Effect of DEET-multiple exposures on behavior and life history traits in the malaria mosquito Anopheles gambiae (s.s.)

Sublethal insecticide exposure of larvae affects the blood-feeding behaviour of adult mosquitoes

BACKGROUND

Due to their widespread use for controlling disease vectors and agricultural pests, insecticides have become ubiquitous in the environment, including in water bodies harbouring mosquito larvae. As a result, these larvae are continuously exposed to sublethal doses. Since this has long-lasting effects on the mosquitoes' physiology and life-history, we expected that it may also affect behaviours that underlie the mosquitoes' population dynamics and disease epidemiology, such as egg-laying preference, blood-feeding motivation and host-seeking behaviour.

METHODS

Using an insecticide-sensitive and a resistant strain of Anopheles gambiae, an important malaria vector, we evaluated the effects of sublethal exposure to permethrin throughout larval development on the resistance to the insecticide in adults, on host-seeking behaviour, on the motivation to blood-feed and on egg-laying behaviour.

RESULTS

Exposure to sublethal doses of insecticide did not affect knock-down or mortality rates. However, it decreased the avoidance of permethrin-treated nets, and it increased the motivation of females to seek blood meals through permethrin-treated nets, regardless of their sensitivity to the insecticide. It also increased the blood-meal size in particular of the sensitive mosquitoes. Finally, exposed females were more likely than unexposed ones to lay their eggs into several sites.

CONCLUSIONS

Sublethal insecticide exposure during larval development changes several aspects of the behaviour of mosquitoes in ways that could enhance disease transmission and may thus have significant epidemiological implications.

A rapid method for screening mosquito repellents on Anopheles pseudopunctipennis and Aedes aegypti

One of the main vectors for malaria in Latin America is Anopheles pseudopunctipennis (Theobald), whereas Aedes aegypti (L.) is the primary vector of dengue, yellow fever, Zika and chikungunya viruses. The use of repellents is recommended as a personal protection method against these mosquitoes. However there are very few studies evaluating the effect of repellents on An. pseudopunctipennis. The use of a Petri dish to study repellence has been applied by several authors on flies, cockroaches, kissing bugs and mosquitoes, being a valuable technique for species that are difficult to breed under laboratory conditions, such as An. pseudopunctipennis. In the present study, we evaluated the repellence of the essential oil of the Eucalyptus nitens (Shining gum), its main component (1,8-cineole) and the commercial repellent DEET on An. pseudopunctipennis and Ae. aegypti adult females using the plaque repellency method coupled to EthoVision XT10.1 video-tracking software. Repellent effect and locomotor activity were studied through a repellence index (RI) together with an axis constructed from the behavioural variables obtained using the tracking software. DEET repellent effect was observed at 0.01 mg/mL for Ae. aegypti and 0.01 and 0.1 mg/mL for An. pseudopunctipennis. In addition, the essential oil showed significant repellence at 1 and 10 mg/mL for Ae. aegypti, and 1, 5, 10 and 25 mg/mL for An. pseudopunctipennis. Neither of these species were repelled at any concentration of 1,8-cineole. This is the first study that evaluates these compounds on An. pseudopunctipennis females and quantifies their effects on the activity of both species.

Transcriptome profiles of Anopheles gambiae harboring natural low-level Plasmodium infection reveal adaptive advantages for the mosquito

Anopheline mosquitoes are the sole vectors for the Plasmodium pathogens responsible for malaria, which is among the oldest and most devastating of human diseases. The continuing global impact of malaria reflects the evolutionary success of a complex vector-pathogen relationship that accordingly has been the long-term focus of both debate and study. An open question in the biology of malaria transmission is the impact of naturally occurring low-level Plasmodium infections of the vector on the mosquito's health and longevity as well as critical behaviors such as host-preference/seeking. To begin to answer this, we have completed a comparative RNAseq-based transcriptome profile study examining the effect of biologically salient, salivary gland transmission-stage Plasmodium infection on the molecular physiology of Anopheles gambiae s.s. head, sensory appendages, and salivary glands. When compared with their uninfected counterparts, Plasmodium infected mosquitoes exhibit increased transcript abundance of genes associated with olfactory acuity as well as a range of synergistic processes that align with increased fitness based on both anti-aging and reproductive advantages. Taken together, these data argue against the long-held paradigm that malaria infection is pathogenic for anophelines and, instead suggests there are biological and evolutionary advantages for the mosquito that drive the preservation of its high vectorial capacity.

