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INTERNATIONAL WORKSHOP ON POPULATION
24-28 September 1995 Chagas disease (american trypanosomiasis) is one of the few vector-borne diseases that can only be controlled by interrupting transmission of the causative agent, Trypanosoma cruzi. Transmission by blood transfusion seems to be increasing in many Latin American cities, but most transmission is still due to domestic populations of the insect vectors - large blood-sucking bugs of the subfamily Triatominae. WHO estimates (1) indicate 16-18 million people infected with T.cruzi, with around 100 million at risk of infection, making Chagas disease by far the most serious parasitic disease in the Americas (2). Since 1991, governments of the most seriously affected countries in southern Latin America have been engaged in an ambitious programme to eliminate all domestic populations of the most important vector, Triatoma infestans. To date, these governments have invested over US$150 million in vector control activities (3) bringing transmission rates close to zero in many areas (3,4,5). This programma, which is part of the Southern Cone Initiative (6,7), is facilitated by the exclusively domestic habits of T. infestans throughout most of its wide distribution. Moreover, T. infestans seems highly specialised, adapted to the stable environment offered by human dwellings, with rather low genetic variability facilitating its control by classic house spraying and long-term community-based vigilance. Outside the southern cone area however, in Mexico, Central America and the Andean Pact countries, there is a range of other vector species of Triatominae, many with widespread silvatic ecotopes. The big question therefore, was to what extent these vectors can also be controlled, leading to a sustainable suppression of Chagas disease transmission throughout Latin America. Meeting in Santo Domingo de los Colorados, in the foothills of the Ecuadorian Andes, specialists from 15 countries debated during three intense days. The workshop, declared to be 'of National Interest' by the Minister of Health of Ecuador, was primarily sponsored by the Commission of the European Communities. As introduction to be discussions, participants presented detailed reviews of the current epidemiological situation in Mexico, Central America, Colombia, Venezuela, Ecuador and Peru. The first conclusions were quickly apparent: in these countries there are almost 6 million people seropositive for T. cruzi infection and around 20-25 million at risk; Rhodnius prolixus is the main culprit, accounting for well over half of the reported cases. There are some evidences in Mexico and Central America that at least R. prolixus could be exclusively domestic with no reported selvatic foci. If this fundings proves to be true, R. prolixus could be a target for complete erradication. As the debate proceeded on technical and political requirements, a parallel working group discussed research needs to support the vector control and surveillance activities. Basic methods for Chagas vector control are established and well proven. Houses and peridomestic habitats are sprayed with wettable powder or flowable formulations of modern pyrethroids such as deltamethrin, lambdacyhalothrin or cyfluthrin. These are approved for use in domestic situations and quickly produce a dramatic reduction in infestation rates. But treated houses then remain vulnerable to reinfestation. In the long term, this can be addressed by gradually improving the quality of rural housing, rendering it less suitable for triatomine bugs. In the shorter term however, well-organized community-based vigilance is the key to reporting reinfestations so they can be quickly dealt with. But herein lies the second major question. Reinfestations following control may represent the original vector species, or another. They may represent the original vector population (i.e., hidden survivors recovering from the control treatment) or they may be due to bugs immigrating from untreated foci. Operationally, it becomes important to distinguish between 'survivors' (indicating control failure) and 'immigrants' (indicating poor geographic coverage). Moreover, land-use changes in several regions have been associated with localized domestic invasions by little-known species of Triatominae, providing a certain imperative to identify bug populations that may present a future risk of domiciliation. Analysis of bug population genetics seems to be providing the means to address these questions, as well as providing a cohesive framework for collaborative research throughout Latin America. Presentations in the meeting discussed the use of morphology eggshell architecture, head and wing morphometrics, sensilla patterns, isoenzymes, cuticular hydrocarbons, cytogenetics and chromosome analysis, and random amplified polimorfic DNA test (RAPD), as well as the results of complex interfertility studies. Each technique offers deferent practical advantages, but each seems to point in a similar direction. Triatominae seem to represent a genetically heterogeneous group, with generalist species displaying higher genetic variation, and specialist species showing relatively reduced population variability - probable associated with a series of genetic bottlenecks followed by modest selection. Populations tend to show strong geographic structuring, which can be detected, for example, by consistent morphometric, cytogenetic and isoenzyme markers and already there are several outstanding results of operational importance. In Colombia for example, calculated gene flow between silvatic and domestic populations of R. prolixus seems remarkable low giving optimism that control of domestic populations may not abe heavily influenced by silvatic invasions. Similarly in Bolivia, there is increasing evidence of isolation between silvatic and domestic T. infestans again giving optimism for unimpeded elimination of domestic populations of this vector. And most importantly, isoenzymes and morphometrics have already provided a means to identify the source populations of reinfestant T. infestans following control interventions. The workshop ended with a feeling of accomplishment, and of optimism, having identified clear operational and technical goals as well as administrative steps toward their implementation. There was a feeling of brotherhood too, of Latin Americans building on collective experience to resolve a uniquely Latin American problem. A summary of the meeting has been presented to the Minister of Health of Ecuador, and the full report will be published early in 1996.
References
Source: Division of Disease Prevention and Control, Communicable Diseases Program, HCP/HCT, PAHO.
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