The impact of temperature on insecticide toxicity against the malaria vectors Anopheles arabiensis and Anopheles funestus.

BACKGROUND: It is anticipated that malaria elimination efforts in Africa will be hampered by increasing resistance to the limited arsenal of insecticides approved for use in public health. However, insecticide susceptibility status of vector populations evaluated under standard insectary test conditions can give a false picture of the threat, as the thermal environment in which the insect and insecticide interact plays a significant role in insecticide toxicity. METHODS: The effect of temperature on the expression of the standard WHO insecticide resistance phenotype was examined using Anopheles arabiensis and Anopheles funestus strains: a susceptible strain and the derived resistant strain, selected in the laboratory for resistance to DDT or pyrethroids. The susceptibility of mosquitoes to the pyrethroid deltamethrin or the carbamate bendiocarb was assessed at 18, 25 or 30 °C. The ability of the pyrethroid synergist piperonyl-butoxide (PBO) to restore pyrethroid susceptibility was also assessed at these temperatures. RESULTS: Temperature impacted the toxicity of deltamethrin and bendiocarb. Although the resistant An. funestus strain was uniformly resistant to deltamethrin across temperatures, increasing temperature increased the resistance of the susceptible An. arabiensis strain. Against susceptible An. funestus and resistant An. arabiensis females, deltamethrin exposure at temperatures both lower and higher than standard insectary conditions increased mortality. PBO exposure completely restored deltamethrin susceptibility at all temperatures. Bendiocarb displayed a consistently positive temperature coefficient against both susceptible and resistant An. funestus strains, with survival increasing as temperature increased. CONCLUSIONS: Environmental temperature has a marked effect on the efficacy of insecticides used in public health against important African malaria vectors. Caution must be exercised when drawing conclusions about a chemical's efficacy from laboratory assays performed at only one temperature, as phenotypic resistance can vary significantly even over a temperature range that could be experienced by mosquitoes in the field during a single day. Similarly, it might be inappropriate to assume equal efficacy of a control tool over a geographic area where local conditions vary drastically. Additional studies into the effects of temperature on the efficacy of insecticide-based interventions under field conditions are warranted.

Long-lasting insecticidal nets no longer effectively kill the highly resistant Anopheles funestus of southern Mozambique.

BACKGROUND: Chemical insecticides are crucial to malaria control and elimination programmes. The frontline vector control interventions depend mainly on pyrethroids; all long-lasting insecticidal nets (LLINs) and more than 80% of indoor residual spraying (IRS) campaigns use chemicals from this class. This extensive use of pyrethroids imposes a strong selection pressure for resistance in mosquito populations, and so continuous resistance monitoring and evaluation are important. As pyrethroids have also been used for many years in the Manhiça District, an area in southern Mozambique with perennial malaria transmission, an assessment of their efficacy against the local malaria vectors was conducted. METHODS: Female offspring of wild-caught Anopheles funestus s.s. females were exposed to deltamethrin, lambda-cyhalothrin and permethrin using the World Health Organization (WHO) insecticide-resistance monitoring protocols. The 3-min WHO cone bioassay was used to evaluate the effectiveness of the bed nets distributed or available for purchase in the area (Olyset, permethrin LLIN; PermaNet 2.0, deltamethrin LLIN) against An. funestus. Mosquitoes were also exposed to PermaNet 2.0 for up to 8 h in time-exposure assays. RESULTS: Resistance to pyrethroids in An. funestus s.s. was extremely high, much higher than reported in 2002 and 2009. No exposure killed more than 25.8% of the mosquitoes tested (average mortality, deltamethrin: 6.4%; lambda-cyhalothrin: 5.1%; permethrin: 19.1%). There was no significant difference in the mortality generated by 3-min exposure to any net (Olyset: 9.3% mortality, PermaNet 2.0: 6.0%, untreated: 2.0%; p = 0.2). Six hours of exposure were required to kill 50% of the An. funestus s.s. on PermaNet 2.0. CONCLUSIONS: Anopheles funestus s.s. in Manhiça is extremely resistant to pyrethroids, and this area is clearly a pyrethroid-resistance hotspot. This could severely undermine vector control in this district if no appropriate countermeasures are undertaken. The National Malaria Control Programme (NMCP) of Mozambique is currently improving its resistance monitoring programme, to design and scale up new management strategies. These actions are urgently needed, as the goal of the NMCP and its partners is to reach elimination in southern Mozambique by 2020.

Environmental temperatures significantly change the impact of insecticides measured using WHOPES protocols.

