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 battle between humans and mosquitoes, which kill hundreds of thousands of people a year by transmitting malaria, dengue fever and other 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 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 2 million cases of dengue last year in Brazil," said Dr. Artur Timerman, an infectious disease expert in Sao Paulo. "New approaches are critically necessary."
But the new efforts have yet to be proved, and it would take 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.
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 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.
The main mosquito that transmits Zika virus -- and also dengue, chikungunya and yellow fever -- is Aedes aegypti.
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 night-biting malaria mosquitoes, have little effect.
Aedes aegypti is found in the southern part of the United States, so public health authorities say there will be some transmission of Zika in this country, though it will be far less serious than in Latin America. Petersen 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 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.
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.
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.
Tests are underway in Indonesia and Vietnam to see if the technique can reduce the number of people getting dengue fever.
A new and even more powerful tool may be gene drives, which are genetic mechanisms that rapidly propagate a trait through a wild population. In the past few months, scientists have made gene drives that work in mosquitoes in the laboratory.
Anthony 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.
Petersen 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."
A Section on 01/31/2016