Can Breeding Millions of Mosquitoes Help Fight the Dengue Surge?

By Jumana Farouky

The mosquito is known as the world’s deadliest creature, spreading diseases that kill more people than any other animal. Traditionally, efforts to protect people from those diseases have focused on vector control, using pesticides and traps to suppress mosquito populations.

But in north Rio de Janeiro, on a campus owned by Fiocruz, one of the world’s leading public health institutions, there is a building dedicated to doing the opposite. Inside the biofactory run by Fiocruz and the World Mosquito Program (WMP), they are making more mosquitoes.

It is the insect equivalent of fighting fire with fire. The biofactory produces tens of millions of Aedes aegypti mosquitoes every week, each one bred to carry Wolbachia, a naturally occurring bacterium that blocks dengue from replicating inside the mosquito’s body and reduces the insect’s ability to spread the disease to humans.

When the mosquitoes are released into the wild, they pass the bacteria on to the next generation of mosquitoes, gradually reducing transmission across the population.

“Wolbachia is very smart in the way it manipulates the reproduction of mosquitoes,” said Luciano Moreira, leader of the WMP’s Wolbachia method project in Brazil.

“If male mosquitoes with Wolbachia mate with a female in the field that does not have the bacteria, the female becomes sterile. And if a female with Wolbachia mates with any male, she will always produce mosquitoes with the bacteria. So, mosquitoes carrying Wolbachia will eventually outnumber those that don’t.”

Driven by climate change and rising urbanization, mosquito-borne diseases are spreading to new areas and re-emerging in regions where they had long been dormant. Commonly used solutions such as insecticides are not sustainable, as they negatively impact the environment and they are losing their efficacy as mosquito populations build up resistance.

After many many countries experienced record rates of dengue in 2024, current trends are showing an overall decline worldwide. But with the disease still present at alarming rates, and some countries expected to see record-breaking dengue numbers yet again this summer, scientists, health groups, and governments are looking for innovative ways to tackle the epidemic.

The Wolbachia method was born when a team that included Moreira was working on a different project with renowned scientist Dr. Scott O’Neill, who later founded the WMP and currently serves as its CEO. At the time, they were using Wolbachia to shorten the mosquitoes’ lifespans and discovered the bacteria could also block the dengue virus.

Moreira brought the method to Brazil’s health ministry in 2011. By then, the program had already shown some success in countries such as Australia, Viet Nam, and Indonesia, which went on to adopt the Wolbachia method as its intervention of choice and saw a 77% reduction in dengue incidence rates in the first decade of use.

Effective not only against dengue but also Zika and chikungunya, the project offers governments an affordable, long-lasting, and environmentally friendly solution to controlling mosquito populations, Moreira said.

“The method’s key advantage is its sustainability,” he said. “Once established, the Wolbachia mosquito population maintains itself, reducing the need for ongoing interventions. Other methods, like insecticides, require repeated applications and can lead to resistance over time.”

Record-breaking dengue rates

According to the World Health Organization (WHO), there are up to 400 million dengue infections every year, most of them unreported. The viral disease has symptoms that include fever, joint and muscle ache, and nausea. In extreme cases, it can lead to death.

In 2024, there were more than 13 million dengue cases reported in the Americas alone, three times the all-time record of the year prior. About half of those cases were in Brazil, which has emerged as a dengue hotspot since 1981, when the disease returned after a 20-year absence.

As summer approaches, the peak season for dengue transmission, predictions for 2025 say some parts of the country are already on track to exceed last year’s numbers.

“Along with the impacts of climate change and urbanization, arboviral diseases such as dengue are expected to increase in the coming years,” said Dr Raman Velayudhan, Head of the Veterinary Public Health, Vector Control and Environment Unit at WHO’s Global Neglected Tropical Diseases Program, which supports Wolbachia-based interventions, including the one implemented by the WMP.

“We need an integrated, affordable approach utilizing innovative and sustainable tools to reduce the burden of cases and severe cases,” he added.

Funded in Brazil by the country’s health ministry, the WMP’s Wolbachia method was first implemented in Rio de Janeiro in 2014 and then nearby Niterói in 2017. Dengue cases in Niterói dropped from 12,000 in 2013 to fewer than 330 cases in total between 2020 and 2023.

Mosquito releases in both those municipalities stopped a few years ago, but WMP data shows that Wolbachia is still present, and still blocking the spread of dengue, in over 90% of the mosquito population.

Now, the WMP program is running in 11 sites in Brazil, and work has just been completed on the world’s largest Wolbachia biofactory in Curitiba. Due to start operating this month, the new factory is expected to produce about 100 million mosquito eggs per week.

Releasing Wolbachia mosquitoes in Niteroi, Brazil. Credit: Flávio Carvalho / WMP Brasil / Fiocruz

Prioritizing community buy-in

In parts of Brazil where people lack access to vaccines, the Wolbachia project can sometimes be their only defense against the disease. But, making sure the program reaches everyone who needs it can prove tricky.

Moreira recalls the time when daily outbreaks of violence in Rio de Janeiro’s favelas made it difficult for the WMP team to keep to the required weekly release schedule. “There was a big complex of villas that our team just could not enter because of the fighting,” he said.

“We worked in partnership with the local health agents, because they live in the favelas and the people there know them. Even then, sometimes the violence was so bad that three weeks would pass before they could release another batch of mosquitoes.”

Moreira’s story underscores the crucial role that community buy-in plays in the success of the Wolbachia method.

And when people have spent their lives killing mosquitoes to keep themselves safe, convincing them that the solution is more mosquitoes is not always easy, said Diogo Chalegre, who was Leader of Government and Institutional Relations at WMP Brazil before moving to Fiocruz’s Vice-Presidency of Production and Innovation in Health in March 2024.

“Some people think it’s crazy,” said Chalegre. “I was presenting the project at a conference one time, and you could see from some people’s expressions that they didn’t believe it could work at all.”

Knowing that the project can face pushback, WMP spends the three or four months prior to a release holding awareness-raising activities at schools, health clinics, and public events. It also recruits volunteers to help release the mosquitoes when the time comes.

“Then the last step before releasing a single mosquito is conducting surveys to gauge awareness, understanding, and acceptance of the project,” Chalegre said. “Approval rates usually range from 87% to 99%, which is fantastic.”

Reaching for sustainable solutions

The Wolbachia method is not the only solution using mosquitoes to curb the spread of disease. The Transmission Zero team at Imperial College London has come up with genetic technology that makes mosquitoes resistant to the parasite that spreads malaria and ensures future generations are resistant, too.

The British company Oxitec creates genetically modified male mosquitoes that carry a self-limiting gene, so when they mate with wild females their offspring die before reaching adulthood.

Along with vector control techniques, dengue vaccines, and Wolbachia, Brazil’s health ministry is looking at adding the sterile insect technique (SIT) to help push back dengue’s rising tide. With SIT, male mosquitoes are sterilized using radiation and then released into the wild. The females they mate with produce no viable offspring, reducing the mosquito population.

All these interventions will need time to bring mosquito populations low enough to put dengue elimination within reach. But until then, they give people a fighting chance against the animal kingdom’s most notorious killer.