Spatial analyses of the effect of insecticide (permethrin)-treated bed nets (ITNs) on nearby households both with and without ITNs was performed in the context of a large-scale, group-randomized, controlled mortality trial in Asembo, western Kenya. Results illustrate a protective effect of ITNs on compounds lacking ITNs located within 300 meters of compounds with ITNs for child mortality, moderate anemia, high-density parasitemia, and hemoglobin levels. This community effect on nearby compounds without nets is approximately as strong as the effect observed within villages with ITNs. This implies that in areas with intense malaria transmission with high ITN coverage, the primary effect of insecticide-treated nets is via area-wide effects on the mosquito population and not, as commonly supposed, by simple imposition of a physical barrier protecting individuals from biting. The strength of the community effect depended upon the proportion of nearby compounds with treated nets. To maximize their public health impact, high coverage with treated nets is essential.

Abstract. The impact of insecticide (permethrin)-treated bed nets (ITNs) on malaria in pregnancy was studied in a rural area in western Kenya with intense perennial malaria transmission. All households in 40 of 79 villages were randomized to receive ITNs by January 1997. The ITNs were distributed in control villages two years later. Complete data on birth outcome were available on 2,754 (89.6%) of 3,072 deliveries. Women (n ‫ס‬ 780) were followed monthly throughout pregnancy in 19 of 79 villages. Among gravidae 1-4, ITNs were associated with reductions of 38% (95% confidence interval [CI] ‫ס‬ 17-54%) in the incidence of malaria parasitemia and 47% (95% CI ‫ס‬ 6-71%) in the incidence of severe malarial anemia (hemoglobin level < 8 g/dL with parasitemia) during pregnancy. At the time of delivery, mean hemoglobin levels were 0.6 g/dL (95% CI ‫ס‬ 0.01-1.2 g/dL) higher, the prevalence of placental or maternal malaria was reduced by 35% (95% CI ‫ס‬ 20-47%), and the prevalence of low birth weight was reduced by 28% (95% CI ‫ס‬ 2-47%) in gravidae 1-4 from ITN villages. No beneficial impact was observed in gravidae five or higher. In areas of intense perennial malaria transmission, permethrin-treated bed nets reduce the adverse effect of malaria during the first four pregnancies.

Information on the impact of insecticide (permethrin)-treated bed nets (ITNs) from randomized controlled trials in areas of intense perennial malaria transmission is limited. As part of a large-scale, community-based, group-randomized controlled trial of the effect of ITNs on childhood mortality in a holoendemic area in western Kenya, we conducted three cross-sectional surveys in 60 villages to assess the impact of ITNs on morbidity in 1,890 children less than three years old. Children in ITN and control villages were comparable pre-intervention, but after the introduction of ITNs, children in intervention villages were less likely to have recently experienced illness requiring treatment (protective efficacy [95% confidence intervals] ‫ס‬ 15% [1−26%]), have an enlarged spleen (32% [20−43%]), be parasitemic (19% [11−27%]), have clinical malaria (44% [6−66%]), have moderately severe anemia (hemoglobin level < 7.0 g/dL; 39% [18−54%]), or have a pruritic body rash, presumably from reduced nuisance insect bites (38% [24−50%]). Use of ITNs was also associated with significantly higher mean weight-forage Z-scores and mid-upper arm circumferences. There was no evidence, however, that ITNs reduced the risk of helminth infections, diarrhea, or upper or lower respiratory tract infections. The ITNs substantially reduced malaria-associated morbidity and improved weight gain in young children in this area of intense perennial malaria transmission.

As part of a community-based, group-randomized, controlled trial of insecticide-treated bed nets (ITNs) in an area with intense malaria transmission in western Kenya, a birth cohort (n ‫ס‬ 833) was followed monthly until the age of 24 months to determine the potential beneficial and adverse effects of reduced malaria exposure during pregnancy and infancy. Malaria transmission and morbidity were comparable pre-intervention. The ITNs reduced malaria attack rates (force of infection) in infancy by 74%, and delayed the median time-to-first parasitemia (4.5 to 10.7 months; P < 0.0001). The incidence of both clinical malaria and moderate-severe anemia (hemoglobin level <7 g/dL) were reduced by 60% (P < 0.001 for both). Protective efficacy was greatest in infants less than three months old and similar in older infants and one-year-old children. Efficacy was lowest in the dry season. Infants from ITN villages experienced better height and weight gain. In areas of intense perennial malaria transmission, ITNs substantially reduce exposure to malaria and subsequent malaria-associated morbidity in children less than 24 months old. Reduced malaria exposure during infancy did not result, with continued ITN use, in increased malaria morbidity in one-year-old children. * ITN ‫ס‬ insecticide-treated bed net; CI ‫ס‬ confidence interval; WHZ ‫ס‬ weight-for-length Z-score; HAZ ‫ס‬ length-forage Z-score; WAZ ‫ס‬ weight-forage Z-score; MAZ ‫ס‬ mid-upper-arm circumference-forage Z-score; 6-24 months of age only; NA ‫ס‬ not asssessed. † Difference in change in mean Z-score over time in ITN versus control villages and associated P value.

Randomized controlled trials in sub-Saharan Africa have shown that permethrin-treated bed nets and curtains reduce all-cause child mortality by 15−33% in areas with low or high but seasonal malaria transmission. This report describes the study site for a community-based, group-randomized, controlled trial in an area of high and year-round malaria transmission in western Kenya. We outline the development of the human and physical infrastructure required to conduct this trial and discuss some of the difficulties encountered and lessons learned in conducting it.