Influence Of Temperature On The Growth, Development And Susceptibility Of Anopheles Gambiae (S.L.) Mosquitoes To Pyrethroids
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Date
2023-02
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University Of Ghana
Abstract
Background: Anopheles mosquitoes are responsible for transmitting malaria and lymphatic filariasis. They are among the notable vector species for their crucial role in transmitting malaria. The survival of the vector is of great interest as it affects its ability to transmit diseases. The biology and ecology of mosquitoes are strongly dependent on ambient temperature. The mosquito's life cycle includes four stages: egg, larva, pupa and adult. Indeed, the rearing temperature of the immature stages (egg, larva, and pupa) can significantly impact the completion of the life cycle, the overall fitness of the adult, and ability to transmit disease. In recent years, global warming and possible future warmer climate have prompted many studies to focus on the effects of elevated temperatures on both the morphology and the biology of various species, including vectors. Despite the importance of temperature variability on An. gambiae (s.l.) mosquito's development and survival, there is still the need to explore how and whether or not elevated temperatures associated with climate change is likely to reduce or increase the vector's population dynamics by modifying the life cycle, reduce the efficacy of insecticides, and increase the expression of metabolic enzymes in An. gambiae (s.l.) mosquitoes.
Objective: This study aimed to investigate the influence of elevated temperatures on the growth and development of An. gambiae (s.l.) mosquitoes, and the effectiveness of pyrethroid insecticides in such higher temperatures.
Methods: Anopheles gambiae (s.l.) eggs were obtained from colonies established in the laboratory and were incubated, hatched and reared under eight temperature regimes (25, 28, 30, 32, 34, 36, 38 and 40 °C) using climate-controlled incubators (RTOP-1000D, Zhejiang, China), with photoperiod of 12:12 (L:D) and 80 ± 10% relative humidity. Larvae were fed 10 mg of TetraFin goldfish flakes (Tetra Werke, Melle, Germany). All adults were fed with a 10% sugar solution soaked in cotton wool. In addition, female mosquitoes used to estimate fecundity and longevity were blood-fed using a guinea pig on day four (4) post-emergence. Larvae were monitored daily for development to the next stage. The time to pupation, pupation success, number of adults produced, and sex ratio of the newly emerged adult was recorded. Molecular identification of An. gambiae (s.l.) mosquitoes was done using polymerase chain reaction (PCR) to identify the composition of sibling species in the An. gambiae complex. Larval survival and adult longevity were monitored every 24 hours, and data were analyzed using Kaplan-Meier survival analysis. Furthermore, analysis of variance (ANOVA) was used to assess the relationship between temperature and development time, time to pupation, length of the gonotrophic cycle, biting rate and fecundity. Kruskal-Wallis test was also used to assess the relationship between temperature and pupation success, pupal mortality, the number of adults produced, and sex ratio.
Digital images of larvae, pupae, adult wings and proboscis were captured using stereo microscope with inbuilt camera (Leica EZ4 HD, Leica Microsystems Limited, Switzerland) and body parts were measured using Leica Application Software, version 3.4.0 (Leica Microsystems Limited, Switzerland). Data on larval, and pupal weight and size, adult weight, size and proboscis length were log-transformed and analyzed using ordinary least square (OLS) regression with robust standard errors. In addition, three to five-day-old non-blood-fed An. gambiae (s.l.) mosquitoes were used for insecticide susceptibility test following the WHO bioassay protocol. Batches of 20 – 25 non-blood-fed female adult An. gambiae (s.l.) mosquitoes from each temperature regime (25 – 32 °C) were exposed to two pyrethroid insecticides (0.75% permethrin and 0.05% deltamethrin). The knockdown rate after 60 min and mortality at 24 h were recorded. The levels of four metabolic enzymes (MFO, GST, α-EST and β-EST) were examined in both mosquitoes that were not exposed and those exposed to pyrethroids. Results: An. gambiae (s.l.) mosquitoes used in this study consisted of An. gambiae (s.s.) and An. coluzzii. Development time of immature mosquitoes significantly decreased (F(5, 24) = 133.55, P < 0.001) with increasing temperature. Log-rank test showed that larval survival (X2(6) = 5353.12, P < 0.001) decreased with increasing temperature. In addition, Kruskal- Wallis test showed that the number of adults produced (X2(5) = 28.16, P < 0.001) decreased with increasing temperature, with male mosquitoes disproportionately produced at higher temperatures than females. Larval (βlarval size = 0.11, 95% CI; 0.14, 0.09, P < 0.001) and pupal (βpupal size = 0.12, 95% CI; 0.14, 0.10, P < 0.001) size significantly decreased with increasing temperature. Furthermore, longevity of both blood-fed (log-rank test; X2(4) = 904.15, P < 0.001) and non-blood-fed (log-rank test; X2(4) = 1163.60, P < 0.001) mosquitoes decreased with increasing temperature. The results further showed that the fecundity of mosquitoes significantly (F(2,57) = 3.46, P = 0.038) reduced with increasing temperature. Body size and proboscis length also decreased with increasing temperature. The mortality of An. gambiae (s.l.) mosquitoes to pyrethroids decreased at temperatures above 28 oC. Mosquitoes reared at higher temperatures were more resistant to the insecticides tested (deltamethrin and permethrin) and had more elevated enzyme levels than those reared at low temperatures (P < 0.05).
Conclusion: Mosquitoes could not breed beyond temperatures at 36 oC. Therefore, if the ambient environmental temperatures rise to 36 oC, possibly as a consequence of climate change, it is likely to reduce or inhibit malaria transmission and perhaps its eradication in a future warmer temperature. In conclusion, warmer temperature is potentially hostile to a considerable proportion of emerging mosquitoes and may inhibit their survival such that the numbers of potential vectors may decrease. This study contributes to the knowledge on the relationship between temperature and growth and development of An. gambiae (s.l.) mosquitoes and provides helpful information for modelling vector population dynamics in a future warmer climate.
Description
PhD. Public Health
Keywords
Temperature, Anopheles Gambiae (S.L.) Mosquitoes, Pyrethroids