Every weekday at 7 a.m., a van drives slowly through the southeastern Brazilian city of Piracicaba carrying a precious cargo — mosquitoes. More than 100,000 of them are dumped from plastic containers out the van’s window, and they fly off to find mates.
But these are not ordinary mosquitoes. They have been genetically engineered to pass a lethal gene to their offspring, which die before they can reach adulthood. In small tests, this approach has lowered mosquito populations by 80 percent or more.
The biotech bugs could become one of the newest weapons in the perennial battle between humans and mosquitoes, which kill hundreds of thousands of people a year by transmitting malaria, dengue fever and other devastating diseases and have been called the deadliest animal in the world.
“When it comes to killing humans, no other animal even comes close,” Bill Gates, whose foundation fights disease globally, has written.
The battle has abruptly become more pressing by what the World Health Organization has called the “explosive” spread of the mosquito-borne Zika virus through Brazil and other parts of Latin America. Experts say that new methods are needed because the standard practices — using insecticides and removing the standing water where mosquitoes breed — have not proved sufficient.
“After 30 years of this kind of fight, we had more than two million cases of dengue last year in Brazil,” said Dr. Artur Timerman, an infectious disease expert in São Paulo. “New approaches are critically necessary.”
But the new efforts have yet to be proved, and it would take some years to scale them up to a meaningful level. An alternative to mosquito control, a vaccine against Zika, is not expected to be available soon.
So for now, experts say, the best modes of prevention are to intensify use of the older methods of mosquito control and to lower the risk of being bitten using repellents and by wearing long sleeves.
Women are being advised to not get pregnant and to avoid infested areas if pregnant, since the virus is strongly suspected of causing babies to be born with abnormally small heads and damaged brains.
One old method that is not getting serious attention would be to use DDT, a powerful pesticide that is banned in many countries because of the ecological damage documented in the 1962 book “Silent Spring.” Still, it is being mentioned a bit, and some experts defend its use for disease control.
“That concern about DDT has to be reconsidered in the public health context,” said Dr. Lyle R. Petersen, director of the division of vector-borne diseases at the Centers for Disease Control and Prevention. He said the damage to fish and wildlife stemmed from widespread outdoor use of DDT in agriculture, not the use of small amounts on walls inside homes to kill mosquitoes.
Other experts say the old methods can work if applied diligently.
“We’ve had great success using old methods for the last 50, 60 years,” said Dr. Peter J. Hotez, dean of the National School of Tropical Medicine at the Baylor College of Medicine. “We just need to be very aggressive and exercise political will.”
A determined American doctor named Fred L. Soper eradicated a malaria-carrying mosquito in Brazil in the 1930s, even before the widespread use of DDT. And dengue-carrying mosquitoes were eradicated in 18 Latin American countries from 1947 to 1962, Dr. Hotez said.
But Dr. Soper was a fanatic, making sure every house was thoroughly inspected and all standing water removed. In Brazil, he was backed by the government, which made it a crime to deny entry to an inspector. According to a profile of him in The New Yorker, Dr. Soper used to say that mosquito eradication was impossible in a democracy.
Such an autocratic approach might not be feasible in today’s societies. Moreover, Latin American cities have grown tremendously since then, said Carlos Brisola Marcondes, a medical entomologist at the Federal University of Santa Catarina in Brazil.
“The situation is much worse than it was in the past,” he said.
The main mosquito that transmits Zika virus — and also dengue, chikungunya and yellow fever — is Aedes aegypti, a particularly wily foe.
It prefers urban areas and bites mainly people, making it very efficient at spreading disease. It bites in the day, so bed nets, a common way to protect people against the night-biting malaria mosquitoes, have little effect. It breeds in small containers of water, such as flower pots, cans and tires that collect rainwater.
“I’ve seen Aedes aegypti merrily breeding in discarded soda caps,” said Joseph M. Conlon, technical adviser to the American Mosquito Control Association.
Aedes aegypti is found in the southern part of the United States, so public health authorities say there will be some local transmission of Zika in this country, though it will be far less serious than in Latin America. Dr. Petersen of the C.D.C. said he envisioned “almost a SWAT team approach” in which resources would be rapidly deployed to areas of local transmission to control mosquitoes using conventional methods.
The genetically engineered Aedes aegypti mosquitoes were developed by Oxitec, a British company, to fight dengue, but would also work to curtail the spread of Zika.
Since last April, the mosquitoes have been released in one neighborhood of Piracicaba populated by about 5,000 people. By the end of 2015, there was a reduction in wild mosquito larvae — as opposed to larvae inheriting the lethal gene — of 82 percent, the company said.
Oxitec and the city said this month that they would extend the project for another year and expand it to cover an area of up to 60,000 people. Oxitec is building a new factory to rear enough mosquitoes to cover an area with 300,000 people.
The company, which was acquired last year by the American biotechnology firm Intrexon, calls its creation the “friendly Aedes aegypti” and notes that it releases only male mosquitoes, which do not bite. It says its solution is ecologically friendly because only the one species is targeted, whereas chemical spraying can affect many types of organisms.
But critics worry about the long-term effects of releasing genetically modified organisms. Oxitec has run into public opposition to a proposed test in the Florida Keys.
A Brazilian commission that oversees genetically engineered organisms declared the Oxitec mosquitoes safe to release into the environment in 2014. But Oxitec still does not have a license from Brazil’s health regulators that would allow it to actively market its approach to Brazilian cities.
Still, said Hadyn Parry, the company’s chief executive, with the outbreak of Zika, “We’ve had a huge amount more interest from different municipalities.”
Another approach, being tested in one Rio de Janeiro neighborhood, is to infect the mosquitoes with Wolbachia, a bacterium that does not infect them naturally. Once infected, the mosquitoes do not pick up and transmit viruses as easily.
The bacteria can be passed to the next generation through eggs, so they spread through the mosquito population.
“The beauty of it is it is a sustainable method — once you put it out it sustains itself in the environment and gives ongoing protection,” said Scott O’Neill, dean of science at Monash University in Australia. He is the leader of Eliminate Dengue, a Wolbachia project supported by the Bill & Melinda Gates Foundation and others.
Tests are now underway in Indonesia and Vietnam to see if the technique can reduce the number of people getting dengue fever.
Dr. Paulo Gadelha, president of the Oswaldo Cruz Foundation, a scientific institute under the Brazilian Ministry of Health, said initial results in his country were good and there were plans to try it on a larger scale, in Niterói, a municipality across Guanabara Bay from Rio.
“We are planning to scale this up,” he said. “The mayor has already agreed.”
A new and even more powerful tool may be gene drives, which are genetic mechanisms that rapidly propagate a trait through a wild population. Just in the last few months, scientists have made gene drives that work in mosquitoes in the laboratory.
Anthony A. James, a professor at the University of California, Irvine, said it would be straightforward to use a gene drive to spread something like a sterility trait through the Aedes aegypti population to kill them off.
“We have all the blueprints and have demonstrated proofs of principle,” he said. “It’s just public will to do this.”
The public might not be ready to deploy gene drives outside the laboratory because once a new trait is let loose to spread through the population, it would be difficult to reverse it if something went wrong.
Dr. Petersen of the C.D.C. said of all the new approaches, “We don’t know about the efficacy of any of them on a wide enough scale.” He added, “For now, we’ve got to deal with what we have.”