Academic journal article Bulletin of the World Health Organization

Effectiveness of Residual Spraying of Peridomestic Ecotopes with Deltamethrin and Permethrin on Triatoma Infestans in Rural Western Argentina: A District-Wide Randomized Trial

Academic journal article Bulletin of the World Health Organization

Effectiveness of Residual Spraying of Peridomestic Ecotopes with Deltamethrin and Permethrin on Triatoma Infestans in Rural Western Argentina: A District-Wide Randomized Trial

Article excerpt

Voir page 203 le resume en francais. En la pagina 204 figura un resumen en espanol.

Introduction

Triatoma infestans, the main vector of Chagas disease, has been the target of the regional elimination programme "Southern Cone Initiative" since 1991 (1). Based mostly on the residual application of pyrethroid insecticides, this programme has been much more successful in Brazil, Chile, and Uruguay, than in the semiarid Chaco extending over Argentina, Bolivia, and Paraguay. Resurgence of peridomestic T. infestans populations after spraying with pyrethroids has frequently been reported by official control programmes in Argentina (S. Blanco, unpublished data, 2000), but the reasons remain unclear. Pyrethroids have proven much more effective in human habitations (2, 3) than in peridomestic ecotopes housing domestic animals and various species of triatomine bugs (4-10).

The formulation of the insecticide affects its residual activity (2). For example, emulsifiable concentrate (EC) cyfluthrin had a greater impact on sand flies that rested on sylvatic vegetation, probably because the oil carrier allowed a greater adsorption of the insecticide to the substrate (11). In addition, we hypothesized that newer formulations of EC pyrethroids applied with back-pack power sprayers might have greater effect than wettable powder (WP) or suspension concentrate (SC) formulations applied with manual sprayers, because power sprayers might increase the penetration of insecticides into deep refuges and provide a greater initial impact (3). EC or WP deltamethrin suppressed peridomestic infestations by 77 infestans for 500 days (4) though not always (8). Our objectives were to compare the effectiveness on peridomestic populations of T. infestans of new emulsifiable formulations of deltamethrin and cis-permethrin applied with power sprayers relative to that of SC deltamethrin applied with manual sprayers (the standard treatment) or motor sprayers in a district-wide infested area. We also sought m identify factors that modify the local effectiveness of pyrethroids.

Methods and materials

Study area

The study was conducted in rural areas around Olta (30.4[degrees]S, 66.1[degrees]W), Departments General Belgrano and Chamical, Province La Rioja, Argentina (Fig. 1). The area is on a semiarid plain with xerophytic vegetation, and has been described elsewhere (12). The houses had last been sprayed with beta-cyfluthrin approximately 5-6 years before this trial. Most houses were of adobe bricks and thatched roofs, and nearly all had numerous typical peridomestic structures (range, 1-10 per house), for domestic animals (10). The area was selected because it had shown recurrent reinfestation after insecticide spraying and high peridomestic infestation rates; most houses were relatively easy to access, and local authorities were cooperative. The study area was extended to adjacent areas of Chamical to reach about 100 houses per treatment.

[FIGURE 1 OMITTED]

Study design

The trial was divided into a baseline survey, intervention, and assessment. At baseline, three teams numbered all 369 houses with a metal plaque (Fig. 1), and surveyed 362 houses between 22 April and 20 May 1999. Each team was composed of two highly experienced bug collectors from the Nanonal Vector Control Programme (NVCP) staff, one bug collector from the local control programme, and one supervisor. Supervisors explained the objectives and phases of the trial to every household, and canvassed each family head to record the number of residents, type and number of domestic animals, and use of insecticides (type and frequency of use). The number and type of domestic and peridomestic structures were sketched on a map, building materials were noted, and distances from each structure to human habitations were paced out. Two highly experienced NVCP bug collectors concurrently assessed the intensity of peridomestic infestation by T. …

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