Global plan for insecticide resistance management in malaria vectors

PART 3TECHNICALRECOMMENDATIONSFOR COUNTRIESIntroduction to the recommendations.The following recommendations are initial working proposals forIRM strategies. They are valid as of May 2012 and will be revisedas more evidence and research results become available.Updated versions of these recommendations are available athttp://www.who.int/malaria.IRM strategies for specific countries should be planned indiscussion with technical experts, either in-country or with externalsupport. WHO is responsible for making policy recommendationsand for giving advice on the implementation of malaria preventionand control activities by countries and their partners, followingnational policies.In order to produce the recommendations outlined below, anumber of different insecticide resistance scenarios wereconceived. The interpretation of these, together with the designof appropriate responses, was used to develop strategies that arelikely to be applicable to current conditions in different situations.In some cases, countries may request additional detailed advicethat is more closely adapted to local situations. WHO, together withpartners, will endeavour to provide such technical support and,whenever necessary, will convene experts to assist in this.In most cases, the methods of resistance management thatwould, in the long run, be most appropriate and effective for agiven scenario, will not be adopted immediately because the newinsecticide products that would be necessary to do this are notyet available. In such situations, countries may be forced to usethose methods that are available, regardless of their limitations.The recommendations that follow are specific to both of thesesituations - detailing both interim methods of IRM for immediateimplementation, and preferred methods that can be implementedin the future once new tools become available.Thresholds for susceptibility and resistanceWHO has recently updated the Test procedures for monitoringsusceptibility of mosquitoes to insecticides (42). Please refer tothese guidelines for more details (referred to as the ‘WHO testprocedures’ in the following sections). The document is availableat http://www.who.int/malaria.WHO defines susceptibility and resistance to insecticides on thebasis of testing for the mortality rates of vectors exposed to adiscriminating dose:- Susceptibility: An observation of more than or equal to 98%mortality rate among vectors tested for resistance with WHOmethods provides evidence of clear susceptibility.- Possible resistance: An initial observation of less than 98%vector mortality in bioassays conducted with WHO methodsindicates possible resistance. Once this observation hasbeen made, further testing is required to confirm resistance.Additional tests should be conducted to determine whetherthe vector mortality rate is consistently lower than 98% andto understand the extent of resistance. The mosquito samplesize should be extended; the number of bioassays increasedand the geographical scope widened. On the basis of theseadditional tests, resistance will either be confirmed or not, andits geographical distribution will be clarified. Those additionaltests have to be conducted soon after resistance is suspected.Once resistance has been confirmed based on bioassays, themechanisms of resistance should ideally be investigated by biochemicaland molecular testing. The results of additional tests tounderstand the underlying mechanisms of resistance providesvaluable information to further guide the choice of tools for IRM.Nevertheless, action should not be delayed in situations wheresuch testing is not possible. Countries that are not in a positionto conduct biochemical and molecular tests in the short term willhave to make decisions on the basis of bio-assays only.79