Malaria in a rural area of Sierra Leone. III. Vector ecology and disease transmission

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Abstract
Studies were undertaken on the role of Anopheles gambiae and An. funestus in the transmission of malaria in four villages in a high-rainfall, forested area in the Bo district of southern Sierra Leone. Anopheles gambiae s.s., identified chromosomally as the Forest form, was the most important vector, with a mean annual sporozoite rate, based on ELISA, of 7·4%. Anopheles funestus, which was found in considerably lower numbers, was mainly a dry season vector, with an annual sporozoite rate of 11·4%. Despite these relatively high sporozoite rates, vector populations were at a low level, with approximate mean densities of only 1·0 An. gambiae and 0·1 An. funestus resting females per house room, and average biting rates of just 1·1 and 0·1 bites/person/night by these two species, respectively. In the rainy season, biting rates peaked at 9·5 An. gambiae bites/person/night and 1·0 An. funestus bites/person/night. Annual sporozoite inoculation rates by An. gambiae and An. funestus were 0·088 and 0·007 infective bites/person/night, respectively. ELISA showed that both species were highly anthropophagic. Exit-trap collections and outdoors searches showed that An. gambiae exhibited a considerable degree of exophily. Light traps inside houses caught nine anopheline species, whereas pyrethrum spray collections in houses caught only An. gambiae, An. funestus and An. hancocki.