The Kiss of Death

Triatomines are biologically equipped to find blood meals. Heat-sensitive antennae and pheromonal communication direct them to sleeping mammals and birds at night. Frequently, they glide back and forth across a room, detect warmth from a human and gently land upon his or her body or face. Probing the warm surface to sample underlying fluids, sensilla in the insect’s proboscis search for adenosine triphosphate (ATP) or other appropriate engorgement factors. If successful, the vinchuca injects its needle-sharp proboscis coated with a general anesthetic and decoagulant so that it can easily and leisurely (up to thirty minutes) drink blood, as much as seven times the bug’s weight, until its gut is filled to a certain capacity determined by abdominal stretch, which triggers the molting cycle in nymphs (Molyneux and Ashford 1983:79). Abdominal stretch of the midgut with blood also triggers peristalsis and the triatomine defecates on its victim’s face, adding insult to injury (as well as exchange of parasites for blood).

Genetic factors of different triatomine species regulate their susceptibility to parasites, parasite density in the feces, and defecation time (WHO 1991). Defecation time of triatomines is crucial for the transmission of parasites to hosts. Scratching the wound, the victim spreads the fecal matter over the bite site, allowing parasites to enter through the wound or the irritated surrounding skin.

After filling up with blood, T. infestans defecates within minutes near the bite site. Other triatomines defecate some time after the blood meal and away from the bite site, which lessens chances of spreading infection (Wood 1951). Studies of three species have also shown important differences not only between species but also between the different instars and sexes within the species (Dias 1956, Zeledon et al. 1975, Pippin 1970). After a blood meal, R. prolixus was the first to defecate; it also defecates more frequently, within one-, five-, or ten-minute intervals, than other species. The mean times for the first defecation and the percentage of individuals which defecated within this species was surpassed only by the adult stages of T. infestans. T. dimidiata was slower to defecate and did so less frequently after meals than the other two species. However, T. dimidiata defecated more during the meal, taking longer to feed and interrupting the blood meal more frequently to defecate. Females of all three species defecated sooner, more frequently, and in greater number than did males. T. dimidiata females and fifth-instar nymphs had the highest defecation average. T. infestans defecated throughout all instar stages but mostly so during the fourth instar stage and less so as an adult. R. prolixus also defecated throughout all instar stages, with the males defecating decisively less.

A defecation index has been established by Zeledon (1983:329) for each of the instars, which permits an easy visualization of the differences between instars of the same species or of different species. The index is the mean number of defecations in ten minutes, multiplied by the percentage of insects which defecate in ten minutes divided by 100. The high defecation index of T. infestans and R. prolixus is a major reason why these triatomines are so successful in transmitting T. cruzi.

Nonetheless, defecation at later times also presents health hazards in that fecal matter is dragged across bedding, floors, walls, and other parts of the room, where it presents occasions for contamination. T. cruzi remain alive in fecal matter as long as it stays moist, and this depends on the humidity and the amounts of fecal matter. If the fecal matter falls from the ceiling into liquids or food, parasites can survive for long periods and can contaminate a large number of people at one time. Already discussed, schoolchildren in Brazil were infected with Chagas’ disease from contaminated milk (Mazza 1936).

APPENDIX 5 House Infestation in Bolivia

A national epidemiological study of housing and Chagas’ disease in Bolivia was completed by Angel Valencia, Abraham Gemio Alarico, and John Banda Navia in 1990. They studied various localities from 1,000 to 11,500 feet above sea level and populations stratified into urban populations, dense population centers, and dispersed populations. Moreover, they also considered the ecological factors of microenvironments. In all, they studied 109 localities and 3,236 houses. The populations within these houses were 18,172 people (an average of 5.6 per household) and 17,588 animals (25 percent dogs, 13 percent cats, 27 percent pigs, and 35 percent guinea pigs). From the human population, they obtained 9,547 serological samples on filtered paper and also conducted 7,996 electrocardiographic studies (Valencia 1990a:7).

Regarding housing infestation, Valencia (1990a:29-30) and co-workers first noted the exact time that their searches began, then they started in the dormitory, going from left to right in the room, and then proceeded in a clockwise pattern to inspect the other rooms. Employing flashlights, they examined every surface, including walls, ceiling or roof, bedding, furniture, stored clothing, wall hangings, and domestic items. Wearing plastic gloves and using anatomical pinchers, they captured the vinchucas by the thorax and placed them in separate boxes designating where they had been captured. They annotated house number, date, name of the head of the household, locality, province, department, and how the specimen was caught.[78] Likewise, inspectors proceeded outside the house (peridomicile). Wearing thick leather gloves, they removed rocks, uncovered logs, removed firewood, and turned over adobes to capture vinchucas. Averages were computed from how many vinchucas were captured per hour per inspector, not counting time spent explaining procedures to household members or in filling out forms. At no time were irritants used to drive out bugs.

Indices were established in the following manner: the index of infestation for a particular locale was determined by the percentage of the places examined in relation to the places infested; the index of infestation of houses was determined by the percentage of the houses examined in relation to houses infested; the index of density was determined by the percentage of houses examined for triatomines in relation to the number of triatomines captured; and the index of accumulation was the percentage of houses infested with triatomines in relation to the total number of triatomines captured (Valencia 1990a:27).

These indices are at best estimates, given the variables of the investigators’ ability to catch bugs and various environmental factors, including the accessibility of hiding places and the temperature. A more accurate but methodologically unrealistic accumulation index would be one in which the house is enclosed in plastic, sprayed, and dismantled, as was done in Panama for one house, which contained a reported 100,000 triatomines (Sousa and Johnson 1971). Another method would be for entomologists to study the carrying capacity of different environments in Bolivia for vinchucas, which then could be used as a guide. An area’s carrying capacity implies that vinchucas maintain fairly stable populations determined by the availability of hiding places and blood meals. If they overpopulate, they are unable to nourish themselves and do not progress along the instar stages to adulthood, when they are able to lay eggs.

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