In other words, is an 80-percent reduction in the mosquito population enough to stop the spread of Zika? The short answer is: We don’t know.
The level of mosquito suppression that’s needed to stop disease transmission depends on a wide variety of factors, including the mosquito population, the human population, the amount of disease already circulating, and the proportion of people previously infected, and therefore immune.
“Our technology is about controlling the mosquito population, so we don’t make direct claims that we’re controlling disease,” McKemey, the Oxitec scientist, told me. “Zika is so new that we don’t know exactly the population dynamic of mosquitoes and humans that’s needed to sustain an outbreak. But if you can reduce the mosquito population sufficiently, you can break that transmission cycle.”
The other big question is: How scalable is Oxitec’s approach? Genetically modified mosquitoes may have been successful at reducing the Aedes aegypti population in Piracicaba, where Oxitec has expanded its trials to cover an area that’s home to some 60,000 people; but what about more densely populated areas, or the hundreds of miles of cities and towns along the Gulf Coast?
“In less than a year, from one female Aedes aegypti, you can wind up with a billion progeny,” McKemey told me. “What I’m trying to say is the biology of it is incredibly scalable, and ultimately that’s why these bugs are such a problem—because they’re so prolific. This kind of scalability is not just that you could theoretically scale up to country or continental scale; it has been done and is being done agriculturally.”
There are success stories in agriculture—sterility by genetic modification helped wipe out the New World screwworm in the 1960s, which once devastated livestock in the United States. Similar methods have been used to successfully fight fruit flies in the United States. Sterility as the end-result of genetic modification has other benefits. For one, it basically makes a species breed itself out of existence—rather than introducing a new kind of pest that continues to pass along its altered genes for generations. “There’s an inherent safety aspect to what we’re doing,” McKemey said. “The very thing we’re releasing is designed to disappear from the environment without a trace.”
In contrast, other gene-modification methods of mosquito control might attempt to alter a mosquito’s DNA so that it’s not as good at transmitting a virus; but it would still be able to produce viable offspring. Those approaches are, McKemey told me, “more of a genie-out-of-a-bottle situation.”
“The potential benefit of that other approach is you might need to release [the gene-altered mosquitoes] less often, but the downside is something unforeseen happens, and it’s out there,” he said.
Something unforeseen isn’t out of the question for Oxitec’s approach, either, though.