Tuesday, May 12, 2009

Impact of insecticide-treated bed nets on infant mortality

I've done a bit of surveys on the impact of insecticide-treated bed nets (ITNs) on infant mortality. For the uninitiated, ITNs are known to be effective for preventing child deaths due to malaria infection. ITNs do not just protect you from mosquito bites that can transmit malaria parasite into your body. They also kill mosquitos that touch the ITNs as the bed net fabric contains insecticide. As malaria is transmitted only through mosquitos, ITNs reduce the risk of malaria infection in these two ways.

But how much do they reduce infant mortality in a village in Africa?

To identify randomized control trials of high quality on this topic, I look at the Cochrane Review (Lengeler (2004)), which identifies the list of such trials with no bias in the estimates. There are five studies looking at the impact on child mortality, and Lengeler (2004) concludes that ITNs reduce child mortality by 5.5 deaths per 1,000 children.

However, I'm interested in infant mortality (the death rate among those aged under 12 months old), not in child mortality (the death rate for children under the age of 5 years). Also, it is not clear to me whether these estimates refer to the intention-to-treat effect (the impact of distributing ITNs) or the effect of the actual use of ITNs. So I look at the original studies.

It turns out that one study looks at insecticide-treated curtains. Another looks at the impact of treating bed nets with insecticide (because the study is conducted in The Gambia, where the use of bed nets is common). The third study does not report infant mortality. So only two studies actually evaluate the impact on infant mortality. And the intervention they evaluate is actually the distribution of ITNs to a community, not the actual use of ITNs by villagers (though both studies find that the usage rate of distributed ITNs is rather high).

Binka et al. (1996) report that, in Kassena-Nankana district in Ghana, before distributing ITNs, the mortality among children 6 to 11 months old is 49.7 per 1,000 for treated villages while it is 55.1 per 1,000 for control villages. After the intervention, 73.2 for the treated and 90.3 for the control. The difference-in-difference estimate yields a reduction by 11.7 per 1,000. In addition, the authors report that the mortality for children aged 1 to 5 months is comparable between the treated and the control before the intervention (they don't report exact numbers) and that it becomes 77.4 for the treated and 100.7 for the control, implying that the impact is a reduction of 23.3 deaths per 1,000. Combining these figures, the impact on infant mortality is 35 deaths per 1,000, much higher than the effect for all the children under the age of 5 years concluded by Lengeler (2004).

The other study, Phillips-Howard et al. (2003), reports a reduction of infant mortality by 31 per 1,000 due to the distribution of ITNs in Asembo and Gem areas in western Kenya, which is surprisingly similar to what Binka et al. (1996) find. This similarity may be explained by the fact that malaria is endemic (ie. there is risk of infection throughout the year) in both study sites.

By the way, Hawley et al. (2003), another piece of paper by the same research team as Phillips-Howard et al. (2003), reports the reduction of child mortality by similar magnitude in villages without ITNs but near those villages with ITNs distributed. This finding suggests that the main mechanism through which ITNs reduce child mortality is not the protection of children (and pregnant mothers, as malaria infection during pregnancy can cause the malfunctioning of placenta and thus reduce the birth weight of newborns, which increases the risk of infant death) by bed nets but the eradication of mosquitos by insecticide.

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