With the World Health Organisation calculating that half the world is now at risk from dengue fever, a genetically modified mosquito offers a solution
Though perhaps not as well-known as Malaria, Dengue fever in its various forms effects 400 million people each year with symptoms that range from feelings of mild flu and nausea, to skin bleeding, sore muscles and joint pain in its more serious form – it is why it has been sometimes called breakbone fever. Dengue is only second to malaria. ‘Mosquito-borne diseases are absolutely on the rise,’ explains Haydn Parry, Chief Executive Officer of Oxitec, a biotechnology company spun out of the Zoology Department in Oxford University in 2002. One person dies every 12 seconds from a mosquito-borne illness. ‘We tend to think about malaria, but Dengue fever, Chikungunya and Zika have grown tremendously over the last few years. It’s a big issue, you can’t control these insects with chemicals.’ The most severe form of Dengue fever – Dengue haemorrhagic fever (DHF) – can cause the blood vessels to collapse resulting in dengue shock syndrome and death.
‘We are pioneering the area of genetic engineering to control insect pests that spread diseases and damage crops,’ says Parry. ‘We are most advanced in Brazil, where our mosquitoes are known as “the friendly mosquitoes”, the press have given them that name.’ In Oxford, and now in Brazil, Oxitec and their partners have labs producing male mosquitoes that cannot reproduce effectively. In the lab, the larvae of the genetically engineered mosquito are fed with a special diet which keeps them fertile, but when they are released into the open they become essentially infertile. ‘The male mosquitoes go out, they mate with females, the females will lay eggs that hatch out but the offspring will die,’ explains Parry. `The female mosquito which is fertile can lay anything up to 500 eggs in its lifetime. In a normal situation that’s 500 more live mosquitoes – if she has mated with one of ours then it’s none.’ The modified gene doesn’t survive. Because the released mosquitoes and their offspring die, the insects and their genes do not stay in the environment. There are two genes – a self-limiting gene which prevents the offspring from surviving (they die before they can reproduce and before they can spread disease), and a fluorescent marker gene for simple monitoring.
Ineffective chemical treatments
Currently the mosquitoes are released from a truck driving through town, which has been mapped and divided into sectors. ‘There are some areas where there are a lot of mosquitoes, like bus stops where people come together, where you have a lot of standing water, poorer areas, those areas will tend to have a lot higher mosquito concentrations. You would release more in those areas and less in others.’ The male mosquitoes disperse and over a six-month period it has been shown that the mosquito population is reduced by over 90 percent. The best that can be achieved with chemical treatments says Parry is around 30 percent and the mosquitoes can also develop resistance to chemicals.
The problem with the traditional methods such as chemicals was they could be extremely harmful to the environment or were simply not effective at targeting and reducing the mosquito population carrying disease – mosquitoes live in and around people’s homes, where they can feed. The mosquito carrying dengue fever is called Aedes aegypti – it came out of Egypt – and though mosquitoes typically only travel 200 yards in a lifetime, the larvae and eggs have piggybacked on goods exported across the world. It is now in over 100 countries spreading diseases such as Dengue, Chikungunya and Zika virus. ‘If you take Brazil for example after the Second World War,’ says Parry, ‘they used so much DDT that they wiped this insect out. From an insect control point of view no one has ever had anything as good as DDT but it wasn’t good for the environment.’ When the use of DDT was stopped the mosquitoes returned.
Oxitec releases male mosquitoes which don’t bite or spread disease. Helping people understand the human and the wider environmental impact of the mosquitoes is a core feature of the project. ‘When we do public engagement before we release mosquitoes in the town, we’ll have people in the tent, we’ll have the mosquitoes there,’ says Parry. ‘People can put their hands in a cage and not get bitten, that’s a very easy way of explaining what we are doing.’
Because the mosquito is invasive, ‘you don’t actually rob any major predator or insect of anything key in the food chain,’ explains Parry. Nothing exclusively depends on it. For instance in the Florida Keys where we have been asked to do a trial, Aedes aegypti makes up less than one percent of the mosquito biomass there. So it’s not an important food source. It doesn’t knock out a major part of the food chain and there’s no evidence that some other insect will take its place.’ And importantly, the self-limiting gene and fluorescent marker used for simple monitoring are non-toxic and non-allergenic – if an animal ate an Oxitec mosquito it would be no different to eating a wild one.
The mosquito has been approved for trials in several countries by regulatory agencies deeming it safe for people and the environment. In Brazil the regulatory process has been signed off, and now the world’s first municipal partnership releasing Oxitec mosquitoes is taking place in the city of Piracicaba near São Paulo. They are expecting regulatory bodies to approve in other countries soon. ‘Then we are doing the same thing with agricultural pests as well. So we are just doing trials in the US as well for protection against a moth that attacks cabbages, broccoli and green vegetables and we will do another trail in Brazil for an insect that attacks avocadoes and fruit.’