The continued threat of mosquito-transmitted diseases, such as malaria, dengue fever and Zika, combined with the development of resistance to insecticides, has driven the need to develop new methods for insect control. Two of the most encouraging approaches to have been developed in recent years include the introduction of bacteria into mosquitoes to render them resistant to certain virus infections, and the use of genetic control strategies.
Endosymbiotic bacteria
Endosymbiotic bacteria are bacteria that live inside other organisms. A group of bacteria found living in up to 60% of all insects are Wolbachia. Studies have shown that Wolbachia taken from fruit flies and introduced into Aedes mosquitoes (the vector of Zika and dengue) are maternally inherited and can directly inhibit the replication of the dengue virus within these mosquitoes (1). Wolbachia have also been shown to limit the capacity of mosquitoes to harbour and transmit other medically important pathogens, including the yellow fever virus, chikungunya virus, and Plasmodium parasites.
A trial has been performed in which Wolbachia-infected Aedes aegypti were released over a series of weeks at selected locations in Queensland, Australia. Monitoring of the mosquito population following the releases revealed that the Wolbachia infection had successfully invaded the natural population and that infection rates approached near-fixation within a few months (2). What effect this approach will have on the transmission and epidemiology of pathogens is currently unknown, but further releases of Wolbachia-infected mosquitoes are ongoing in countries endemic for dengue, including Indonesia, Vietnam, and Brazil.
The biggest concern for the Wolbachia approach will be any long term evolution of the pathogen: like any control method, evolutionary responses are expected and over time it is possible that dengue, Zika or other pathogens may overcome the inhibitory effect that the Wolbachia have, enabling transmission by the mosquito once again.
Genetic modification
Sterile Insect Techniques in the past have used radiation to sterilise male mosquitoes that are released to mate with wild females and reduce the reproductive potential of the population. An advancement on this system that has reached the field evaluation stage involves creating a strain of mosquitoes with an engineered lethal transgene (3). Males of the genetically modified mosquitoes can be released into the wild to mate with wild females, and because their offspring won’t live to adulthood the mosquito population is expected to decline. A recent field trial near Juazeiro in Brazil involved releasing the mosquitoes in a sustained effort, and reduced the local Ae. aegypti population by 95% (4). This implies that the release of genetically modified mosquitoes with a lethal gene could be a highly effective method of preventing disease epidemics.
The two new methods outlined above are species-specific, and because they don’t rely on the use of chemical insecticides they are considered environmentally friendly options for insect control. Furthermore, as Ae. aegypti is the vector for several related viruses, suppressing the population of this species or rending it unable to transmit viruses offers the potential to simultaneously tackle Zika, dengue, chikungunya, and yellow fever.
Robert and colleagues are running an online course entitled “Preventing the Zika virus: understanding and controlling the Aedes mosquito” commencing on the 8th of August 2016. Sign up and take part at: https://www.futurelearn.com/courses/preventing-zika
References
- Walker, T. et al., The wMel Wolbachia strain blocks dengue and invades caged Aedes aegypti populations. 2011. Nature. 2011 Aug 24; 476(7361):450-3.
- Hoffmann AA, et al. Successful establishment of Wolbachia in Aedes populations to suppress dengue transmission. Nature 2011; 476: 454–57.
- Thomas D.D. et al. Insect population control using a dominant, repressible, lethal genetic system. Science. 2000;287(5462):2474–6.
- Carvalho, D. O. et al. Suppression of a Field Population of Aedes aegypti in Brazil by Sustained Release of Transgenic Male Mosquitoes. PLoS Neglected Tropical Diseases. July 2015
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