|Home | About | Journals | Submit | Contact Us | Français|
Policymakers should remember that indoor residual spraying is highly effective
About 40% of the world's population, most of whom live in the poorest countries, are at risk from malaria. In Africa alone, malaria kills nearly a million children each year.1 Although we have the tools to fight malaria, such as insecticides for indoor residual spraying, environmentalist campaigns and some ill conceived decisions on public health policy have limited their use.
A renewed effort is under way to control malaria in sub-Saharan Africa. While heartening, the lead agencies have neglected to rebuild the technical expertise necessary to run effective vertical malaria control programmes. Still cautious of DDT (dichlorodiphenyltrichloroethane) and indoor residual spraying, such programmes have focused on the distribution of bed nets impregnated with insecticides. In this week's BMJ, Hill and colleagues assess the effect of combining an insect repellent with insecticide treated bed nets on Plasmodium falciparum or P vivax malaria in Bolivia.2 The trial found that people who used treated nets and repellent had an 80% reduction in P vivax episodes compared with those who used treated nets alone (incidence rate ratio 0.2, 95% confidence interval 0.11 to 0.38). The number of cases of P falciparum during the study was small and, after adjustment for age, a protective effect of 82% was seen, although this was not significant (0.18, 0.02 to 1.4).
The authors describe the many barriers that mosquitoes must overcome to bite and transmit malaria indoors. The first barrier, the house wall, deters most mosquitoes—the more tightly enclosed the house, the more effective this barrier is. If the house wall is sprayed with a spatial repellent like DDT, then it becomes an even better barrier. However, Hill and colleagues' study focused on two other barriers. The first was lemon eucalyptus—a topical insect repellent—which was applied to the skin in the early evening to deter insects from biting. Lemon eucalyptus is a contact irritant, which agitates mosquitoes when they come into physical contact with the chemical residue (J Grieco and N Achee, personal communication, 2007). The second barrier comprised bed nets treated with a pyrethroid insecticide. The first and dominant action of this insecticide is contact irritancy. Mosquitoes become agitated when they land on the netting.3 4 They might then leave the house without biting or bite an unprotected person in the house before leaving. In addition, the insecticide kills mosquitoes that have more prolonged contact with netting.
However, despite the strengths in the theory and design of the study, the results show that using impregnated bed nets alone did not adequately reduce rates of malaria. The annual parasite index (number of cases of malaria per 1000 people each year) is a standard measure of the extent of malarial infection in human populations. If the numbers of cases of P vivax for the intervention and control groups are converted to annual parasite indexes, they are unacceptably high—more than 21 for the combined intervention group and more than 32 for the group using bed nets alone. Even these high values are underestimates because no data were presented for infections in children under 10 years. In addition, only the first infection with vivax malaria was included in the analysis, and subsequent infections were not analysed.
To illustrate how high the annual parasite indexes are in the study, when DDT was used for indoor residual spraying, malaria endemic areas of Bolivia typically had values below one. Index values increased only during years of declining use of DDT.5 This remarkable effectiveness was attributable mostly to the spatial repellent action of DDT on house walls.6 7 So, although Hill and colleagues' trial looks at the long neglected use of repellents for controlling malaria, indoor residual spraying with DDT is a much more effective way to control this disease. Unfortunately, spray programmes have gradually been eliminated. As a result, high rates of malaria have returned to Bolivia, as the Hill study shows.
The free distribution of insecticide treated bed nets halved child deaths in Kenya.8 This encouraging result led the World Health Organization to endorse the free distribution of treated bed nets. People and organisations who promote treated bed nets often suggest that indoor residual spraying is not sustainable because of the need for infrastructure, but even a programme that promotes the distribution of free bed nets needs infrastructure. Bed nets have to be procured and distributed, people need to be told how to use them properly, user compliance needs to be monitored, and disease surveillance is required. In addition, the number of child deaths, not the amount of disease within populations, is used to assess the success of treated bed nets. As suggested by Hill and colleagues' study, evaluations of bed nets that rely on amount of disease show far less efficacy than evaluations that use child deaths as an end point. Hill and colleagues did not record child deaths, and their results show that treated bed nets do not adequately control malaria. This contrasts sharply with the high and continuous levels of control achieved in Bolivia when house walls were sprayed with DDT.
All available tools are needed to control malaria. However, those who promote treated bed nets should stop using the need for infrastructure as an argument against the use of indoor residual spraying, because infrastructure is needed for bed nets too. Furthermore, child deaths and disease rates should both be used when assessing the effectiveness of control programmes.
The Bill and Melinda Gates Foundation stands alone today in investing in research, through the innovative vector control consortium, for new chemical tools to control malaria. But much more is needed and public funds should be made available for research to develop new spatial and topical repellents, contact irritants, and innovative modes of toxic action.
The trial by Hill and colleagues raises intriguing questions, such as can high levels of control be achieved through the spatial repellent activity or contact irritant activity of non-toxic chemicals? And, will non-toxic chemicals that modify behaviour select for resistance? Instead of supporting impregnated bed nets alone, public health agencies should invest more in finding the answers to these questions and increasing our arsenal of chemicals to control malaria.
This article was posted on bmj.com on 25 October 2007
Competing interests: None declared.
Provenance and peer review: Commissioned; not externally peer reviewed.