Stalled malaria control – root causes and possible remedies

Abstract Background Malaria control has been stalled for some years in many African countries. We suggest reasons for the stalling, and ways of remedying the situation. Methods We analyse malaria data from Kenya and Tanzania using mathematical analysis and a deterministic model for mosquito and malaria population dynamics. The model was produced in Microsoft Excel and is usable by persons who are neither mathematicians nor specialised modellers. Results In Kenya, there was no significant decline in malaria incidence during the last decade, despite 50-80% of the human population owning and using insecticide-treated bed-nets (ITN). Similar situations exist in Tanzania and Uganda. There were only limited declines in malaria incidence in Kenyan counties where indoor spraying of residual insecticides covered about 90% of the dwellings. This is especially surprising since, in the earlier decade, a rapid drop in malaria incidence followed much less intense control operations. While there have been well documented increases in resistance of anopheline mosquitoes to various pyrethroids, these are not sufficient to explain the stalled control. Instead, we suggest that this is largely due to a change in the species composition and behaviour of the vector populations, consequent on the widespread use of ITN. Quantitative support for this view is offered by our mathematical analyses and modelling of published data on changes in malaria incidence and levels of access to, and use of, ITN. The modelling also suggests that a resumed decline in the incidence of malaria might best be achieved by increasing ITN coverage as close as possible to 100% and supplementing this effort with limited application of control measures that kill mosquitoes attempting to feed off non-humans. Particular attention deserves to be given to the insecticide treatment of cattle hosts and the refinement of artificial baits for outdoor deployment. Conclusions Current levels of indoor residual spraying (IRS) and ITN will not result in any significant improvement. If, however, ITN ownership and effective use can be increased closer to 100%, modest levels of additional control outdoors should result in substantial reductions in incidence. Treatment of adult mosquitoes visiting livestock is a promising option.