It's just before dawn in Piracicaba, a small city in southeastern Brazil, when a large white van pulls over to the side of the road. A door slides open, revealing stacks of crates jammed with plastic pots. Each pot is buzzing with mosquitoes.
"There's about 1,000 mosquitoes in each of those pots," says Guilherme Trivellato, who works for Oxitec, a biotech company that owns the van. All together, there are more than 300,000 mosquitoes swarming inside those pots.
"That's how many we're going to release today," he says.
And now Brazil is fighting an epidemic of Zika, which is spread by the same species of mosquito. Doctors think Zika may be causing microcephaly, a condition that leaves babies with abnormally small heads and damaged brains, as well as other birth defects and a neurological condition known as Guillain-Barre syndrome.
But the mosquitoes in the van aren't just any mosquitoes. They're genetically engineered to help prevent the spread of Zika and the other viruses.
"That's the idea — to stop any diseases that are transmitted by these specific mosquitoes," Trivellato says.
First of all, the mosquitoes in Trivellato's van are all males, which don't bite and therefore can't transmit diseases. But more important, they are engineered to carry a kind of self-destruct gene — a gene that causes them to quickly die along with all of their progeny.
"All the offspring will die before they reach adult phase," which means they'll never be capable of spreading disease, Trivellato says.
Trivellato climbs back into the van to demonstrate how he releases the genetically engineered mosquitoes, which he has been doing five days a week for months.
As the van pulls out into traffic, he launches an app on a tablet that beeps when it's time to open each pot and shove it into a big tube sticking out a side window.
"Now they will fly around looking for the females," he says.
The idea is that if enough of these males mate with wild Aedes aegypti mosquitoes, the population will plummet.
Oxitec claims the mosquitoes have been able to suppress the wild population by more than 80 percent in one section of Piracicaba. It's an area the size of about 56 football fields where more than 5,000 people live.
Based on the results, the company wants the Brazilian government to considering letting Oxitec release its mosquitoes more widely.
In the United States, the Food and Drug Administration announced on March 11 that the agency had concluded that the mosquitoes would not cause any harm to the environment. The FDA is taking public comment on a proposal to allow the mosquitoes to be let loose in Key Haven, Fla.
But some critics question how effective the mosquitoes really are and whether suppressing the Aedes aegypti population necessarily translates into reducing the spread of disease.
The releases could enable another species of mosquito that can transmit the same diseases, Aedes aldopictus, to proliferate, critics say. Another concern is that the engineered mosquitoes could somehow survive and become better disease-transmitters themselves, they say.
"The worst thing is if they reproduce in the environment and are more efficient in transmitting the virus than a native mosquito," says Jose Maria Guzman Ferraz, an ecologist at the University of Sao Carlos in Campinas.
"I'm worried because the population shouldn't be lab rats," Ferraz says. "They're not lab rats. They're people."
Some argue that there are other new approaches that look more promising for controlling mosquitoes. Other scientists are experimenting with mosquitoes infected with bacteria called wolbachia, for example, which also suppress the wild population. Others are trying mosquitoes exposed to radiation to do the same thing.
The company argues that the genetically engineered mosquitoes are superior to the alternatives, and that its own extensive testing in Brazil and elsewhere has demonstrated that there is no evidence they survive in the wild, cause any harm to the environment or prompt any other disease-transmitting mosquitoes to proliferate.
"We've done exhausting kinds of testing," says Karla Tepedino, who runs a nearby Oxitec facility that produces the mosquitoes. "It is safe, both for the environment and for people."
Both Tepedino and Trivellato argue the Oxitec mosquito is better for the environment than traditional mosquito-control efforts because no pesticides are involved.
"It's environmentally friendly. It has no effect on any other species," Trivellato says.
An informal survey of a handful of residents in the area where the mosquitoes are being released in Piracicaba found mixed reaction. Some residents share Ferraz's fears. Others are glad the mosquitoes are being released.
Elder Santin, a chemist at a local paper company, says he has a grandson who was hospitalized because of dengue and a daughter-in-law who's pregnant, which makes him worry about Zika.
"People think, 'Oh, I don't have a pregnant person in the family.' But as soon as you do have a pregnant person in the family it's an immense worry," he says.
Santin admits it took time for him to get used to having genetically engineered mosquitoes buzzing his house.
"Normally, when you have insects in your house the first thing that you do is spray insecticide. You pulverize them. But not in this case. You don't pulverize them. You leave them," he says. "So it's weird because of that."
Oxitec's mosquito production facility in Campinas produces thousands of the genetically engineered insects every week.
Thousands of genetically engineered female Aedes aegypti mosquitoes are kept in plastic and canvas cages, where they mate with males to produce eggs. The eggs are hatched in trays containing fish food to nourish them and tetracycline. Tetracycline is normally used as an antibiotic but in this case keeps the insects alive by suppressing the self-destruct gene.
"It's an artificial lethal gene that produces a protein that blocks cells from working properly," Tepedino says. "So they die if they don't receive tetracycline, which is the antidote."
Technicians use strainers to separate the immature genetically engineered males from the females — males are smaller and float through the strainers to the top. They are then placed on those plastic pots to emerge as flying insects for release.
Technicians regularly sample the immature insects to make sure no females are accidentally being released. In addition, they routinely examine mosquitoes captured in the field to gauge the impact of the releases. They do that by looking for a fluorescent gene that the mosquitoes were also engineered to carry.
Oxitec plans to open a bigger mosquito breeding facility in Piracicaba to expand its mosquito suppression program to the entire city of more than 60,000 residents.