The Aedes aegypti mosquito can infect humans with dengue (Image: CDC/Phanie/Rex)

Time to unleash the mozzies? Genetically modified mosquitoes will be raised on a commercial scale for the first time, in a bid to stem outbreaks of dengue fever in Brazil. But it is unclear how well it will work.

Next week biotech company Oxitec of Abingdon, UK, will open a factory in Campinas, Brazil, to raise millions of modified mosquitoes. Once released, they will mate with wild females, whose offspring then die before adulthood. That should cut the number of dengue-carrying Aedes aegypti mosquitoes. In April, Brazil’s National Technical Commission for Biosecurity (CTNBio) approved their commercial use.

The mosquitoes could be an important step forward in controlling dengue, which affects more than 50 million people every year, with a 30-fold increase in the last 50 years. There is no vaccine or preventive drug, so all anyone can do is to spray insecticide on a large scale in a bid to kill dengue-carrying mosquitoes.


The Brazilian state of Bahia is one of the affected areas. A state of alert, declared in February, is in force in 10 rural districts. Oxitec plans to release millions of modified mosquitoes in the Bahia town of Jacobina, as part of an expanded research programme. A larger release could follow if the Brazilian Health Surveillance Agency also lends its approval, as expected.

Question mark

But no one is sure if the insects will succeed. Margareth Capurro at the University of São Paulo has studied the effects of a trial release in Jacobina last year. She plans to submit her report this month.

Capurro says her data show the number of mosquito eggs falling by an impressive 92 per cent in Jacobina. But so far this has not led to a drop in the incidence of dengue.

That may be because the study was too small, says Capurro. Only after a full epidemiological study next year will we know for sure if the GM mosquitoes are working.

“In every trial we’ve demonstrated excellent control of the dengue mosquito in an urban setting,” says Hadyn Parry of Oxitec. For now they are only measuring success in terms of mosquito numbers.

Dangerous precedent

The CTNBio set a dangerous precedent by approving the commercial release of the mosquitoes before full epidemiological studies had been completed, says agronomist Leonardo Melgarejo, who works for Brazil’s Ministry of Agrarian Development, and economist Antonio Inacio Andrioli of the Regional Northwest University of Rio Grande do Sul in Brazil.

In a technical paper presented before the authorisation of the GM mosquitoes, they argued that they should only be released commercially once the technology had been fully evaluated.

In the long run the modified mosquitoes might be stymied by their high cost, says Thomas Unnasch of the University of South Florida in Tampa. The technology depends on sterile males, who by definition cannot pass on the genetically engineered trait. So Unnasch says it would be necessary to release huge numbers of them year after year, at a cost of millions of dollars.

Nevertheless the US authorities are considering using the mosquitoes in Florida, where dengue is also a serious problem.

Additional reporting by Andy Coghlan