Genetically modified mosquitoes can put a dent in dengue cases. The first evidence of the health effects of releasing the insects into the real world comes from a year’s worth of disease data from Brazil, says biotech company Oxitec, the mosquitoes’ engineer.
Over much of the city of Piracicaba, where conventional methods of mosquito control were used, cases of the debilitating virus dropped 52 percent from mid-2015 to mid-2016. But in neighborhoods where Oxitec released GM Aedes aegypti mosquitoes as an extra control, the results were even better. Dengue cases there dropped 91 percent, from 133 to 12, according to a press statement from Oxitec, based in Abingdon, England.
Oxitec’s genetically modified line of Ae. aegypti mosquitoes attack a wild population by romantic deception. The GM males sire offspring with built-in self-destruct DNA that kills the new generation off in the wild before they begin to bite. This is the modern biotech twist on a decades-old strategy for controlling insects by releasing sterile males in such numbers that many females waste their reproductive effort, and a population eventually breeds itself out of existence.
In tests around the world before this, Oxitec has published or released evidence that mosquito numbers go down when the GM decoys swarm through a neighborhood. But this is the first claim that reducing those mosquitoes indeed means less disease.
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That information — though not the result of a full epidemiological study — could address a gap in the debate in the Florida Keys over a proposed test release there. Opponents of introducing GM organisms, even ones pretty reliably programmed to die, have objected that there has been no evidence the measure brings any health benefit.
Brazil, where dengue and now Zika have wreaked havoc, has been much more open to the use of GM mosquitoes. In this case, Oxitec looked at the numbers of dengue cases reported mid-year to mid-year from Piracicaba’s epidemiologic surveillance program. The GM mosquito test focused on an area, called CECAP/Eldorado, of about 5,000 residents where the dengue rates were higher than in the rest of the city in 2014‒2015 — 2.66 percent incidence rate versus 0.902 percent. After a year of control measures including releasing the GM mosquitoes, the 2015‒2016 numbers show the test area now fares better than the rest of the city. Its dengue incident rate dropped to 0.24 percent compared with the municipality incidence rate of 0.437 percent.
Data on mosquito populations and diseases are rare and important, says Grayson Brown, who directs the Public Health Entomology lab at the University of Kentucky in Lexington. He wonders how far down the GM mosquitoes drove the wild population before dengue rates started dropping. (Oxitec reports that mosquito numbers dropped 82 percent, but, Brown asks, 82 percent of what?) Such a useful number turns out to be virtually unknown for most mosquito-borne diseases and their countermeasures, except for malaria, he says. Plenty of programs monitor disease outbreaks as they treat mosquitoes, but ethically and politically, “you can’t just leave a section of the city untreated.” Adding the extra measure of the GM treatments offers a way to fill that data gap.