By Nitara Wijayatilake
Less than half an inch in size, the mosquito is the world’s deadliest animal. Although historically feared in tropical countries, the infamous mosquito is gaining more and more recognition in the West. Dengue, Zika, Chikungunya and West Nile viruses use different species of mosquito as their vectors of transmission, much like the plasmodium parasite that causes malaria, and cause over a million human deaths annually (WHO,2020). Makeshift methods of preventing mosquito-borne disease transmission include bed nets and coils, although this has moved to mass spraying of insecticide in some areas. Now, big biotech companies are trialling the use of Genetically Modified Organisms (GMOs) to win the war against the mosquito but the repercussions of wiping out this species are yet to be discovered.
Mosquitoes branch off the family Culicidae. Due to the increasing global temperatures caused by climate change, mosquitoes are completing their life cycles much faster than the usual 2-week period. This means more and more mosquitoes are born every year. Blood-seeking mosquitoes are female and require Haemoglobin and ferric-transferrin proteins from blood to produce eggs. The female will lay eggs on the surface of water; this is preferably a site of collected water where predators are less able to access. In the second stage of the mosquito life cycle they become larvae, feeding on bacteria and algae, growing rapidly and moulting into the pupa stage. After rest and development in the pupa stage, the adult mosquito is formed. Living for approximately a week, mosquitoes actually feed on plant nectar, not blood (Palermo, 2018). The fatal female bite is simply for her own reproductive survival.
To bite, the mosquito needs to find a human. Through the use of a number of different chemical pathways, mosquitoes are able to detect exhaled carbon dioxide and body heat which initially draws them in. Upon getting closer to a victim, olfactory receptors like IR8a detect human odour by recognition of lactic acid in sweat (Greenfield Boyce, 2019). Once the mosquito lands on the target skin, it will use its sharp proboscis to penetrate the flesh. After exposing an open capillary, the mosquito injects its saliva, which contains anticoagulant proteins, into the blood to prevent clotting (Freudenrich, 2008). This is how a virus enters its new host.
Aedes aegypti is the mosquito responsible for the transmission of Zika, dengue and chikungunya viruses. Anopheles gambe carries the malaria virus, Plasmodium. Aedes albopictus, known commonly as the Asian tiger mosquito, is also responsible for epidemiological transmission of many viruses (Discovery, 2017). However, the most common, the house mosquito Culex pipiens, is the principal vector of West Nile fever. Countries that have become accustomed to mosquito-borne disease epidemics are skilled in prevention techniques. These include using Gambusia affinis, mosquito fish in water sources to eat mosquito larvae, or using mechanical barriers like mesh on the outside of houses (CDC, 2019). Ironically, humans are the biggest cause of the success of mosquitoes. Human activity leading to climate change has disastrous effects on an abundance of species, including Homo sapiens, but not the mosquito prospers.
With the aim of reining in mosquito populations, efforts now include using genetically modified mosquitoes as a means of control. These ‘Jurassic Park experiments’ are bold. An interesting trial showing major success genetically modified male and female Aedes aegypti mosquitoes by introducing the bacteria Wolbachia pipientis into them, a bacterium harmless to humans and other animals. These GMOs are released and, through mating with wild populations, increase the number of mosquitoes with Wolbachia over time. This is useful in combating viral diseases because the viruses compete with the bacterium for a place in the host, making transmission much less likely (WMP, 2020). Another biocontrol approach, taken by the company Oxitec, is RIDL (release of insects carrying a dominant lethal gene). In this, male mosquitoes are rendered sterile as they carry a piggyBac-based transposon gene which contains a tetracycline-repressible transcription factor that is lethal when hyper expressed. Male GMOs mate and spread this gene to females which results in their offspring dying at the larval stage (Gilbert & Melton, 2018). This month, the decision to release 750 million of the experiment mosquito OX5034 into Florida Keys has been approved, but this may come at a great risk (Desai, 2020).
Backlash from these experiments include fears on whether the hybrids produced from genetic mixing of lab and wild type populations will actually be more resistant to insecticide and robust in disease transmission (Servick, 2019). Another concern is what the evolutionary costs will be of trials like these. Interfering in the natural world can have detrimental effects. Whilst the mosquito is responsible for the most global deaths, for many it is prey. Damselflies consume mosquitoes in both adult and larval stages; taking away this food source could destabilise ecosystems (Discovery, 2017).
The war against the mosquito is everlasting. Scientists’ efforts often become futile when the concerned virus mutates, rendering research on vaccines and disease prevention useless. Wiping out the whole species could seem like the best idea but may have serious effects on our environment. Although mosquitoes have been villainised for centuries, the real problem is the much smaller virus. Unfortunately for the mosquito, removing the host removes the problem.
References:
World Health Organisation. (2020) Mosquito borne diseases. Available from: https://www.who.int/neglected_diseases/vector_ecology/mosquito-borne-diseases/en/ [Accessed 30/08/2020]
Elizabeth Palermo. (2018) Mosquitoes: Biology, Repellent and Bite Relief. Available from: https://www.livescience.com/45404-mosquito-bites.html [Accessed 30/08/2020]
Nell Greenfieldboyce. (2019) How mosquitoes sniff out human sweat to find us. Available from https://www.npr.org/sections/health-shots/2019/03/28/706838786/how-mosquitoes-sniff-out-human-sweat-to-find-us?t=1598818379573 [Accessed 30/08/2020]
Centers for Disease Control and Prevention. (2019) Controlling Mosquitoes at Home. Available from: https://www.cdc.gov/zika/prevention/controlling-mosquitoes-at-home.html [Accessed 30/08/2020]
Devika Desai. (2020) ‘Jurassic Park experiment’: Florida state government to release millions of GMO mosquitos despite outcry. Available from: https://nationalpost.com/news/world/jurassic-park-experiment-florida-state-government-to-release-millions-of-gmo-mosquitos-despite-public-backlash [Accessed 30/08/2020]
Gilbert, J., Melton, L. Verily project releases millions of factory-reared mosquitoes. Nat Biotechnol 36, 781–782 (2018). https://doi.org/10.1038/nbt0918-781a
Craig Freudenrich. (2008) How Mosquitoes Work. Available from: https://animals.howstuffworks.com/insects/mosquito.htm
[Accessed 30/08/2020]
Kelly Servick. (2019) Study on Dna spread by genetically modified mosquitoes prompts backlash. Available from: https://www.sciencemag.org/news/2019/09/study-dna-spread-genetically-modified-mosquitoes-prompts-backlash [Accessed 30/08/2020]
World Mosquito Program. (2020) Wolbachia method. Available from: https://www.worldmosquitoprogram.org/en/work/wolbachia-method [Accessed 30/08/2020]
Imperial Bioscience Review 