Chagas' Disease: an Ecological Appraisal with Special Emphasis on its Insect Vectors

ZELEDÓN, R. AND J. RABINOVICH

ANNUAL REVIEW OF ENTOMOLOGY

ANNUAL REVIEWS

Lugar: Palo Alto, California; Año: 1981 p. 101 - 133

0066-4170

In spite of being spread widely throughout the Western Hemisphere, Chagas’ disease varies considerably in prevalence in different areas according to ecological factors. At present, probably 13 to 14 million persons suffer from the infection. The disease is closely related to low socioeconomic and educational conditions and, although the insect vectors are becoming established in the slums of important cities, it is mainly rural. It probably existed as a disease involving a great variety of wild mammals and the triatomine bugs associated with them long before man arrived in this part of the world. Disturbance of these natural foci enabled some of the insect vectors to become adapted to man’s artificial ecotopes, a mechanism that is still in progress. Furthermore, some of the most ancient reservoirs of the parasite, such as opossums, are very close to man, thus facilitating the linkage of the wild and the domestic cycles of the disease. Analysis of the stomach contents of numerous insect species by means of the precipitin test has shown that most of them are euryphagic, particularly those better adapted to artificial ecotopes. T. infestans and R. prolixus are today the most epidemiologically important species because of their broad ecological valence, wide geographical distribution, high densities, good adaptation to households, and vector efficiency. The former species has been confined to the southern part of South America where its range extends over a Series of countries, but it seems now to be advancing in northeastern and northwestern directions, mainly through passive dissemination by man. R. prolixus covers a wide belt in northern South America and in most of the Central American countries. It is better represented in wild ecotopes, particularly in palm trees, from which it is constantly being introduced into palm thatched huts. Other species in some more restricted areas can be important but are as yet not as well adapted to human dwellings. Ecological studies on wild triatomines have contributed to the knowledge and understanding of their primitive ecotopes, host associations, and the mechanisms of interchange between natural and artificial ecotopes. These studies are critical from the standpoints of both the epidemiology of Chagas’ disease and the population dynamics of the insects. For instance, the food resources in an artificial ecotope, represented by a household, are in general much more stable than the ones found in the natural ecotopes, mainly because host availability in the latter is irregular. Also the peridomestic environment, as well as the household, usually harbors domestic and sylvatic animals and hence provides an ideal physical refuge and food supply for the triatomines. A key piece of information still needed for both natural and artificial ecotopes is an estimate of population trends, including temporary fluctuations, as determined by the continued sampling of the population. More studies with marked insects will help to clarify some details, particularly on the displacement abilities of the insects. Also, new experimental studies on the effect of controlled climatic and nutritional factors are necessary to explain the apparent inability of some species to adapt to different ecological conditions. Life cycle studies in the laboratory have been useful in showing important differences among species, although disparities in experimental conditions have produced some contradictory results. However, laboratory development may not be the same as that which actually occurs in nature. The effect on natural population regulation of the interactions of physical factors, feeding patterns, dispersal behavior, and enemies deserves more attention, and more work is required on natural enemies for possible biological control of triatomine populations. Several aspects of the insect-parasite relationship also need further clarification. The mechanisms involved in the apparent resistance of the insect hind gut to colonization by the parasite in certain instances and the conditions under which infected insects sometimes become negative are not well understood. Contradictory information on the higher susceptibility of a given species of insect to the local strains of the parasite needs to be clarified.