Methods in Anopheles Research - MR4

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Chapter 5 : Insecticide Resistance5.1 Insecticide Resistance Bioassays5.1.3 Guidelines for Evaluating Insecticide Resistance in Vectors using the CDC Bottle BioassayPage 13 of 24Figure 5.1.3.9: Determination of resistance threshold.Mortality (%)1009080706050403020100SusceptibleResistant0 15 30 45 60 75 90 105 120Time (min)Resistance surveillanceBackgroundAlthough resistance data are often collected as part of vector control programs, this is often not done asregularly as it should be in a true resistance surveillance effort. Surveillance requires the regularcollection and interpretation of epidemiological data to support changes in public health programs. It isimportant to consider the CDC bottle bioassay an instrument to collect information to support aninsecticide resistance surveillance system. Resistance data are most valuable when collected over timeto allow for comparisons and for monitoring of trends.It is important to consider how information collected as part of an insecticide resistance surveillancesystem will be used. Most malaria control programs carefully assess the efficacy of their vector controlprogram by, for example, plotting incidence of malaria cases or by counting adult mosquitoes or larvalcollections at sentinel sites. The integration of insecticide resistance data and other kinds of malariarelateddata needs to be taken into consideration before proposing and implementing remediationstrategies for insecticide resistance.Features of resistance emergenceSeveral genetic, biologic, and operational factors influence the development of insecticide resistance. Inmany respects, resistance is a complex problem, with different outcomes possible in a particular area,depending on the influence of diverse factors on initial conditions. Even so, certain factors affectresistance development throughout the world. Major resistance characteristics are discussed below,showing why each manifestation of resistance is potentially unique and therefore must be evaluated oncase-by-case basis.
Chapter 5 : Insecticide Resistance5.1 Insecticide Resistance Bioassays5.1.3 Guidelines for Evaluating Insecticide Resistance in Vectors using the CDC Bottle BioassayPage 14 of 24Focal nature of resistanceVector control personnel frequently assume that resistance in a particular species occurs throughout theircontrol area, but insecticide resistance can be focal. In Guatemala, sampling sites for Anophelesalbimanus only a few kilometers apart varied not only by presence or absence of resistance, but also bylevel of resistance and by dominant mechanism responsible for resistance. Generally speaking, areas ofongoing vector control activities tend to have higher levels of resistance; when resistance levels inadjacent areas are compared, levels may be higher in areas of more intensive mosquito control.Resistance and disease controlIn some cases, vector control strategies in a given area may not be affected by the level of insecticideresistance. For example, a control program may be able to control only 75% of the vector population. Inthese cases, an insecticide resistance level lower than 10% will likely not affect disease control efforts. Insuch a situation, it would be sufficient to increase surveillance and monitor the level and frequency ofresistance but no change in control strategies would be needed.Guiding principlesIn general terms, resistance surveillance should be conducted in areas where disease transmission is aconcern and where insecticide-based control measures are contemplated, ideally before purchase ofinsecticide. In addition to constraints imposed by economic resources, the number of sites that can besampled is highly dependent on the size of the area contemplated for insecticide use. Due to thepotential focal nature of resistance, efforts must be made to choose spatially distributed sites in the areaof interest, if possible. Areas 20 km or more apart should not be assumed to have similar resistancepatterns. Another means of deciding on surveillance sites is to focus on those areas of active diseasetransmission. Even if only one or a few sites can be monitored, this is far preferable to having nosurveillance sites. In addition, efforts should be made to operate sites for at least a few years, sincecomparative data are the most meaningful information.Ideally each site should be monitored once a year. Where control efforts are seasonal, it may be usefulto monitor at the beginning and at the end of the control season. This does not apply to situations suchas the use of insecticide-treated bednets, where the insecticide exposure is year round. If several vectorsin the area are seasonal, the resistance testing schedule should be adjusted to the species of interest.It is also important to consider that it will be necessary to try to identify resistance mechanisms onceresistance is detected with the CDC bottle bioassay, whether using the CDC bottle bioassay withsynergists, or biochemical and/or molecular methods. Decisions on which insecticide to change to willdepend upon the specific mechanism(s) of resistance.Finally, some countries have found it useful to centralize preparation of bottles and administrativeorganization of surveillance. A central reference laboratory can provide support for technical assistanceand quality assurance. It can also serve as a reference laboratory for training, provision of supplies,species identification, and enzymatic assays and molecular methods for determination of resistancemechanisms.
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