When you start catching blood-seeking mosquitoes predominantly outdoors, and when you start seeing a non-seasonal change in species composition and behavior traits of mosquitoes, you should know that indoor insecticidal interventions are having an effect.
Bednets are believed to work really well. But how do we actually know that? I have had some interesting arguments recently on how scientists should determine if malaria vector interventions are successful. Here is my take:
Experimental hut trials can provide data on intermediary indicators such as: reduction in the number of mosquitoes that successfully bite net users, reduction in numbers of mosquitoes that enter households and the number of mosquitoes that are killed inside those houses. Though it is difficult to directly link these indicators to community level benefits it is logical to assume that preventing mosquito bites, or house entry by mosquitoes would lead to a reduction in malaria. In fact, mathematical models relying on these experimental data in simulated malaria transmission scenarios generate results representative of many field observations.
As a mosquito biologist however, I believe that another way of knowing whether interventions like bednets have worked, is to examine the residual mosquito population in an area. My colleagues and I, as well us a number of vector biologists working in Africa have confirmed that in many villages today Anopheles gambiae s.s., formerly the most prominent malaria vector in the continent has (nearly) disappeared. Today the predominant vector is An. arabiensis in many of these locations, or An. funestus. Moreover, these residual mosquito populations seem to bite people predominantly during those times when the people are unlikely to be indoors or to be using their nets. In other words: you know that bednets have worked when most of the residual mosquitoes are those that actively avoid feeding indoors, when the mosquitoes increasingly feed outdoors and when the species composition shifts towards those can survive on non-human blood sources outdoors.
Mosquitoes that feed outside human dwellings continue to contribute significantly to the transmission of diseases such as malaria, lymphatic filariasis and viral infections. To achieve existing and future goals of eliminating mosquito-borne diseases, new tools that can be used against the outdoor biting mosquitoes, are required to complement existing indoor interventions. Hence many groups including our research teams at the Ifakara Health Institute are emphasizing the need for integrated outdoor mosquito control.