Relationship between temperature and Anopheles gambiae sensu lato mosquitoes' susceptibility to pyrethroids and expression of metabolic enzymes

Abstract

Background: Malaria remains one of the most devastating diseases globally, and the control of mosquitoes as the vector is mainly dependent on chemical insecticides. Elevated temperatures associated with future warmer climates could affect mosquitoes' metabolic enzyme expression and increase insecticide resistance, making vector control difficult. Understanding how mosquito rearing temperatures influence their susceptibility to insecticide and expression of metabolic enzymes could aid in the development of novel tools and strategies to control mosquitoes in a future warmer climate. This study evaluated the effects of temperature on the susceptibility of Anopheles gambiae sensu lato (s.l.) mosquitoes to pyrethroids and their expression of metabolic enzymes.

Methods: Anopheles gambiae s.l. eggs obtained from laboratory-established colonies were reared under eight temperature regimes (25, 28, 30, 32, 34, 36, 38, and 40 °C). Upon adult emergence, 3- to 5-day-old female non-blood-fed mosquitoes were used for susceptibility tests following the World Health Organization (WHO) bioassay protocol. Batches of 20-25 mosquitoes from each temperature regime (25-34 °C) were exposed to two pyrethroid insecticides (0.75% permethrin and 0.05% deltamethrin). In addition, the levels of four metabolic enzymes (α-esterase, β-esterase, glutathione S-transferase [GST], and mixed-function oxidase [MFO]) were examined in mosquitoes that were not exposed and those that were exposed to pyrethroids.

Results: Mortality in An. gambiae s.l. mosquitoes exposed to deltamethrin and permethrin decreased at temperatures above 28 °C. In addition, mosquitoes reared at higher temperatures were more resistant and had more elevated enzyme levels than those raised at low temperatures. Overall, mosquitoes that survived after being exposed to pyrethroids had higher levels of metabolic enzymes than those that were not exposed to pyrethroids.

Conclusions: This study provides evidence that elevated temperatures decreased An. gambiae s.l. mosquitoes' susceptibility to pyrethroids and increased the expression of metabolic enzymes. This evidence suggests that elevated temperatures projected in a future warmer climate could increase mosquitoes' resistance to insecticides and complicate malaria vector control measures. This study therefore provides vital information, and suggests useful areas of future research, on the effects of temperature variability on mosquitoes that could guide vector control measures in a future warmer climate.

Keywords: Anopheles gambiae; Bioassay; Biochemical analysis; Climate change; Insecticide; Metabolic enzyme; Susceptibility.

Fig. 1

Insecticide susceptibility of An. gambiae s.l. (Tiassalé strain) mosquitoes reared at different temperature regimes

Fig. 2

Median MFO level in An. gambiae s.l. mosquitoes reared at different temperature regimes. NB: no mosquito reared at 34 °C survived after being exposed to pyrethroids; hence, no enzyme level was measured

Fig. 3

Median GST level in An. gambiae s.l. mosquitoes reared at different temperature regimes. NB: No mosquito reared at 34 °C survived after being exposed to pyrethroids; hence, no enzyme level was measured

Fig. 4

Median α-esterase level in An. gambiae s.l. mosquitoes reared at different temperature regimes. NB: No mosquito reared at 34 °C survived after being exposed to pyrethroids; hence, no enzyme level was measured

Fig. 5

Median β-esterase level in An. gambiae mosquitoes reared at different temperature regimes. NB: No mosquito reared at 34 °C survived after being exposed to pyrethroids; hence, no enzyme level was measured