Breeding Special Mosquitoes to Fight Dengue

25 September 2023

For many years, preventing dengue fever in Honduras has meant teaching people to avoid mosquito bites. Now, Hondurans are being taught a more effective way to control the disease — and it goes against everything they have learned.

Recently, a small group of people cheered as Hector Enriquez freed mosquitoes from a glass jar. Enriquez did that to bring attention to a program to suppress the viral disease dengue fever. He did so by releasing millions of special mosquitoes in the Honduran capital of Tegucigalpa.

Scientists bred the mosquitoes. The small insects carry bacteria called Wolbachia that stop the spread of the disease. When these mosquitoes reproduce, they pass on the bacteria and reduce future outbreaks.

Lourdes Betancourt prepares a jar containing mosquito eggs to hang from a tree in her yard, in Tegucigalpa, Honduras, Aug. 23, 2023. The mosquitoes that hatch will carry bacteria called Wolbachia that interrupt the transmission of dengue. (AP Photo/Elmer Martinez)
Lourdes Betancourt prepares a jar containing mosquito eggs to hang from a tree in her yard, in Tegucigalpa, Honduras, Aug. 23, 2023. The mosquitoes that hatch will carry bacteria called Wolbachia that interrupt the transmission of dengue. (AP Photo/Elmer Martinez)

The nonprofit World Mosquito Program based in Australia launched the new plan for battling dengue. And it is being tested in more than 10 countries. With more than half the world's population at risk of dengue infection, the World Health Organization is watching the mosquito releases in Honduras and other places closely in order to expand the plan worldwide.

In Honduras, about 10,000 people are known to be sickened by dengue each year. The health organization Doctors Without Borders is partnering with the mosquito program over the next six months to release 9 million mosquitoes carrying the Wolbachia bacteria.

"There is a desperate need for new approaches," said Scott O'Neill, founder of the mosquito program.

Dengue infection rising

Scientists have made progress in recent years to reduce the threat of infectious diseases, including diseases spread by mosquitos, like malaria. But dengue is the exception: Its rate of infection keeps going up.

Public health studies estimate that around 400 million people across about 130 countries get infected each year with dengue. An estimated 40,000 people die each year from it.

Traditional methods of preventing mosquito-borne illnesses have not been effective against dengue.

The Aedes aegypti mosquitoes that most commonly spread dengue are resistant to insecticides. They are most active during the day when people are not protected by bed nets. And the dengue virus is harder to control through vaccines.

"We need better tools," said Raman Velayudhan, a researcher from the WHO's Global Neglected Tropical Diseases Program. "Wolbachia is definitely a long-term, sustainable solution."

Velayudhan and other experts from the WHO plan to promote further testing of Wolbachia bacteria in other parts of the world.

Wolbachia bacteria

The Wolbachia bacteria exist naturally in about 60 percent of mosquitoes but not in the Aedes aegypti.

About 40 years ago, scientists aimed to use Wolbachia to drive down mosquito populations. Because male mosquitoes carrying the bacteria only produce offspring with females that also have it, scientists would release infected male mosquitoes into the wild to breed with uninfected females.

But they made a surprising discovery: Mosquitoes carrying Wolbachia did not spread dengue — or other related diseases, including yellow fever, Zika and chikungunya. And since infected females pass on Wolbachia to their offspring, they will eventually "replace" a local mosquito population with one that carries the virus-blocking bacteria.

This involves a change in thinking. Scientists are now replacing mosquitoes instead of killing them.

O'Neill's laboratory first tested the replacement plan in Australia in 2011. The World Mosquito Program has run tests affecting 11 million people across 14 countries, including Brazil, Mexico, Colombia, Fiji and Vietnam.

In 2019, a large field test in Indonesia showed a 76 percent drop in reported dengue cases after Wolbachia-infected mosquitoes were released.

Scientists are still not sure how Wolbachia bacteria blocks the spread of the virus. And it is not clear whether the bacteria will work equally well against all versions, or strains, of the virus. It also is not clear if some strains might become resistant over time.

Bobby Reiner is a mosquito researcher at the University of Washington. He said, "It's certainly not a one-and-done fix, forever guaranteed."

Raising Wolbachia-infected mosquitoes

The World Mosquito Program breeds 30 million mosquitoes a week at a factory in Medellín, Colombia. Once workers confirm that the new mosquitoes are infected with Wolbachia, their eggs are dried and sent off to test areas for release.

The Doctors Without Borders team in Honduras recently went around Tegucigalpa to enlist local help in incubating mosquito eggs bred in the Medellin factory.

They received permission to hang glass jars from tree branches that contained water and mosquito eggs. After about 10 days, the mosquitoes would hatch and fly off. That same day, workers from Doctors Without Borders released thousands of dengue-fighting mosquitoes into the air.

Some of the most common questions from the community were about whether Wolbachia would harm people or the environment. Workers said that the bites from the special mosquitoes were harmless.

María Fernanda Marín is a 19-year-old student working for Doctors Without Borders. She showed neighbors a photo of her arm covered in bites to help earn their trust.

Lourdes Betancourt is a 63-year-old volunteer with the Doctors Without Borders team. She had been sickened by dengue several times and was not sure about the new plan at first. But Betancourt now urges her neighbors to let the "good mosquitoes" grow in their yards.

"I tell people not to be afraid, that this isn't anything bad, to have trust," Betancourt said. "They are going to bite you, but you won't get dengue."

I'm Faith Pirlo. And I'm Bryan Lynn.

Maria Verza and Maddie Burakoff reported this story for the Associated Press. Hai Do adapted it for VOA Learning English.


Words in This Story

jar –n. a glass container that usually comes with a screw on top

breed –v. to raise and keep animals or plants to produce more of the same kind

borne –adj. carried by or spread by

insecticide –n. a chemical poison used to kill insects

bed net –n. a material that is woven to let air through but to keep things like insects out

offspring –n. the young of a plant or animal

one-and-done –adj. (informal) done once and not again

incubate –v. to keep eggs or very young living things in a condition that is good for growth and development

hatch v. to be born from an egg