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The suicide swarm: mosquitoes to halt the spread of disease

ByHannah Smith

Sep 30, 2017

Mosquitoes are an annoying, if mostly harmless, summer threat in the UK and Europe. However, they are a major disease carrier in other parts of the world, spreading Dengue fever and Chikungunya. They were the primary vector for transmitting the Zika virus during its 2015 outbreak, and remain a serious threat in South and Central America.
Many methods have been trialled to reduce or remove the mosquito population, with limited success. Pesticides can be carried further into the food chain or can kill off other, harmless organisms. However, one company, Oxitec, has decided to target the disease transmitting Aedes aegypti mosquito strain directly, using genetic modification.

The company, based in Oxfordshire, have genetically modified the mosquito to carry a self-limiting gene. This gene produces a protein that kills the mosquito by “tying up” its cellular machinery, preventing the cells from growing or replicating properly.

The modified mosquitos survive to adulthood as they are raised on a special diet which represses that protein, but their offspring in the wild will die before they can breed themselves. The company releases the modified males only, as it is the females which bite humans, so the modified mosquitos cannot spread disease and instead breed with multiple females. In this way, the males will father several broods of doomed larvae and reduce the mosquito population for the next generation.

This will be combined with standard trapping programmes, as the genetically altered mosquitos also carry a fluorescent tag in their cells, so scientists can distinguish the altered mosquitos, releasing them and killing the wild, fertile ones.

This also allows the company to follow the movement of their released insects. The protein produced that limits the lifespan of the mosquito is non-toxic, so birds and other predators can eat the dying mosquitoes without any ill-effects in the food chain.

Oxitec has trialled these genetically modified insects in several small-scale locations. It claims to have seen an 80 per cent reduction in mosquito populations.

However, several questions over this method remain. Although the reported 80 per cent reduction in population is impressive, it is unknown if this is sufficient to affect the transmission of disease. A near eradication of the mosquito population could be necessary.

There is risk of genetic drift, where the mutation evolves out of the population, but it is unlikely to be possible if the mutation is lethal within one generation. One group doubtful of the method is GeneWatch UK, which criticised several aspects of Oxitec’s trial in the Cayman Islands.

GeneWatch UK doubts the 80% population reduction claimed by Oxitec, as this is calculated using egg traps to count eggs laid, rather than counting through adult trapping. GeneWatch UK also warned against the accidental release of genetically modified females along with the males, which can become disease vectors as they bite humans.

Despite these concerns, Oxitec is scaling up their mosquito production. A new facility is opening soon, and the company plans to roll out their mosquito programme across the Caymans and Brazil, to combat the Zika virus.

The company is also branching out into genetically engineered moths, to fight the crop-destroying diamond-backed moth. Whether it will be successful in eliminating disease transmission is yet to be seen, but genetic manipulation may prove to be more effective than the current methods to kill these tiny, persistent pests.

Image: James Gathany

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