MEM INST OSWALDO CRUZ, RIO DE JANEIRO, 91(3) May/Jun 1996
PAGES: 271-272 DOI: Technical notes
Current Spread of Triatoma infestans at the Expense of Triatoma sordida in Bolivia

François Noireau +, Frédérique BrenièreI, Lucyla CardozoII, Marie France BossenoI, Fernando Vargas, Carlos PeredoII, Marlon MedinacelliII

Institut Français de Recherche Scientifique pour le Développement en Coopération (ORSTOM), Casilla Postal 9214, La Paz, Bolivia
IUMR CNRS/ORSTOM, BP 5045, 34032 Montpellier Cedex, France
IICentro Nacional de Enfermedades Tropicales (CENETROP), Casilla Postal 2974, Santa Cruz, Bolivia

RESEARCH NOTE

Triatoma infestans is the most widespread synanthropic species of Triatominae and the most important vector of Chagas disease in South America. The progressive geographical expansion of this species from its silvatic focus of origin in the Cochabamba valley of Bolivia (JP Dujardin et al. 1987 J Med Entomol 24: 40-45) was probably associated with human migration into the continent (CJ Schofield 1988 Biosystematics of haematophagous insects, p. 285-312. MW Service Oxford). Triatoma sordida, a species associated with silvatic and peridomestic habitats, is currently found in houses in some rural areas of Bolivia, particularly in La Paz and Santa Cruz departments (E De Muynck 1977 Bol Inform CENETROP 3: 41-51, F Noireau et al. 1995 J Med Entomol 32: 594-598). The analysis of the present distribution of T. sordida in Bolivia, in comparison to 20 years ago data, gives a clear indication of the reduction of the area within which this species is found in houses and its substitution by T. infestans, the most important vector of Chagas disease in this country.

Our first study area was the Yungas in the department of La Paz. The Yungas are steep-sided valleys covered with subtropical forest in the foothills of the Eastern Andean Cordillera. Both T. infestans and T. sordida were reported more than 50 years ago in several main villages such as Chulumani, Coripata and Coroico; T. sordida was the only triatomine species collected in many houses in Chulumani and Villa La Fuente (S Mazza 1942 Prensa Med Arg 29: 1-15). In spite of the competitive superiority of T. infestans over T. sordida in houses, probably due to its faster development coupled with the greater accessibility to blood offered by man and domestic animals, the coexistence of both species in this region was likely to have been due to the relative human stability in this period (E Juarez & EP de C Silva 1982 Rev Saúde Públ 16 (supl.): 1-36, Schofield 1988 loc. cit., ME Bar et al. 1994 Rev Saúde Públ 28: 59-68). An entomological survey performed 50 years later, in 1992, in the small towns of Coroico, Coripata and Chulumani, as well as villages situated in the valleys surrounding them, pointed out the dispersal of T. infestans. 795 domestic triatomines were collected and taxonomically identified by morphology (all specimens) and genetics (confirmation of the taxonomic status for nymphal instars by isozyme analysis). All specimens belonged to T. infestans species except one 4th-instar nymph of the T. sordida species. The spread of T. infestans and the associated virtual disappearance of T. sordida from houses had very likely occurred in association with the intense human migratory flow from the endemic regions of Bolivia to the Yungas during the 1950-1976 period (in the Yungas, the yearly increasing rate of population was 3.62% compared to 2.05% for Bolivia as a whole; Censo Nacional de Población y vivienda, Bolivia, 1992).

The Velasco province, located in the north of Santa Cruz department, was the second study area. This province, which forms part of the Chiquitania region, is made up of a mosaic of woodlands and wooded savannas. Until recently T. sordida was the only triatomine species reported to colonize houses in this province (De Muynck 1977 loc. cit., J Salcedo et al. 1980 Bol Inform CENETROP 6: 2-10). An entomological survey was performed in 1994 following the same process of taxonomic identification as described above. It pointed out the rapid penetration of T. infestans to this region during the last 15 years: among 26 investigated localities in the Velasco Province, T. sordida was found as the only species infesting houses in 18 localities (69%), while T. infestans was collected in 8 other localities associated or not with T. sordida. The few localities infested by T. infestans are situated in areas which have recently experienced substantial human migrations (during the 1976-1992 period, the yearly increasing rate of Velasco population was 3.50% compared to 2.11% for Bolivia as a whole; Censo Nacional de Población y vivienda, Bolivia, 1992). On the contrary, isolated villages located off the lines of communication and consequently less affected by migration are still free of T. infestans. Nevertheless, such isolated villages may be considered as being at risk to the short-term spread of T. infestans. Such is the case in Tacoigo, located in the middle of the area considered as free of T. infestans, one specimen of this last species was recently collected in association with 65 T. sordida. These surveys show that in Bolivia as elsewhere T. infestans, when introduced into new areas, quickly displaces T. sordida from human dwellings.

Acknowledgments: to J Johnson for reviewing the manuscript; to N Yaksic, J Telleria, J Ordoñez and S Garcia H for technical assistance.

+Corresponding author. Fax: 591-2-39.1854
Received 21 September 1995
Accepted 29 January 1996

Financial support from the UNDP/World Bank/WHO Special Programme for Research and Training in Tropical Diseases (TDR).

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