Compensatory mechanisms in resistant Anopheles gambiae AcerKis and KdrKis neurons modulate insecticide-based mosquito control

In the malaria vector Anopheles gambiae, two point mutations in the acetylcholinesterase (ace-1R) and the sodium channel (kdrR) genes confer resistance to organophosphate/carbamate and pyrethroid insecticides, respectively. The mechanisms of compensation that recover the functional alterations associated with these mutations and their role in the modulation of insecticide efficacy are unknown. Using multidisciplinary approaches adapted to neurons isolated from resistant Anopheles gambiae AcerKis and KdrKis strains together with larval bioassays, we demonstrate that nAChRs, and the intracellular calcium concentration represent the key components of an adaptation strategy ensuring neuronal functions maintenance. In AcerKis neurons, the increased effect of acetylcholine related to the reduced acetylcholinesterase activity is compensated by expressing higher density of nAChRs permeable to calcium. In KdrKis neurons, changes in the biophysical properties of the L1014F mutant sodium channel, leading to enhance overlap between activation and inactivation relationships, diminish the resting membrane potential and reduce the fraction of calcium channels available involved in acetylcholine release. Together with the lower intracellular basal calcium concentration observed, these factors increase nAChRs sensitivity to maintain the effect of low concentration of acetylcholine. These results explain the opposite effects of the insecticide clothianidin observed in AcerKis and KdrKis neurons in vitro and in vivo.

Prior contact with permethrin decreases its irritancy at the following exposure among a pyrethroid-resistant malaria vector Anopheles gambiae

Insecticide-treated nets (ITNs) remain major components for vector control despite the spread of resistance mechanisms among mosquito populations. Multiple exposures to pyrethroids may induce physiological and behavioral changes in mosquitoes, possibly reducing efficacy of control tools. Despite epidemiological relevance, the effects of multiple exposures to pyrethroids on their efficacy against pyrethroid-resistant mosquitoes has received little interest. In the present study, we assessed the effects of a blood-meal successfully obtained upon a permethrin-treated net on the success at taking a second blood-meal in presence of permethrin in Anopheles gambiae, carrying pyrethroid resistance alleles. We also measured the impact of exposure to permethrin on life-history traits to address the delayed efficacy of ITNs. Our results showed that females that successfully blood-fed upon a permethrin-treated net were no longer inhibited by permethrin at the following exposure. Blood-meal inhibition due to permethrin was not affected by female size nor by exposure of mothers when testing the offspring, allowing to discard the effect of genetic or physiological selection. Besides, in our assays, exposure to permethrin did not affect mosquito fecundity, fertility nor survival. These results give insights to understand the long-term efficacy of ITNs, and allow to reevaluate the criteria used when choosing compounds for fighting malaria mosquitoes.

Efficacy of vector control tools against malaria-infected mosquitoes

Within mosquito vector populations, infectious mosquitoes are the ones completing the transmission of pathogens to susceptible hosts and they are, consequently, of great epidemiological interest. Mosquito infection by malaria parasites has been shown to affect several traits of mosquito physiology and behavior, and could interplay with the efficacy of control tools. In this study, we evaluated, in pyrethroid resistant Anopheles gambiae, the effect of mosquito infection with the human malaria parasite Plasmodium falciparum on the efficacy of nets treated with either the insecticide deltamethrin or the repellent DEET, measuring (i) mosquito success to pass through the net, (ii) blood-feeding on a host and (iii) chemicals-induced mortality. Infection of mosquitoes at non-infectious stage did not affect their success to pass through the net, to blood-feed, nor chemicals-induced mortality. At infectious stage, depending on replicates, infected mosquitoes had higher mortality rates than uninfected mosquitoes, with stronger effect in presence of DEET. This data evidenced a cost of infection on mosquito survival at transmissible stages of infection, which could have significant consequences for both malaria epidemiology and vector control. This stresses the need for understanding the combined effects of insecticide resistance and infection on the efficacy on control tools.