The biotech bugs could become one of the newest weapons in the perennial battle between humans and mosquitoes, which have been called the deadliest animal in the world because of the diseases they transmit.
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.
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The battle has abruptly become more pressing by what the World Health Organization (WHO) 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 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 by 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 the use of 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 Petersen, director of the division of vector-borne diseases at the Centers for Disease Control and Prevention (CDC). 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 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.”
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 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.
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.
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 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.
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.
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 ONeill, 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 under way in Indonesia and Vietnam to see if the technique can reduce the number of people getting dengue fever.