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 their randomised controlled trial on bmj.com, 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...
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- WHO. Malaria. Fact sheet 94. 2007. www.who.int/mediacentre/factsheets/fs094/en/
- Hill N, Lenglet A, Arnéz AM, Carneiro I. Plant based insect repellent and insecticide treated bed nets to protect against malaria in areas of early evening biting vectors: double blind randomised placebo controlled clinical trial in the Bolivian Amazon. BMJ 2007 doi: 10.1136/bmj.39356.574641.55 [Abstract/Free Full Text]
- Grieco JP, Achee NL, Chareonviriyaphap T, Suwonkerd W, Chauhan K, Sardelis MR, et al. A new classification system for the actions of IRS chemicals traditionally used for malaria control. PLoS ONE 2007;2:e716.[CrossRef]
- Grieco JP, Achee NL, Andre RG, Roberts DR. A comparison study of house entering and exiting behavior of Anopheles vestitipennis using experimental huts sprayed with DDT or deltamethrin in the southern district of Toledo, Belize, C.A. J Vector Ecol 2000;25:62-73.[ISI] [Medline]
- Pan American Health Organization. Status of malaria programs in the Americas. XLII report. Washington, DC: PAHO, 1994.
- Roberts DR, Alecrim WD. Behavioral responses of Anopheles darlingi to DDT-sprayed house walls in Amazonia. Bull Panam Health Organ 1991;25:210-7.
- Roberts DR, Alecrim WD, Hshieh P, Grieco JP, Bangs M, Andre RG, et al. A probability model of vector behavior: Effects of DDT repellency, irritancy, and toxicity in malaria control. J Vector Ecol 2000;25:48-61.[ISI] [Medline]