Abstract
BackgroundThe mosquito Anopheles arabiensis is the primary vector of Plasmodium falciparum in Macha, Zambia. A major portion of Zambia's current malaria control programme relies on long-lasting insecticide-treated nets (LLINs) and indoor residual spraying (IRS) with insecticides. Currently, the efficacy of these measures against An. arabiensis in Macha is unknown, and previous data has shown that An. arabiensis has continued to feed on human hosts, despite high ITN coverage. It is possible that this could be due to either decreased efficacy of ITNs in used in Macha, or pyrethroid resistance in the vector.MethodsF1 offspring of field-collected adult An. arabiensis were tested for insecticide resistance, using CDC bottle bioassays and deltamethrin ITN susceptibility assays. The mosquitoes were characterized for the knock-down resistance (kdr) allele by PCR. LLINs that had been in use for two years in nearby villages were collected and tested for residual deltamethrin concentration and net quality, and were used in bioassays against susceptible colonized Anopheles gambiae s.s. Keele. Additionally, a survey on ITN use and care was conducted among LLIN owners.ResultsIn the F1 An. arabiensis field population, low levels of resistance to DDT and deltamethrin-treated net material were detected by bioassay, although the knock-down resistance (kdr) allele not present in the population. ITN evaluations revealed high variability in residual deltamethrin concentration, quality of the nets, and mosquito mortality in bioassays. Mortality against An. gambiae s.s. in bioassays was correlated with residual deltamethrin concentration, which was dependent upon the number of washes each net had received.ConclusionsProper LLIN care was a strong determinant of LLIN efficacy, indicating that education on the importance of LLIN use and care is key when distributing nets. As there is little insecticide resistance in the local vector population, degradation of LLINs most likely allowed for continued human feeding by An. arabiensis. Continued monitoring and assessment of both the vector population and the efficacy of LLINs in use is necessary in order to appropriately modify vector control operations and prevent the development of pyrethroid resistance.
Highlights
The mosquito Anopheles arabiensis is the primary vector of Plasmodium falciparum in Macha, Zambia
One long-lasting insecticide-treated nets (LLINs) recommended by the World Health Organization Pesticide Evaluation Scheme (WHOPES) is Permanet®, which contains 55 mg/m2 deltamethrin ± 25%, and should retain 25 mg/m2 deltamethrin after 6 standard washes, enough to still be effective against mosquitoes [9]
Our aim was to both investigate the insecticide susceptibility status of the An. arabiensis population in Macha, as well as to evaluate the condition of LLINs distributed in Macha, in order to determine their potential efficacy after two years of typical use in the community
Summary
The mosquito Anopheles arabiensis is the primary vector of Plasmodium falciparum in Macha, Zambia. Insecticide-treated nets (ITNs), introduced over 20 years ago [2,3], are an important tool to protect individuals against the morbidity and mortality caused by malaria [4,5]. One LLIN recommended by the World Health Organization Pesticide Evaluation Scheme (WHOPES) is Permanet®, which contains 55 mg/m2 deltamethrin ± 25% (minimum 45 mg/m2), and should retain 25 mg/m2 deltamethrin after 6 standard washes, enough to still be effective against mosquitoes [9]. In laboratory tests, it provides 80% functional mortality for up to 20 laboratory washes [9]
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