Bioefficacy of long-lasting insecticidal nets against pyrethroid-resistant populations of Anopheles gambiae s.s. from different malaria transmission zones in Uganda

Abstract

BackgroundThere are major concerns over sustaining the efficacy of current malaria vector control interventions given the rapid spread of resistance, particularly to pyrethroids. This study assessed the bioefficacy of five WHO-recommended long-lasting insecticidal nets (LLINs) against pyrethroid-resistant Anopheles gambiae field populations from Uganda.MethodsAdult An. gambiae from Lira, Tororo, Wakiso and Kanungu districts were exposed to permethrin (0.75%) or deltamethrin (0.05%) in standard WHO susceptibility tests. Cone bioassays were used to measure the bioefficacy of four mono-treated LLINs (Olyset®, Interceptor®, Netprotect® and PermaNet® 2.0) and one combination LLIN (PermaNet® 3.0) against the four mosquito populations. Wireball assays were similarly conducted to determine knockdown rates. Species composition and kdr mutation frequency were determined for a sample of mosquitoes from each population. Chemical assays confirmed that test nets fell within target dose ranges.ResultsAnopheles gambiae s.s. predominated at all four sites (86 - 99% of Anopheles spp.) with moderate kdr L1014S allelic frequency (0.34 – 0.37). Confirmed or possible resistance to both permethrin and deltamethrin was identified for all four test populations. Reduced susceptibility to standard LLINs was observed for all four populations, with mortality rates as low as 45.8% even though the nets were unused. The combination LLIN PermaNet®3.0 showed the highest overall bioefficacy against all four An. gambiae s.l. populations (98.5 - 100% mortality). Wireball assays provided a more sensitive indicator of comparative bioefficacy, and PermaNet 3.0 was again associated with the highest bioefficacy against all four populations (76.5 – 91.7% mortality after 30 mins).ConclusionsThe bioefficacy of mono-treated LLINs against pyrethroid-resistant field populations of An. gambiae varied by LLIN type and mosquito population, indicating that certain LLINs may be more suitable than others at particular sites. In contrast, the combination LLIN PermaNet 3.0 performed optimally against the four An. gambiae populations tested. The observed reduced susceptibility of malaria vectors to mono-treated LLINs is of particular concern, especially considering all nets were unused. With ongoing scale-up of insecticidal tools in the advent of increasing resistance, it is essential that those interventions with proven enhanced efficacy are given preference particularly in areas with high resistance.

Efficacy of PermaNet® 3.0 and PermaNet® 2.0 nets against laboratory-reared and wild Anopheles gambiae sensu lato populations in northern Tanzania

“…These findings indicate that PermaNet® 2.0 and improved PermaNet® 3.0 (with PBO) are advanced tools for protection against resistant populations of An. gambiae mosquitoes [8, 17, 37]. This efficacy has also been observed in Ethiopia [9] and Cote d’Ivoire [18].…”

“…gambiae ) across Africa could represent a threat to the tools currently used for vector control [8–14]. Resistance to every currently used insecticide has been found and many factors are believed to increase vector resistance including the extensive use and misuse of the same classes of insecticides in agriculture and public health sectors [7].…”

Increasing insecticide resistance in Anopheles funestus and Anopheles arabiensis in Malawi, 2011–2015

“…However, to date, there is limited evidence that pyrethroid resistance is undermining the effectiveness of LLINs [47]. Laboratory experiments in which wild, resistant Anopheles strains are directly exposed to net samples have indicated reduced efficacy of LLINs against resistant strains [48–50], while experimental hut studies and field studies have demonstrated that resistant mosquitoes are more likely to enter LLINs, feed upon the occupants and survive [15, 16]. However, to date there is insufficient evidence on the epidemiological significance of the observed pyrethroid resistance on LLINs in the community.…”

Zooprophylaxis: A Strategy for Effective Delivery of Endectocides for Vector Control

“…To date long lasting insecticidal nets (LLINs) and indoor residual spray (IRS) have remained at the core of malaria intervention in endemic areas. The LLINs provide both a physical barrier to mosquitoes and insecticidal effects [13,14]. The IRS has an effect on the mortality of malaria vectors when they rest on treated surfaces and are therefore susceptible to the insecticides applied on them.…”