub-Andean region, around 6,400 feet in elevation, with a warm, dry winter season. Temperature fluctuates from 64 to 77°F, and the area receives 28 inches of rain a year, beginning in October and subsiding in March (Muñoz 1977). This climate is preferred by triatomines, as there are no severe cold spells to immobilize the insects. The principal cultivation is of semitropical fruits, coca, coffee, grapes, and cocoa, and many of these plants provide nesting sites for triatomines. Some of this region has been cleared for cattle range, destroying wildlife and plants and forcing triatomines to invade houses. Less hospitable to vinchucas, the Department of Cochabamba is a mesothermic valley (at 8,310 feet) of the Cordillera Cochabamba. Temperatures fluctuate from 54 to 72°F, and rainfall is 26 inches, falling mainly between November and March. Principal cultivation is of corn, potatoes, wheat, barley, fruits, and vegetables. The Department of Chuquisaca is a sub-Andean temperate zone with a median temperature of 59°F and an average altitude of 8,200 feet above sea level. This de
partment also includes lower regions, which might explain its high triatomine infestation and infection rates discussed in Chapter 8.
APPENDIX 6
House Infestation in Latin America
Similar statistics to those in Bolivia (see Appendix 5: House Infestation in Bolivia) are found in endemic areas for Chagas’ disease in other countries of Central and South America (Briceño-León 1990). For Central America, equally high infestation rates of from 30 to 70 percent are reported for three-fourths of El Salvador (OPS 1982:3); 25 percent of vinchuca bugs examined there were infected with T. cruzi. In endemic areas of Honduras 15 percent of houses are infested (Ponce 1984), and one vector, T. dimidiata, has spread to urban settings of the capital, where a family of middle-class professionals reported an acute case of Chagas’ disease in 1989 (Briceño-León 1990:24). In Costa Rica, 35 percent of the houses are infested, with an average of 22 triatomines per house (Zeledón et al. 1975). Some 31 percent of the triatomines and 12 percent of the people of Costa Rica are infected with T. cruzi. Six percent of the population in Guatemala are infected with T. cruzi; and, in Panama, 3 to 22 percent are infected (WHO 1985). (See Appendix 7.)
In South America, 100 percent of the houses in endemic areas of Argentina are infested with T. cruzi, and 8 percent of children under eight years are infected with Chagas’ disease (Pavlone et al. 1988:103-5). Around 30 percent of the houses in the northern half of Chile are infested with vinchucas, and 17 percent of the insects carried T. cruzi (Schofield, Apt, and Miles 1982; Flores et al. 1983; Schenone et al. 1985). In Uruguay, Chagas’ infection rates range from 1 to 7 percent for people over twelve years old (Salvatella 1986); and, in Paraguay, all rural areas are endemic for the disease, with infection rate percentages between 22 and 72 percent being reported (Arias et al. 1988; WHO 1985).
Even though Brazil has made a concerted effort to prevent Chagas’ disease, it ranks as one of the most endemic areas in Latin America. About 4.5 percent of the rural population have antibodies against the parasite; 5 million Brazilians have Chagas’ disease and another 25 million are at risk. In the Federal District of Brazil, 4.3 percent of all deaths are attributed to Chagas’ disease (Dias 1987, Pereira 1984).
In Peru, endemic regions are Arequipa, Moquegua, and Tacna, with 12 percent infection rates. Colombia has an even higher infection rate of 16 percent of the houses infested with triatomines, and 30 percent in the Departamento del Norte de Santander.
Venezuela has more than 1 million people with Chagas’ disease. Roberto Briceño-León (1990) presents a carefully researched analysis of the relationship of housing to Chagas’ disease in Venezuela. Within the last fifty years, Venezuela has had zones where 54 percent of the people were seropositive for Chagas’ disease. One epidemiological study conducted between 1959 and 1965 indicated that within the 35-to-44-year-old age group 79 percent of those tested had antibodies against T. cruzi, and within the 5-to-14-year-old age group 15 percent were infected. Fortunately, these figures have dropped significantly since the 1970s because of the use of insecticides: within the group from birth to 9 years of age infection rates have lowered from 20.5 percent in 1959-1965 to 1.3 percent in 1980-1982; the rates have dropped from 28.4 percent to 2.7 percent in the group aged from 10 to 19 years. Levels of house infestation have also lowered considerablyfrom 73.2 percent of the huts and 31.1 percent of the houses at the beginning of the program in 1970 to 22.1 percent of the huts and 5.6 percent of the houses being infected in 1976 (Sequeda et al. 1986).
In Venezuela, initial success in the late 1970s led to diminishing insecticide use in the 1980s; it dropped from 74 percent coverage in 1980 to 11 percent in 1984 (Briceño-León 1990:29). Houses again became increasingly infested, in part because depletion of forests and sylvatic animals in Venezuela pressured triatomines to search for domestic sites for blood meals. This has been demonstrated in the Municipal Bergantín, Estado Anzoátegui, where insecticides were used from 1970 until 1973, followed by three years without spraying insecticides. The rate of house infestation by triatomines increased from 2.8 percent to 11.4 percent, and hut infestation went from 10 percent to 50 percent.
APPENDIX 7
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, B
olivia, 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).
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