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Think globally: act ... area-wide

November 20, 2009 - 22:15 -- Mark Benedict

MalariaWorld Newsletter recipients would need to have their heads buried in the sand if they were not well aware of numerous threats to the current methods for reducing malaria transmission. Whether the intervention is insecticides or drugs, their sustainability is threatened by failures which - it is hoped - will not become widespread. Of greater concern for those who are responsible for implementing programs are the complexities of applying control methods in different cultures, education in different languages and achieving sufficient compliance.

Few would argue that new anti-malaria tools are not needed to solve the technical problems, but the social, economic, political and ethological problems will remain substantial, particularly for all methods that depend on uniform individual and community level implementation. When centralized health authority and financial resources are lacking, these issues are magnified. Program management issues are problematic when malaria control is the goal, but they become critical for elimination programs. The stringency and extent of control is then greater, and uniform individual behavior cannot be relied upon for success. Therefore, developing control methods that do not depend upon community or individual compliance and armies of technical expertise are likely to be most successful.

 

It is essential to consider alternative methods that minimize such inherent logistical uncertainty. Bednets, drugs and indoor residual spraying all have been demonstrated to be highly effective in reducing malaria transmission. Each of these methods requires highly intrusive and sometimes risky engagement on a person-by-person level. This means that their potential effectiveness will always be determined by the interaction between the technology and humans. This is not necessary: there must be investment in developing methods whose effectiveness is not limited by individuals. In agriculture, “area-wide pest control” methods often fit this criterion. Area-wide vector control implements methods that are usually particularly effective over large areas, but more importantly, it allows the use of control tools that become more effective when applied with consideration of spatial factors that will increase their effect: the extent of the area over which control is implemented itself becomes a new tool. While not inconsistent with community and individual methods, area-wide programs do not require personally intrusive fine-scale methods. Area-wide methods are often made effective and sustainable by initial reductions in pest populations achieved by conventional control or seasonal variation in vector abundance.

 

It is in this context that we have published a supplement to the Malaria Journal (http://malariajournal.com/supplements/8/S2) on a neglected area-wide method that is well-known against agricultural pest insects, the sterile insect technique (SIT). SIT is one of many methods that are implemented on an area-wide basis and in which the extent of area under control itself synergizes the effects of the component methods. SIT is a form of biological control that introduces reproductive sterility into wild populations of insects. Usually this is done by releasing large numbers of sexually sterile males, however other methods that reduce adult fecundity exist. SIT is a critical first step into a new era of genetic control methods against mosquitoes made possible by recombinant DNA technology, advances in which have been funded by NIH, Grand Challenges in Global Health and WHO-TDR. SIT will provide a foundation of expertise and experience in mass-rearing, release methods and female elimination that will be nearly identical when applied to sophisticated genetic methods on the horizon. One prominent SIT program at the International Atomic Energy Agency against Anopheles arabiensis builds upon their expertise against screwworm, Mediterranean and other fruit flies and tsetse.

 

Coincidentally, one of the current success stories for malaria control is the site of a successful and well-sustained elimination of tsetse by SIT on Zanzibar. Is there an opportunity there for a repeat of success? Genetic control programs are fraught with pitfalls, and many of these have been used to argue against significant investment. These worries must be weighed against the scalability of methods for aerial release, production and common management methods that can be uniformly applied in numerous programs and whose effect can be expected to depend more on a relative handful of well-understood operational variables than on the vagaries of community and personal compliance. When elimination is the goal, genetic control methods will be particularly effective. They are one of the very few methods whose effectiveness increases as target populations decrease. Moreover, all genetic methods being considered at this time are consistent with the use of vaccines, drugs and integrated vector management by IRS, ITNs, source reduction and larval control. The ability of released insects to disperse, locate and control isolated populations without regard to boundaries, roadways or property access is a two-edged sword that suggests an unwelcome invasive nature to some, but a powerful and beneficial force to others. Regardless, the first methods to be implemented must, like SIT, be self-limiting to minimize unanticipated negative effects. Quite selfishly, as one whose professional opportunities would increase by greater funding for genetic control, the existing investment in their development has been welcome, essential and inadequate. Objectively, considering the kinds of control tools that are most likely to fill the criteria for global malaria elimination, previous investment has largely overlooked a technology that offers potency that will not and cannot be achieved otherwise.

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