Control of disease due to perennially transmitted malaria in children of rural Sierra Leone.

The effects of the community-wide use of lambdcyhalothrin-impregnated mosquito nets and fortnightly Maloprim/Vplacebo prophylaxis, singly or in combination, were assessed in a randomised controlled trial as control measures to reduce disease due to perennially transmitted Plasmodium falciparum in children of Bo district, Southern Sierra Leone. Age-specific illness thresholds of 2000 Plasmodium falciparum parasites/ul for children younger than 24 months and a corresponding level of 5000 Plasmodium falciparum parasites for older children, together with fever, were used as case definitions of clinical malaria. Using an active case detection scheme, children were clinically screened and thick smears for parasitological diagnosis collected from those fulfilling any one or more of the set of sampling criteria. A series of cross-sectional surveys (pre-rain and immediately post rainy season) were also conducted during which, in addition to clinical and parasitological data, spleen size and haematocrit level were assessed for all children irrespective of health status. A 49% protective efficacy against cases of Plasmodium falciparum clinical malaria was demonstrated in children using the insecticide-impregnated mosquito nets. The impact of combining MaloprimR prophylaxis with use of the lambdacyhalothrin-impregnated mosquito nets resulted in a 72% protective efficacy against disease due to Plasmodium falciparum. The interventions unexpectedly demonstrated a significant impact on some of the traditional malariometric indices; reducing the average spleen rate and the geometric mean parasite density. It was found that children using the impregnated mosquito nets exhibited the largest increase in the mean haematocrit level documented during this trial (which lasted a year). These results provide additional evidences that synthetic pyrethroid-impregnated mosquito nets have the potential of serving as an alternative strategy for the control of disease due to perennially transmitted Plasmodium falciparum.