Efficacy of mosquito nets treated with permethrin in Suriname.

In the rain-forest of Suriname, where malaria is endemic, 95% of the Maroons (who call themselves bush-negroes) and all Amerindians use mosquito nets made of cotton cloth or, less frequently, nylon or cotton gauze over their hammocks or beds. Bush-negroes usually wash their nets weekly; Amerindians wash nets at 1-4 month intervals. Females of the principal local malaria vector, Anopheles darlingi Root, were seen blood-feeding through cotton cloth netting (at 22.30-23.30 hours) on a person sleeping in a hammock; others fed successfully after the net was opened in the morning. Cotton cloth impregnated with permethrin at a rate of 0.5 g/m2 killed all An. darlingi females exposed for 2 min, but after the material had been washed twice in soapy water the bioassay mortality fell to only 21.4%. Exit traps on a hut with a single sleeper protected by a permethrin-impregnated net yielded 185 An. darlingi females (12% blood-fed) in 74 nights, compared with 276 females (19% blood-fed) from another hut with a sleeper using an untreated net on the same nights (P less than 0.001). No An. darlingi females remained resting alive indoors in these huts during the daytime, and very few were found dead on the floor in the mornings (one treated, seven untreated). The 24 h mortality rate for those collected in exit traps was 58.4% for the test hut and 27.1% for the control hut (P less than 0.001). Bioassays of permethrin-treated cotton cloth using laboratory-reared sugar-fed Culex quinquefasciatus Say females showed that sprayed nets were less effective than nets impregnated by soaking (at equivalent dosages of 0.16-1.34 g/m2 measured by chemical assay) and confirmed that washing causes severe decline in insecticidal activity. The feasibility of local mass treatment of mosquito nets is discussed.

Behavioral responses of Anopheles darlingi in Suriname to DDT residues on house walls.

A study was conducted in Suriname on the effects of DDT residual house spray on the main vector of malaria, Anopheles darlingi. Results obtained with an experimental hut sprayed with DDT wettable powder (2 g/m2 AI) were compared with results obtained with an unsprayed control hut. In the sprayed hut, entry rates were reduced by 32% and feeding success by 43.6%. The 24-h mortality of mosquitoes which entered the sprayed hut was 95% (range 85-100%) over the 10-month study period. After furnishing the sprayed hut according to local custom, no reduction in mortality was observed despite an abundance of unsprayed resting surfaces. Bioassays showed that the DDT deposits remained effective for at least 14 months. With excito-repellency test boxes an irritant effect caused by DDT on recently fed An. darlingi females was shown. With a choice-box experiment no preference for unsprayed over sprayed resting places could be demonstrated.