Spatial analysis of the impact of insecticide-treated nets (ITNs) for malaria control in Tanzania

Malaria is the primary cause of ill-health in Africa, causing an enormous health and economic burden. Insecticide-treated nets (ITNs) are a very promising preventive intervention against malaria. On the basis of their demonstrated impact in the frame of randomized controlled trials they are now implemented on a large scale in Africa. However, little is known about their impact under programme conditions. The protection they confer can be divided into a personal protection component (the individual sleeping under a net is not bitten) and a community component (the “mass effect” – in which the overall transmission is reduced and even unprotected individuals benefit from the presence of ITNs). It is likely that the health impact of ITNs will be disproportionately higher with high coverage rates because both individual and community protection will contribute. The interaction of both components cannot be predicted from available empirical data, but elucidating this interaction is of great practical importance for the strategic planning of ITN programmes. Hence, understanding the relationship between ITN coverage rates and health impact is crucial. If such a “high coverage effect” is found, national control strategies should then ensure that such high rates are achieved as soon as possible, rather than rely exclusively on market approaches to provide the nets and the insecticide.

A second important scientific question that we plan to address is the long-term impact of ITNs, as opposed to the impact seen after 1-2 years in randomized controlled trials. It has been postulated on the basis of indirect evidence that reducing malaria transmission with ITNs might delay immunity acquisition in children, and that the short-term mortality gains measured by trials of 1-2 years duration are actually offset by later mortality. We intend to provide direct long-term evidence on this question.

Our work will focus on the short- and long-term impact of differing ITN coverage rates on: (1) overall child mortality, and (2) severe malaria morbidity. This will be undertaken in an area covered by a large social marketing ITN programme since 1996 and which presently covers a total rural population of 350,000 (the KINET programme). As a result of this programme, appropriate monitoring systems are, at least in part, operational. They will be further developed and adapted to our needs. For mortality monitoring a full demographic surveillance system (DSS) has been running since 1996. This DSS allows us to register all births, deaths, in- and out-migrations in the study area (55,000 people). For mild morbidity, a health service-based case monitoring will be established, while the severe morbidity surveillance system will be hospital-based. Further, an in-depth entomological impact assessment will provide data to investigate the links between ITN coverage and health impact in more detail.

This work will also help to develop methods to measure the impact of preventive interventions in resource-poor countries, an area with numerous practical applications for health programming. The work will be entirely conducted in Tanzania, as part of a long-term partnership between the Ifakara Health Research and Development Center (IHRDC) in Tanzania and the Swiss Tropical Institute (STI) in Basel.


Contact:
Dr. Christian Lengeler
Swiss Tropical Institute
4002 Basel
Switzerland
Christian.Lengeler@unibas.ch


Dr. Med. MSc ABDULLA Salim
Ifakara Health Research and Development Center,
P.O. Box 53
Ifakara
Tanzania
ifakara.ihrdc@twiga.com