Abstract
BackgroundThe most potent malaria vectors rely heavily upon human blood so they are vulnerable to attack with insecticide-treated nets (ITNs) and indoor residual spraying (IRS) within houses. Mosquito taxa that can avoid feeding or resting indoors, or by obtaining blood from animals, mediate a growing proportion of the dwindling transmission that persists as ITNs and IRS are scaled up.Presentation of the hypothesisIncreasing frequency of behavioural evasion traits within persisting residual vector systems usually reflect the successful suppression of the most potent and vulnerable vector taxa by IRS or ITNs, rather than their failure. Many of the commonly observed changes in mosquito behavioural patterns following intervention scale-up may well be explained by modified taxonomic composition and expression of phenotypically plastic behavioural preferences, rather than altered innate preferences of individuals or populations.Testing the hypothesisDetailed review of the contemporary evidence base does not yet provide any clear-cut example of true behavioural resistance and is, therefore, consistent with the hypothesis presented.Implications of the hypothesisCaution should be exercised before over-interpreting most existing reports of increased frequency of behavioural traits which enable mosquitoes to evade fatal contact with insecticides: this may simply be the result of suppressing the most behaviourally vulnerable of the vector taxa that constituted the original transmission system. Mosquito taxa which have always exhibited such evasive traits may be more accurately described as behaviourally resilient, rather than resistant. Ongoing national or regional entomological monitoring surveys of physiological susceptibility to insecticides should be supplemented with biologically and epidemiologically meaningfully estimates of malaria vector population dynamics and the behavioural phenotypes that determine intervention impact, in order to design, select, evaluate and optimize the implementation of vector control measures.
Highlights
The most potent malaria vectors rely heavily upon human blood so they are vulnerable to attack with insecticide-treated nets (ITNs) and indoor residual spraying (IRS) within houses
Existing front line tools for malaria vector control, namely insecticide-treated nets (ITNs) and indoor residual spraying (IRS), have greatly reduced the malaria burden [1,2] because the most important mosquito vectors feed predominantly upon people at times when they are inside their houses so that insecticidal contact is
The widespread and exceptionally efficient African vector Anopheles funestus, which feeds almost exclusively upon human blood and predominantly feeds and rests indoors [6,21], was eliminated from the Pare-Taveta study area in Tanzania during the 1960s following three years of IRS with dieldrin [10]. This species took six years to reestablish itself in the area, during which time it was replaced by Anopheles rivulorum and Anopheles parensis, two morphologically similar species from the same group that prefer to feed outdoors and are generally thought to be of secondary relevance to transmission because they prefer to obtain blood from animals [6,10]
Summary
Existing front line tools for malaria vector control, namely insecticide-treated nets (ITNs) and indoor residual spraying (IRS), have greatly reduced the malaria burden [1,2] because the most important mosquito vectors feed predominantly upon people at times when they are inside their houses so that insecticidal contact is. Implications of the hypothesis No entomological survey can measure the physiological or behavioural characteristics of dead mosquitoes that have been removed by successful intervention programmes Caution should be exercised before over-interpreting most existing reports of increased frequency of behavioural and physiological resistance traits: this may be the result of suppressing the most physiologically susceptible and behaviourally vulnerable of the vector taxa that constituted the original transmission system None of these field studies can unambiguously attribute these observations to altered frequencies of heritable behavioural preference traits, rather than altered expression of phenotypically plastic behavioural traits in an environment that has been changed by intervention coverage.
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