Methods for entomological evaluation of insecticide treated bed net trials

This thesis is divided into 3 parts. The first part reports the effect of community-wide use of bed nets treated with lambdacyhalothrin (10mg/m2), on malaria vector Anopheles gambiae in Southern Sierra Leone. In the first year of the trial, 16 villages were randomly allocated to either remain without treated nets or to receive treated nets for all the inhabitants. During the first year of the trial, the treated nets provided personal protection for their users, but had very little impact on densities of An. gambiae mosquitoes. An. gambiae parous rates were significantly reduced in all intervention villages, but malaria sporozoite rate fell in only 7 of the 8 villages with nets. In the second year of the trial, there was clear evidence for a mass effect, shown by reduction of biting, parous and sporozoite rates in the villages that had had nets for two years, compared to controls. The interpretations and significance of these results are discussed. The first part of this thesis also compares the relative sampling efficiency of two sampling methods, namely, light trap catches, and counting of blood fed mosquitoes with human bait catches, in estimating biting rates. Result showed that biting rates obtained from light trap catches (in both villages with and without treated bed nets) can replace those obtained from human bait catches. In contrast, counting of blood fed mosquitoes cannot replace human bait catches in estimating biting rates in villages without treated bed nets. The second part of the thesis describes, analyses, and discusses the spatial and temporal distribution of An. gambiae mosquitoes in two Tanzanian villages. Data for this study were collected by carrying out an intensive mosquito sampling programme, using light traps in two Tanzanian villages. Taylor’s power law showed that aggregation indices for the spatial and temporal distribution of An. gambiae mosquitoes were not significantly different. This suggests that sampling effort should be equally allocated to spatial and temporal parameters (houses and night of sampling, respectively) when estimating mosquito abundance. The results also showed that for a given amount of sampling effort, the estimates of village-level mosquito abundance are more precise when sampling is carried out in randomly selected houses on each sampling night, than when the same houses are used on each occasion. However, in the case of estimating parous rates, it does not depend on whether the sampling was carried out in the same or a random selection of houses. The implications of these findings for designing sampling routines for entomological evaluation of treated bed nets are discussed. The final part of the thesis describes the development of an immunoassay based on polyclonal antibodies for quantitative determination of pyrethroid insecticide on bed nets. This test is capable of determining in a semi-quantitative manner if the amount of permethrin, deltamethrin or lambdacyhalothrin on a piece of mosquito netting is up to the level required for effectiveness. The test can be carried out in a modestly equipped field laboratory. The use of this test and the direction for future work are discussed.