Oxitec's OK513A mosquito has been released and monitored in the Cayman Islands, Malaysia, and Brazil, with promising results, Parry says.
"In every open-air trial we've done in urban environments—even cities filled with buckets of standing water because there is no running water—the results are the same. We recently crashed the local aedes aegypti population in Mandacaru, Brazil by 96 percent in just six months," he says.
That success has been tempered by criticism, mainly from anti-GMO activists concerned about the unintended consequences of meddling with Mother Nature. GeneWatch U.K. accuses Oxitec of skimping on risk assessments required by the Cartagena Biosafety Protocol and of releasing genetically modified mosquitoes on an unsuspecting public overseas without their informed consent.
"As a U.K. company, Oxitec has legal obligations here to produce risk assessments before releasing any mosquitoes. We are concerned that they haven't been following those requirements, which means people in other countries are being used as guinea pigs," says Helen Wallace, GeneWatch U.K.'s director.
Critics are also concerned about a complex ecosystem response. "Reducing the numbers of aedes aegypti could increase the population of aedes albopictus, and we might not only get more of that species, but we may find that they evolve to be a more effective transmitter of disease," Wallace says.
Furthermore, the disease itself could be affected by population-suppression techniques. "A partial or temporary reduction in mosquito numbers can make dengue worse, because when people are bitten frequently, starting at a young age, they are more likely to develop cross-immunity to different serotypes of the dengue virus," she explains. "So, any method that reduces frequency of biting can make dengue hemorrhagic fever worse."
Dr. Anthony James, mosquito researcher and distinguished professor of microbiology and molecular genetics at the University of California, Irvine, shares Wallace's skepticism of population suppression techniques, but for different reasons.
"In large urban areas, we'll never knock down enough mosquitoes to eliminate dengue. We've tried. It's not sustainable. If you control dengue with population suppression, the mosquitoes will come back and you'll have to start over. So we have to go after the disease, not just the mosquitoes."
He separates vector-borne illness prevention strategies into two distinct camps: bite and no-bite.
No-bite strategies focus on preventing mosquitoes from biting people, either by suppressing the population with pesticides and "sterile" mosquitoes, or by keeping mosquitoes away from people with window screens, insect repellents, and protective clothing.
Bite strategies, on the other hand, "recognize that it is the pathogen and not the mosquito that is important here," James says. Before his funding dried up, he was developing a strain of genetically engineered aedes aegypti mosquitoes that still carried the dengue virus, but couldn't transmit it to humans.