Let loose in Piracicaba Victor Moriyama/Getty Images

Releasing genetically modified mosquitoes appears to have helped reduce cases of dengue in a town in Brazil. The news comes as the US is considering whether to approve the use of the same mosquitoes.

The trial involved Aedes mosquitoes that had been modified to kill off wild mosquitoes of the same species, and was carried out in the town of Piracicaba. Just by eliminating the standing water where the mosquitoes that carry dengue and other diseases like Zika breed, Piracicaba was able to halve the number of dengue cases during the 2015-16 dengue season, compared with last year.

But in the areas where the GM mosquitoes were released too, cases of dengue fell by more than 90 per cent.


This result is significant because regulators have been demanding evidence that this control method not only reduces wild mosquito numbers – as previous trials have shown – but also brings down disease incidence.

Piracicaba has a population of 400,000 people, and the areas where the GM mosquitoes were trialed are home to only around 5,000. Although this small trial doesn’t provide rigorous evidence of the standard that epidemiologists require, it shows potential, says Hadyn Parry, chief executive of Oxitec, the UK firm that developed the mosquitoes.

Florida next?

Independent experts agree. “It is very encouraging,” says Philip McCall of the Liverpool School of Tropical Medicine in the UK. However, it is not a randomised controlled trial, he adds.

Oxitec is now working with the World Health Organisation to plan a much larger trial that will provide more rigorous evidence.

Their method involves releasing millions of mosquitoes – all of which are male, as these do not bite. These males seek out and mate with wild females, but have been genetically modified to ensure that any offspring they produce then die. Unlike with methods like insecticide fogging, no other wildlife is harmed.

Brazil is set to start commercially licensing this method for wider use, Parry says. The US Food and Drug Administration is considering whether to approve the technology. If it does, the Florida Keys will hold a referendum in November to decide whether to use it.

Read more: The new regions at risk from mosquitoes carrying diseases