The epidemiology of multiple Plasmodium falciparum infections. 9. Effect of insecticide-treated bed nets on haemoglobin values, prevalence and multiplicity of infection with Plasmodium falciparum in a randomized controlled trial in Tanzania

A randomized controlled trial of insecticide-treated bed nets (ITNs) was conducted in an area of high malaria transmission in Tanzania in order to assess the effects of ITNs on infection and anaemia. One hundred and twenty-two children, aged 5 to 24 months, were randomly allocated to 2 groups, one of which received ITNs. Outcome measures were assessed in 6 consecutive months with monthly cross-sectional surveys. These measures were haemoglobin values, Plasmodium falciparum prevalence and density, and multiplicity of infection determined by polymerase chain reaction-restriction fragment length polymorphism analysis (PCR-RFLP) of the msp2 locus.There was a significant increase in mean heamoglobin values and a significant decrease of 16.4% in microscopically determined P.falciparum prevalence in children in the ITN group six months after the start of the trial. Both effects were more pronounced in younger children. However, no significant difference was observed in parasite density or multiplicity of infection among infected children. Comparison with PCR results indicated that microscopically subpatent parasitaemia was more frequently found in children in the ITN group. This, together with the observed similar multiplicity in the 2 groups, suggests that infections are maintained despite ITN use, owing to the chronicity of infections. This study shows that ITNs reduce the risk of anaemia in highly exposed young children. The virtually unchanged multiplicity of infection indicates that the potentially protective concomitant immunity is not compromised.