BACKGROUND: Insecticides are critical components of malaria control programmes. In a variety of insect species, temperature plays a fundamental role in determining the outcome of insecticide exposure. However, surprisingly little is known about how temperature affects the efficacy of chemical interventions against malaria vectors. METHODS: Anopheles stephensi, with no recent history of insecticide exposure, were exposed to the organophosphate malathion or the pyrethroid permethrin at 12, 18, 22, or 26°C, using the WHO tube resistance-monitoring assay. To evaluate the effect of pre-exposure temperature on susceptibility, adult mosquitoes were kept at 18 or 26°C until just before exposure, and then moved to the opposite temperature. Twenty-four hours after exposure, mosquitoes exposed at <26°C were moved to 26°C and recovery was observed. Susceptibility was assessed in terms of survival 24 hours after exposure; data were analysed as generalized linear models using a binomial error distribution and logit link function. RESULTS: Lowering the exposure temperature from the laboratory standard 26°C can strongly reduce the susceptibility of female An. stephensi to the WHO resistance-discriminating concentration of malathion (χ2(df=3) = 29.0, p < 0.001). While the susceptibility of these mosquitoes to the resistance-discriminating concentration of permethrin was not as strongly temperature-dependent, recovery was observed in mosquitoes moved from 12, 18 or 22°C to 26°C 24 hours after exposure. For permethrin especially, the thermal history of the mosquito was important in determining the ultimate outcome of insecticide exposure for survival (permethrin: pre-exposure temperature: F1,29 = 14.2, p < 0.001; exposure temp: F1,29 = 1.1, p = 0.3; concentration: F1,29 = 85.2, p < 0.001; exposure temp x conc: F1,29 = 5.8, p = 0.02). The effect of acclimation temperature on malathion susceptibility depended on the exposure temperature (exposure temp: F1,79 = 98.4, p < 0.001; pre-exposure temp: F1,79 = 0.03, p = 0.9; pre-exp temp x exp temp F1,79 = 6.0, p = 0.02). CONCLUSIONS: A single population of An. stephensi could be classified by WHO criteria as susceptible or resistant to a given chemical, depending on the temperature at which the mosquitoes were exposed. Investigating the performance of vector control tools under different temperature conditions will augment the ability to better understand the epidemiological significance of insecticide resistance and select the most effective products for a given environment.

Empirical and theoretical investigation into the potential impacts of insecticide resistance on the effectiveness of insecticide-treated bed nets.

In spite of widespread insecticide resistance in vector mosquitoes throughout Africa, there is limited evidence that long-lasting insecticidal bed nets (LLINs) are failing to protect against malaria. Here, we showed that LLIN contact in the course of host-seeking resulted in higher mortality of resistant Anopheles spp. mosquitoes than predicted from standard laboratory exposures with the same net. We also found that sublethal contact with an LLIN caused a reduction in blood feeding and subsequent host-seeking success in multiple lines of resistant mosquitoes from the laboratory and the field. Using a transmission model, we showed that when these LLIN-related lethal and sublethal effects were accrued over mosquito lifetimes, they greatly reduced the impact of resistance on malaria transmission potential under conditions of high net coverage. If coverage falls, the epidemiological impact is far more pronounced. Similarly, if the intensity of resistance intensifies, the loss of malaria control increases nonlinearly. Our findings help explain why insecticide resistance has not yet led to wide-scale failure of LLINs, but reinforce the call for alternative control tools and informed resistance management strategies.

The effects of age, exposure history and malaria infection on the susceptibility of Anopheles mosquitoes to low concentrations of pyrethroid.

Chemical insecticides are critical components of malaria control programs. Their ability to eliminate huge numbers of mosquitoes allows them to swiftly interrupt disease transmission, but that lethality also imposes immense selection for insecticide resistance. Targeting control at the small portion of the mosquito population actually responsible for transmitting malaria parasites to humans would reduce selection for resistance, yet maintain effective malaria control. Here, we ask whether simply lowering the concentration of the active ingredient in insecticide formulations could preferentially kill mosquitoes infected with malaria and/or those that are potentially infectious, namely, old mosquitoes. Using modified WHO resistance-monitoring assays, we exposed uninfected Anopheles stephensi females to low concentrations of the pyrethroid permethrin at days 4, 8, 12, and 16 days post-emergence and monitored survival for at least 30 days to evaluate the immediate and long-term effects of repeated exposure as mosquitoes aged. We also exposed Plasmodium chabaudi- and P. yoelii-infected An. stephensi females. Permethrin exposure did not consistently increase mosquito susceptibility to subsequent insecticide exposure, though older mosquitoes were more susceptible. A blood meal slightly improved survival after insecticide exposure; malaria infection did not detectably increase insecticide susceptibility. Exposure to low concentrations over successive feeding cycles substantially altered cohort age-structure. Our data suggest the possibility that, where high insecticide coverage can be achieved, low concentration formulations have the capacity to reduce disease transmission without the massive selection for resistance imposed by current practice.