The Resistance Phenomenon in Microbes and Infectious Disease Vectors Implications for Human Health and Strategies for Containment: Workshop Summary (2003) / Chapter Skim
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3. Vector Resistance
3. Vector Resistance
Pages 79-106
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From page 79... ...
The focus of this session of the workshop was an examination of the role that vectors play in a variety of diseases and how management efforts are being used to better control vector populations. Malaria represents a classic example of the difficulties associated with vector-control efforts.
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From page 80... ...
Among their benefits, models enable researchers to examine trends in data, explore questions of population dynamics, compare various management options, and generate new hypotheses for study. A major roadblock to progress in this area is the lack of interdisciplinary work among modelers studying insect resistance and those studying antimicrobial resistance in pathogens.
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From page 81... ...
Today the major emphasis in research into resistance is on the molecular mechanisms of resistance, and rational resistance management, with a view to controlling the spread and development of resistant vector populations. The level of resistance in insect populations is dependent on the amount and frequency of insecticides used, and the inherent characteristics of the insect species selected.
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From page 82... ...
gambiae is particularly important given the recent emphasis on the use of pyrethroid impregnated bednets for malaria control (Vontas et al., 2001~. The peridomestic vectors of Leishmania are primarily controlled by insecticides throughout their range.
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From page 83... ...
The superiority of insecticide binding in enzymes from resistant insects suggests that there has been a positive insecticide selection pressure to maintain elevation of favorable esterase. In contrast to the situation in Culex a number of Anopheles species have a non-elevated esterase mechanism that confers resistance specifically to malathion through increased rates of metabolism (Hemingway, 1982, 1983, 1985; Malcolm and Boddington, 1989~.
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From page 84... ...
Target Site Resistance The OPs, carbamates, organochlorines, and pyrethroids all target the nervous system. Newer classes of insecticides are now coming onto the market place for vector control, but the high cost of developing and registering them inevitably means that insecticides are developed initially for the agricultural market and then utilized for public health vector control, where their activities and safety profile are appropriate.
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From page 85... ...
A reduction in the sensitivity of the voltage gated sodium channel to insecticide binding causes the kdr resistance phenotype. Kdr mutants are more variable than those in the GABA receptors, but are still limited to a small number of regions on this large channel protein.
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From page 86... ...
GST-based DDT resistance is common in Anopheles, reflecting decades of heavy use of DDT for malaria control. Molecular characterization of GSTs is most developed in An.
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From page 87... ...
In An. gambiae multiple class I and class III GST genes are clustered in separate single locations.
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From page 88... ...
Information resulting from such large-scale trials may allow us to establish rational strategies for long-term insecticide use. As our ability to manipulate the insect genome improves and our understanding of the regulation of insecticide resistance mechanisms increases, new strategies should be devised for incorporation into these control programs.
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From page 89... ...
Insecticide control failure may have less obvious impact on disease control. The continued survival of a vector species may be less noticeable against a background of pest species that have been controlled, giving the illusion of adequate vector control.
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From page 90... ...
Insecticide Resistance Surveillance The most fundamental feature of insecticide resistance problems may be that each such problem is potentially unique, given the broad variety of resistance mechanisms available to the insect and the great disparity in insecticide use history and thus selection pressure on natural populations of vectors. Given the diversity of problem types and the focal nature of resistance, it becomes the primary goal of resistance surveillance to measure resistance as it exists, at a particular place, at a particular time.
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From page 91... ...
Biochemical microplate assays are available for the oxidase, esterase, glutathione S-transferase, and insensitive acety~cholinesterase resistance mechanisms. These assays give clear evidence of resistance mechanisms, provided their results are carefully correlated to bioassay data from the same vector populations.
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From page 92... ...
These techniques are making it possible to deal with the greatest emerging problem in resistance surveillance, that of multiple resistance. Multiple resistance is the resistance to an insecticide through multiple resistance mechanisms.
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From page 93... ...
The most inexpensive strategy, it involves establishment of susceptibility baselines, periodic susceptibility testing, correlation of changes with control efficacy, and change of control strategy when the data indicate the necessity of doing so. Response choices are to switch chemicals, to apply chemicals focally, or where chemicals are ineffective, to use source reduction or concentrate on personal protection.
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From page 94... ...
Modern vector control methods have not measurably slowed re-conquest of the Americas by this important vector of dengue and yellow fever viruses. The insecticide and methods previously used to exert decades of control over malaria and to eliminate Ae.
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. WHO moved malaria control into PHC system 1.
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From page 96... ...
entitled "The Excitant and Repellent Effects on Mosquitoes of Sub-Lethal Contacts with DDT." This paper warrants careful consideration because its 1947 publication date means that the research was actually conducted before wide-scale use of DDT in malaria control (which started in 1946~. Kennedy's experiments demonstrated repellent and irritant actions of DDT against a vector of malaria (Anopheles maculipennis atroparvus)
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From page 97... ...
It acts as a lethal agent on the one hand, and as an excitant and thereby sometimes a repellent on the other" (Kennedy, 1947~. loo 80 60 OWL 40 20 o % Remaining % Escaping FIGURE 3-3 Percent of Anopheles maculipennis atroparvus females escaping from DDT-treated and untreated chambers in just three minutes post-exposure.
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From page 98... ...
Six years later, in 1953, Macdonaid and Davidson published what became the founding principles for using insecticides in malaria control. Their analyses were based on field studies from 1947 to 1952.
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From page 99... ...
virtually guaranteed that research and expansion of knowledge, particularly regarding chemical mode of action, would not occur. Macdonald and Davidson allowed that DDT killed mosquitoes that entered houses and reduced longevity of malaria vector populations below what is needed to maintain malaria transmission.
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From page 100... ...
. sequentially with repellent actions that prevent house entering; irritant actions that promote premature exiting and prevent biting indoors; and toxic actions that produce mortality (Roberts et al., 2000~.
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From page 101... ...
We have shown that repellent and irritant actions of DDT did not suddenly surface after decades of DDT use; but were actually recognized as major actions of DDT residues from the very start of DDT's use as a public health insecticide. In conclusion, funding agencies should formally recognize behavioral actions of vectors to insecticides as a priority area for future support.
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From page 102... ...
1997. Resistance to insecticides in insect vectors of disease: Estoc3, a novel amplified esterase associated with estpls from insecticide resistant strains of the mosquito Culex quinquefasciatus.
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1997. Resistance management strategies in malaria vector mosquito control.
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From page 104... ...
1998. Resistance management strategies in malaria vector mosquito control.
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From page 105... ...
. Cloning and sequence of the full length cDNA for a major insecticide resistance gene worldwide in the mosquito Culex quinquefasciatus.
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From page 106... ...
1995. The independent gene amplification of indistinguishable esterase B electromorphs from the insecticide resistant mosquito Culex quinquefasciatus.
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