The way forward for vector control.

P revention, diagnosis, and treatment of vector-borne diseases such as malaria, leishmaniasis, and dengue fever is a complex problem. Major reductions in transmission require multiple different interventions, including both disease treatment and vector control. This is particularly crucial in the high-transmission areas where conditions are optimal, or when epidemics are triggered. However, insect-control interventions are becoming less effective; development, evaluation, and introduction of new interventions are slow; and there is limited understanding of just how important these interventions are. To sustain and optimize disease control efforts, it will be necessary to develop more informative models that can inform the timely introduction of new control interventions. Recent impact-modeling studies have highlighted the importance of vector control. Concerted efforts to reduce malaria in Africa in the past two decades have benefitted from better diagnosis, new combination drug therapy, and large increases in the use of bed nets treated with pyrethroid insecticides and the use of pyrethroid-based indoor residual spraying (IRS). Using data on the timing and scale of the changes, Bhatt et al. were able to estimate the relative contributions of the different interventions. To the surprise of many, vector control accounted for 81% of the gains (1). Similarly, Le Rutte et al. recently predicted that increasing IRS coverage from 60 to 80% of all households would have a much greater impact on reducing the time taken to eliminate visceral leishmaniasis than halving the time taken to start treatment from 40 to 20 days, although the greatest benefit would come from combining the two interventions (2). It follows that insect control is crucial to reduce human suffering. However, heavy reliance on any single intervention— whether an insecticide or a drug—will lead to the selection of resistance. The international community has adopted the use of malaria combination drug therapy to slow the selection of drug resistance, but reliance on pyrethroid insecticides alone has exposed them to intense selection. Pyrethroid resistance, almost nonexistent in the major malaria, Zika, and dengue vectors in the 1990s, is now the norm, and the situation is rapidly degenerating. Pyrethroid resistance is already affecting the IRS efficacy and malaria prevalence in several countries. For example, resurgence of malaria in South Africa coincided with the re-emergence of Anopheles funestus after it became resistant to pyrethroids (3). A World Health Organization (WHO)–led multicountry study on the impact of resistance confirmed that pyrethroid IRS was failing in Sudan (4). Resistance of A. coluzzii to pyrethroids also triggered an increase in malaria in Bioko Island, Equatorial Guinea in 2015 (5). Efforts to sustain and further reduce child mortality due to malaria will be compromised unless pesticide resistance is addressed (6).