Spraying insecticides on walls and ceilings (a method known as indoor residual spraying, or IRS) is a major way of killing malaria vectors resting inside houses. To be effective, though, insecticides should be applied where the mosquitoes actually are. Last January, a team of researchers of the Ifakara Health Institute in Tanzania reported that a high proportion of mosquitoes rest on surfaces which are not normally sprayed with insecticides.

Using a battery-powered aspirator, the researchers collected mosquitoes resting on various surfaces inside different house types in rural Tanzania in the early morning, evenings and at night. The key finding was that, overall, around 40% of all mosquitoes were found on furniture, utensils, clothes or just on the floor. Since these surfaces are not tipically sprayed with insecticides, the survey raises some questions about the actual efficacy of IRS.

I asked Dr Julie-Anne Akiko Tangena, public health entomology researcher at the Liverpool School of Tropical Medicine, to comment on this study.

What do you think about these findings and their importance for IRS?

This study highlights how little we actually know about the behaviour of malaria-transmitting mosquitoes, and how essential this information is. IRS might be less effective if a large proportion of the mosquitoes do not land on the insecticide treated walls (and are subsequently not exposed to the insecticides). The study described here collected snapshots of mosquito resting behaviour at different time points, but the question of how mosquitoes behave throughout the night is not yet fully answered. Is there no contact with the wall? Or is contact limited? And what is the minimum time the mosquito needs to be in contact with the wall for the insecticide to be effective? To answer these questions more behavioural studies are necessary. Unfortunately, collecting uninterrupted mosquito behaviour in the field is very challenging (if not impossible) with current methods. This study is clearly opening the door to a plethora of additional questions that need to be answered before IRS use is limited or its methods adjusted.

What is the future of IRS, particularly with the issue of insecticide resistance and the deployment of genetically-modified mosquitoes?

I believe IRS will remain an important part of malaria control for the foreseeable future. Contrary to insecticide treated bednets, which can only be treated with one insecticide class (the pyrethroids), for IRS at least five different classes of insecticides can be used. This diverse collection of insecticides can help control and prevent further development of insecticide resistance. However, IRS is not a silver bullet and it needs to be combined with other control measures such as insecticide treated nets, house screening, control of mosquito breeding sites and pro-active diagnosis and treatment of malaria cases. Further research into novel strategies that block malaria transmission, such as symbiotic microbes like the recently described Microsporidia MB or Wolbachia (which has shown success for other mosquito transmitted diseases), is also of great importance. The deployment of genetically modified mosquitoes is still far from being implemented in the field, but, if successful, it could change the outlook we have on malaria control and prevention.

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Msugupakulya, B.J., Kaindoa, E.W., Ngowo, H.S. et al. Preferred resting surfaces of dominant malaria vectors inside different house types in rural south-eastern Tanzania. Malaria Journal 19, 22 (2020). DOI: 10.1186/s12936-020-3108-0.