Why we need new interventions to battle mosquito-borne diseases
Mosquitoes are the most dangerous creature in the world. They kill one person every 12 seconds and infect hundreds of millions of people each year with serious diseases. Aedes aegypti is a particularly harmful species because it mainly bites people and lives in and around homes. This makes it very effective at spreading diseases like dengue fever and chikungunya.
This non-native species originated in Africa and is now in more than 100 countries and is still spreading. There is no cure or available vaccine for either disease so the World Health Organization recommends controlling the mosquito itself.1
Why is Aedes aegypti so hard to control?
Biting females hide in nooks and crannies around the home so they can stay close to their human meals.
Insecticides need to touch these blood-feeding females to kill them, and it is remarkably difficult to reach all of those hiding places. This mosquito is also developing resistance to the chemicals used to kill it. So there is an ‘arms race’ to find, test and implement better mosquito control methods as quickly as possible to prevent the next epidemic. Using currently available methods, mosquito control groups cannot reduce
Ae. aegypti numbers low enough to prevent the spread of disease. World-leading mosquito control groups with fleets of planes and helicopters can only reduce Ae. aegypti by about 50%. With that many disease vectors still around, it is possible for dengue, chikungunya, Zika virus – anything that Ae. aegypti can carry – to be introduced at any time. This mosquito is a ticking time bomb.
Public health and mosquito control groups urgently need access to better tools to help them in their fight to protect people against the Aedes aegypti mosquito. This global health challenge is on such a large scale that progress will depend on the public, scientists, industry and governments working together towards solutions.
New tools as the way forward
The battle against mosquito-borne diseases can be fought on multiple fronts. Companies are racing to find working vaccines and to test them in clinical trials to determine whether they will work. But viruses mutate and new strains appear.
A compatible but different approach is to reduce the numbers of the mosquito spreading the disease below the transmission threshold2, so that whatever the virus does, there simply aren’t enough mosquitoes around to spread it.
New technology by Oxitec does exactly that by using genetically engineered mosquitoes to control their own species. Since trials started in 2009, Oxitec has reduced target Ae. aegypti populations by more than
90%. This is an unprecedented level of control and is a key reason why mosquito control groups around the world are keen to evaluate it for use in their countries. Another reason is its light ecological footprint because it’s a highly targeted control method that is non-toxic and pesticide-free.
How genetically engineered mosquito control works
Oxitec mosquitoes are genetically engineered so their offspring die before they can reproduce and before they can become transmitters of disease. It’s an approach similar to the Sterile Insect Technique (SIT) where male insects are sterilised by radiation and released to mate with pest females. With successive releases the there are fewer offspring each generation and the pest population crashes.
SIT has been used worldwide for more than 50 years and has been successful in tackling pests such as
New World Screw Worm.3 The Oxitec mosquito is inspired by the SIT approach but doesn’t rely on radiation, which can affect many genes and the insect’s competitiveness. Instead, Oxitec uses one gene to induce ‘sterility’ and a colour marker gene for monitoring the results. Like SIT, this approach requires successive releases, but there are major environmental benefits such as the disappearance of the insects and their genes from the environment once releases are stopped. The approach is also species-specific, controlling only the non-native Ae. aegypti, so other native species can flourish.
Evaluating new tools against the status quo
New tools need to be tested, and human health and environmental impacts are just as important to consider as developing a mosquito control solution that works. This means rigorously evaluating new tools before implementation. The Oxitec mosquito has been scientifically evaluated with laboratory studies, case studies, and then field studies of increasing size. Results of these evaluations are available in peer-reviewed scientific publications and regulatory documents online. 4
In every country where Oxitec is invited to do a mosquito control project, there is a review process by independent experts in accordance with national legislation and regulations. These reviews evaluate the
Oxitec mosquito for its safety and environmental impact and releases only go ahead once these experts are fully satisfied. Since 2009, mosquito trials have been completed in Cayman, Malaysia, Brazil and Panama, without adverse effects, and more projects are being planned.
The evaluation process for new control methods is vital to progress in the war against mosquitoes. An important consideration for such evaluations is weighing the potential risks of a new intervention against the risks of maintaining the status quo. Public health depends not only on protection from potential future problems but also from existing threats such as mosquitoes spreading disease.
Investing in the future
In Brazil, the dengue burden is so great that they’ve called in the army to help educate people on how to fight the Aedes aegypti mosquito. Brazil has a lot of experience and expertise in tackling dengue and recognises the need for new approaches to complement and improve on current methods. This forward-thinking has led to the first municipal project of genetically engineered mosquito control following approval by the national biosafety group (CTNBio) for release of Oxitec mosquitoes throughout the country. The city leading the way is Piracicaba, in São Paulo state.5 The Oxitec mosquito and its lead inventor have also been nominated for a prestigious international award for this biotechnology breakthrough in battling disease-carrying mosquitoes. 6 The hope is to build on the success of the Oxitec mosquito to tackle the mosquitoes that transmit malaria next. Award nominee, Dr Luke Alphey, also hopes to inspire the next generation of scientists to carry on this important work and join in the fight against vector-borne diseases.
1 World Health Organization Dengue and Severe Dengue Factsheet www.who.int/mediacentre/factsheets/fs117/en/
2 Focks et al. 2000. Transmission thresholds for dengue in terms of Aedes aegypti pupae per person with discussion of their utility in source reduction efforts. www.ncbi.nlm.nih.gov/pubmed/10761719
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