Abstract
BackgroundAppropriate monitoring of vector insecticide susceptibility is required to provide the rationale for optimal insecticide selection in vector control programs.MethodsIn order to assess the influence of mosquito age on susceptibility to various insecticides, field-collected larvae of An. gambiae s.l. from Tiassalé were reared to adults. Females aged 1, 2, 3, 5 and 10 days were exposed to 5 insecticides (deltamethrin, permethrin, DDT, malathion and propoxur) using WHO susceptibility test kits. Outcome measures included the LT50 (exposure time required to achieve 50% knockdown), the RR (resistance ratio, i.e. a calculation of how much more resistant the wild population is compared with a standard susceptible strain) and the mortality rate following 1 hour exposure, for each insecticide and each mosquito age group.ResultsThere was a positive correlation between the rate of knockdown and mortality for all the age groups and for all insecticides tested. For deltamethrin, the RR50 was highest for 2 day old and lowest for 10 day old individuals. Overall, mortality was lowest for 2 and 3 day old individuals and significantly higher for 10 day old individuals (P < 0.05). With permethrin, the RR50 was highest for 1 to 3 day old individuals and lowest for 10 day old individuals and mortality was lowest for 1 to 3 day old individuals, intermediate for 5 day old and highest for 10 day old individuals. DDT did not display any knockdown effect and mortality was low for all mosquito age groups (<7%). With malathion, the RR50 was low (1.54 - 2.77) and mortality was high (>93%) for all age groups. With propoxur, no knockdown effect was observed for 1, 2 and 3 day old individuals and a very low level of mortality was observed (< 4%), which was significantly higher for 5 and 10 day old individuals (30%, P < 0.01).ConclusionResults indicate that for An. gambiae s.l. adults derived from wild-collected larvae, there was an influence of age on insecticide susceptibility status, with younger individuals (1 to 3 days old) more resistant than older mosquitoes. This indicates that the use of 1 – 2 day old mosquitoes in susceptibility assays as recommended by the WHO should facilitate detection of resistance at the stage where the highest rate of the resistance phenotype is present.
Highlights
Appropriate monitoring of vector insecticide susceptibility is required to provide the rationale for optimal insecticide selection in vector control programs
Since outcomes from resistance tests can influence decisions in insecticide-based intervention programs, we investigated the extent that different ages of wild population of An. gambiae adult mosquitoes can influence phenotypic resistance to insecticides of four different classes used in malaria control, and whether the current methodology is likely to result in variable classification of resistance status
LT50 values were 164.0, 179.01 and 193.31 minutes respectively for 1, 2 and 3 day old mosquitoes, corresponding to a more than 10-fold higher LT50 that observed with the susceptible strain (RR50 = 11.24 13.25); LT50 was 69.64 minutes for 10 day old individuals corresponding to about 5-fold the LT50 of the susceptible colony (RR50 = 4.77); the values were intermediate for 5 day old mosquitoes (LT50 = 129.18 min; RR50 = 8.85)
Summary
Appropriate monitoring of vector insecticide susceptibility is required to provide the rationale for optimal insecticide selection in vector control programs. Knowledge on vector insecticide resistance status, changing trends of resistance in target vectors and their operational implications remain basic requirements to guide insecticide use in disease control programmes. This knowledge can provide a basis for selecting insecticides and for ascertaining continued susceptibility to insecticides already in use. There are three general factors that can affect outcomes from WHO susceptibility test: (i) the physiological status of mosquitoes used in assays, i.e., whether adult females are unfed, blood fed, semi-gravid or gravid; (ii) the age of the adult mosquitoes used; and (iii) the temperature/ humidity under which insecticide exposure is conducted. Reduced phenotypic resistance in older mosquitoes has been observed in several laboratory studies [10,11], but very few studies have explored this phenomenon in natural populations
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