Last week, the New York Times reported on recent findings that the combination of one inexpensive antibiotic pill each day and sleeping under an insecticide-treated mosquito net can reduce the incidence of malaria by 97 percent in HIV-positive children in Uganda. If the results hold up to further analysis and critical review, this study can be understood as further support for the proposition that a combination of malaria interventions can do substantially better against malaria than any single intervention by itself. A non-experimental study that I and co-authors performed on data from the Solomon Islands similarly showed that mosquito bednets worked much better in combination with indoor DDT spraying and other interventions than by themselves.
It's not surprising that bednets need help in protecting children from malaria. They protect children against (most) bites while the child is under the net, but anyone who has tried a bednet has had the experience of being bitten through the net if it has any holes or even if you just roll too closely against it in your sleep. So even under the net, protection is not perfect. Furthermore, children will always be outside of their bednets and exposed to mosquitoes for a part of the evening hours when biting is most frequent. The DDT spraying in the Solomon Islands and the cotrimoxazole in Uganda both serve to prevent malaria infection from the bites that the child will get regardless of bednet use.
Like all chemical interventions, both cotrimoxazole and insecticide can stimulate the appearance and speed the spread of resistant forms of disease agents. The NY Times article mentions the possibility that increased use of cotrimoxazole will might lead to cotrimoxazole-resistant strains of pneumonia and the other illnesses against which it protects. Some of these illnesses infect people without HIV as well as those with it. Thus, heavy use of cotrimoxazole to prevent malaria must be balanced against the possible loss of the efficacy of this highly beneficial and inexpensive drug. Similarly DDT and other insecticides engender the development of insecticide-resistant varieties of mosquitoes. (DDT has additional harmful effects on the environment, but those are minimized when the spraying is restricted to the interiors of houses, close to the floor.)
It's not clear whether the combination of bednets plus insecticide spraying was assessed as an alternative to bednets plus cotrimoxazole. In a separate study on the resistance of the malaria parasite to anti-malarials, Laxmaman Ramanayan, David Smith and I suggested that one way to slow the development of resistance would be to alternate between two interventions over time in the same local area. For example, the public health authority could combine the bednets with cotrimoxazole for a year or two and then, as soon as resistance begins to appear, drop the cotrimoxazole and replace it with insecticide spraying. When the mosquitoes start to become resistant, the public health authority can investigate whether the cotrimoxazole-resistant microbes have been supplanted by the re-emergent, wild, non-resistant strains. If that is so, the government can again switch away from bednets plus insecticide spraying back to bednets plus preventive cotrimoxazole, at least until the reappearance of cotrimoxazole resistance.
Such a strategy is not a perfect solution to resistance. Once drug-resistance has appeared in a population, it is likely to remain, lurking in the genetic assets of the population, ready to re-emerge rather quickly in response to a renewal of selective pressure from the drug that encouraged it in the first place. However, in the endless arms race between man and disease agent, switching can be a useful tactic for keeping the diseases off-balance until new drugs or vaccines can be developed and distributed.
CGD blog posts reflect the views of the authors, drawing on prior research and experience in their areas of expertise.
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