PURSE 2000

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Browsing PURSE 2000 by Subject "Biology"

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    Insecticide resistance and the vectorial capacity of culex quinquefasciatus, the vector of filariasis
    (University of Peradeniya, 2000-12-16) Karunaratne, S. H. P. P.; Hemingway, J.
    Filariasis. caused by the nematode parasite Wuchereria bancrofti is endemic in Sri Lanka. The mosquito Culex quinquefasciatus is the vector of filariasis in Sri Lanka and other tropical countries. Control of this mosquito is mainly by organophosphorus insecticides. Resistance mechanism of Culex mosquitoes to organophosphorous insecticides is the elevation (increased quantities) of mosquito carboxylesterases and occurs in more than 80% of resistant Culex worldwide. Biochemical characterisation of these carboxylesterases. purified from C. quinquefasciatus from Colombo. Sri Lanka. revealed that they bind and inactivate organophosphorous insecticides very fast. Antiserum was raised against these esterases and respective genes were isolated. It was found that gene amplification was responsible for the increased production and the resistant insects have upto 80 copies of the gene. Electron- microscopic studies. using mosquito tissues and the antiserum. revealed that the enzymes in resistant insects are expressed at very high levels in the mosquito gut and sub-cuticular layer. As the parasites must pass through some of these tissues to complete their development. parasite survival and hence vectorial capacity can be affected by the insecticide resistance status of the insect. However, insecticide resistance is assumed to increase the likelihood of disease transmission by increasing the population size and allowing the mosquitoes to he longer in the presence of insecticides. This hypothesis was tested for the first time. Blood fed female C quinquefasciatus were collected from seven filariasis endemic areas in Sri Lanka. Mosquitoes were individually analysed for parasite loads by quantitative PCR and insecticide resistance by biochemical assay with para-nitrophenyl acetate (pNP A). Almost 80% of the mosquitoes were infected with W bancrofti. However. there was a strong negative correlation between esterase activity levels with pNP A and parasite RNA levels in all seven localities. Membrane feeding of the insecticide resistant and susceptible mosquito colonies with If'. bancrofti infected blood produced infective parasite larvae after twelve days in 76% of the susceptible. and none of the resistant females. Results suggest that elevated esterase activity affects the development of W bancrofti larvae which may be arrested in the gut cells of insecticide resistant. but not susceptible mosquitoes. Filarial worm infection of mosquitoes cause significant damage to the mosquito host. and high microfilaria infections result in insect mortality. The predominance of the elevated esterase-based resistance mechanism in field populations of r. quinquefasciatus may therefore be influenced by the dual roles of insecticide detoxication and reduction of microfilarial burdens.