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Citation Information

Type Journal Article - Jama
Title Sustainability of reductions in malaria transmission and infant mortality in western Kenya with use of insecticide-treated bednets: 4 to 6 years of follow-up
Author(s)
Volume 291
Issue 21
Publication (Day/Month/Year) 2004
Page numbers 2571-2580
URL http://jama.jamanetwork.com/article.aspx?articleid=198845&resultclick=1
Abstract
Context Insecticide-treated bednets reduce malaria transmission and child morbidity and mortality in short-term trials, but this impact may not be sustainable. Previous investigators have suggested that bednet use might paradoxically increase mortality in older children through delayed acquisition of immunity to malaria.

Objectives To determine whether adherence to and public health benefits of insecticide-treated bednets can be sustained over time and whether bednet use during infancy increases all-cause mortality rates in older children in an area of intense perennial malaria transmission.

Design and Setting A community randomized controlled trial in western Kenya (phase 1: January 1997 to February 2000) followed by continued surveillance of adherence, entomologic parameters, morbidity indicators, and all-cause mortality (phase 2: April 1999 to February 2002), and extended demographic monitoring (January to December 2002).

Participants A total of 130 000 residents of 221 villages in Asembo and Gem were randomized to receive insecticide-treated bednets at the start of phase 1 (111 villages) or phase 2 (110 villages).

Main Outcome Measures Proportion of children younger than 5 years using insecticide-treated bednets, mean number of Anopheles mosquitoes per house, and all-cause mortality rates.

Results Adherence to bednet use in children younger than 5 years increased from 65.9% in phase 1 to 82.5% in phase 2 (P<.001). After 3 to 4 years of bednet use, the mean number of Anopheles mosquitoes per house in the study area was 77% lower than in a neighboring area without bednets (risk ratio, 0.23; 95% confidence interval [CI], 0.15-0.35). All-cause mortality rates in infants aged 1 to 11 months were significantly reduced in intervention villages during phase 1 (hazard ratio [HR], 0.78; 95% CI, 0.67-0.90); low rates were maintained during phase 2. Mortality rates did not differ during 2002 (after up to 6 years of bednet use) between children from former intervention and former control households born during phase 1 (HR, 1.01; 95% CI, 0.86-1.19).

Conclusions The public health benefits of insecticide-treated bednets were sustained for up to 6 years. There is no evidence that bednet use from birth increases all-cause mortality in older children in an area of intense perennial transmission of malaria.

The burden of malaria in sub-Saharan Africa remains intolerable, with more than 20% of all deaths of children younger than 5 years attributed to malaria,1 resulting in up to 11.9 deaths per 1000 children living in malaria-endemic settings per year.2 The Roll Back Malaria global partnership, founded by the World Health Organization, the United Nations Development Program, the United Nations Children's Fund, and the World Bank, aims to halve malaria mortality by 2010 through implementation of 4 key technical strategies: insecticide-treated bednets, improved case management, control of malaria in pregnancy, and early warning and containment of epidemics.3 Reductions in all-cause child mortality by an average of 17% have been demonstrated in controlled efficacy trials of insecticide-treated bednets.4 With high coverage in malaria-endemic settings, insecticide-treated bednets could save 6 to 8 child lives (1-59 months) per 1000 protected each year, resulting in significant progress toward Roll Back Malaria goals.1,5

Efforts to increase insecticide-treated bednet coverage in Africa are being made6 but questions remain concerning the long-term durability of this strategy. First, reductions in all-cause child mortality rates due to short-term bednet use may not be sustainable, because initial reductions in mortality occur as a result of the combination of reduced malaria transmission and preexisting partial immunity developed under the formerly higher levels of transmission; after transmission declines and immunity wanes, mortality rates may increase.7 Second, pyrethroid resistance in Anopheles mosquitoes might compromise the long-term effectiveness of insecticide-treated bednets in killing mosquitoes.8 Third, it is not clear whether a population of insecticide-treated bednet users will maintain proper use and deployment of bednets (adherence) over long periods, particularly when nets are distributed free of charge.9

Additionally, some investigators have suggested that in areas of intense perennial malaria transmission, a partial reduction in transmission due to control measures such as insecticide-treated bednets might paradoxically increase child mortality through delayed acquisition of immunity to malaria.10- 12 In areas of high perennial transmission, first malaria infections usually occur during early infancy, when maternal antibodies and physiological factors provide moderate protection from life-threatening illness.13,14 Children who survive malaria in infancy usually gain sufficient malarial immunity to reduce the severity of later infections.15 Reducing malaria transmission through use of bednets may shift the age of first infection out of this period of protection16 and cause a rebound in mortality at older ages. Studies17,18 from areas with highly seasonal transmission have not found increases in mortality in older children as a result of protection by bednets or insecticide-treated curtains during early life.

A community randomized controlled trial of insecticide-treated bednets in an area of intense year-round malaria transmission in western Kenya provided the opportunity to address questions related to the sustainability of the impact of bednets.19 During the 2 years of the trial, insecticide-treated bednet use was found to reduce malaria transmission by 90 ; all-cause mortality of children aged 28 days to 11 months was reduced by 23%.5 We report herein the results of an extended evaluation of mortality and morbidity that followed the conclusion of that trial. We assess changes in adherence to insecticide-treated bednets over time, the impact on mosquito vectors and malaria transmission rates, the effect on morbidity indicators in infants, and the cumulative impact on all-cause child mortality after up to 6 years of use.

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