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Spatial mosquito repellents may provide opportunities for increasing preventive coverage against malaria

November 29, 2009 - 08:31 -- Fredros Okumu

Though mosquito repellents are among the best known personal protection measures, their use outside the deployed-warfighter-communities is limited. The need for active user participation, discipline, consistent applications and their high costs make civilian applications of mosquito repellents a less appreciated option for use against malaria or other mosquito borne infections. there has also been the argument that repelling mosquitoes without killing them might divert the host seeking mosquitoes to bite persons within the household/environment who do not apply the repellents.

At the just concluded ASTMH Annual meeting in Washington DC, a case was made that spatial repellents may provide a new option for civilian use since they would enable protection of not only the users but also the prople who are within a certain radius of application but who do necessarily use the repellents. Moreover, the repellents woud not necessarily have to be applied on humans themeselves but could be strategically placed in areas so that the radius of effecrtiveness covers all susceptibles. Perhaps scientists and insustrialists could explot such compounds for public health use more actively.

Indeed past evidence suggests that some of the most effective insecticides ever used for malaria control as well as those that are currently being used, function mainly as repellents and only limitedly as toxic agents killing mosquitoes. DDT, perhaps the best known is mainly a mosquito repellent preventing at least 70 percent of An. gambiae mosquirtoes from entering peoples dwellings. Similarly, new generation pyrethroids such as Lambda cyhalothrin and Permethrin (the active ingredient of Africa's most widely used long lasting insecticide net, Olyset) are mainly repellent as well. Based on previous studies on mosquito populations including those that have high frequencies of insecticide resistance, these compounrs repell between 50 and 60 percent of all mosquitoes approaching households. Of the few mosquitoes that eventually manage to get into the huts thse compounds continue to act on them by irritating them so that they exit houses earlier than they normally would and ofcourse that that persist to go on and feed are killed by the toxic ation of the insecticide, (which some recent upublished work show to be limited anyway against some mosquito species). Thus spatial repellents can protect not only individuals but actually the entire households and perhaps neighbours too. These datasets, which have been generated from seeveral studies worldwide suggest that compounds with spatial repellence effects do have a real chance of not only preventing direct contact between humans and disease transmitting mosquitoes, but they also can starve out mosquitoes by keeping them out of reach of hosts for longer time thereby being more likely to die from other environmental conditions such as predation, high or low outdoor temps, heat as well as mere lack of food for extended periods of time.

There are no known compounds in the market that areexpressely registed are spatial repellents. However compounds such as transfluthrin and Metafluthrin as well as some plant based repellents such as Para methane 3,8, diol (PMD) and of course the insecticides commonly used for malaria control do have significant spatial repellency properties. This suggests it would not be a bad idea after all to actively bioprospect for compounds that have high spatial repellency properties against disease transmitting arthropods. The fact that such repellents would protect more than just the person near them but also the people around there, means that the problem of diverting mosquitoes to the nearesrt susceptibles will be limited and also that less material and lower costs will be requred to implement the strategies.