Malaria World this week has a review article by Irish et al published in Malaria Journal, ‘A review of selective indoor residual spraying for malaria control’. Indoor residual spraying (IRS) is described by the authors as one of the most effective malaria control tools. However, its application has become limited to specific contexts due to the increased costs of IRS products and implementation programmes. They review articles on selective spraying targeted to particular areas/surfaces of dwellings, which has been proposed to maintain the malaria control and resistance-management benefits of IRS while decreasing the costs of the intervention.
The basis for the technique is to put insecticides on places within dwellings where mosquitos are likely to rest. When the mosquito lands on the contaminated place it will be poisoned by the insecticide. The article examines if the technique could be more efficient if only the areas, either high or low on the walls or furniture where mosquitos are likely to land are sprayed. It includes references to reviews about where different species of mosquito are likely to land, but most are European studies. The entomological references are similar to the studies carried out by Battista Grassi of the four Anopheles species in Italy that I translated in Studies of a zoologist about malaria.
The studies of this to date for the major African malaria vectors, Anopheles gambiae and Anopheles funestus, have contradictory findings in the literature. In some studies they were mostly on the lower part of the wall and in others on the ceiling. Some of the reported studies compared spraying of DDT at different levels on internal walls in countries including Ghana, Taiwan, Indonesia and the Philippines.
The Taiwan research, a WHO article by Pletsch and Demos from 1954 compared partial and full spraying and reported that Malaria rates dropped from 20% to <1% in Chi san by spraying walls, roofs, ceilings, and undersides of furniture with DDT (2 g/m2). I suspect there were other factors at play in this study. The study was excluded from the 2019 Cochrane review, Indoor residual spraying for preventing malaria by Pluess et al.
Only six of 134 potential studies were included in the Cochrane review article. The Cochrane review concluded that the current evidence is insufficient to quantify properly the effect of IRS in high transmission settings, but stated it seems clear that IRS leads to health benefits. Available good quality evidence confirms that IRS works in reducing malaria in unstable malaria settings. However, no study investigated the effect of IRS on reducing child mortality. Indeed, it is not clear if the effect on any other health issue other than malaria was considered in these studies.
The belief underlying the research is that IRS played an important role in the elimination of malaria from Europe and USA in 1950s. As I discussed in my book, Malaria is spread by mosquitos?, there were many other improvements to people’s lives that could have caused this improvement in health at that time. I wonder what effect confirmation bias may have had on any of these conclusions? If mosquitos are not responsible for spreading malaria, how would spraying potent poisons inside dwellings aid overall health?