Dengue rates plunge after release of lab-altered mosquitoes

The world sees up to 100 million cases and 22,000 deaths due to dengue every year. Researchers may have found a way to cut those numbers significantly.

The Aedes aegypti mosquito can be recognized by its polka-dot covered legs. It can spread dengue fever, chikungunya, Zika fever and yellow fever, among other viruses | Photo Credit: Image: H. Schmidbauer/blickwinkel/picture alliance

Mosquitoes infected with Wolbachia bacteria may be associated with a 97 per cent drop in dengue infections in three cities in Colombia’s Aburra Valley. Researchers for the non-profit World Mosquito Program released the results at the annual meeting of the American Society of Tropical Medicine and Hygiene in late October.

The World Mosquito Program is looking to stop the spread of a number of life-threatening vector-borne diseases—among them dengue, Zika virus and yellow fever—by dispersing millions of Wolbachia bacteria-infected mosquitoes in places where these diseases are common.

How does the Wolbachia bacteria prevent dengue?

The Wolbachia bacteria significantly reduces Aedes aegypti mosquitoes’ ability to spread disease. Aedes aegypti is one of the most notorious spreaders of harmful vector-borne diseases.

After a 2015 pilot release in the Colombian city of Bello, the researchers expanded their operations to the nearby cities of Medellín and Itagua. Although researchers have conducted experiments like this across the world, these releases marked the programme’s largest yet.

By April 2022, they found that around 80 per cent of all mosquitoes in Bello and Itagui had been infected by the Wolbachia mosquitoes (through cross-breeding), and around 60 per cent in Medellín.

To see whether this infiltration had actually impacted dengue levels in the three cities, the researchers evaluated the number of cases reported over the course of the releases until July 2022.

They found that the introduction of the infected mosquitoes into local mosquito populations was “associated with a significant reduction” in dengue of up to 97 per cent in each city compared to ten years prior to the start of the experiment. They also conducted a case control study in Medellín. There, they said they found a causal association between the deployment of the infected mosquitoes and reduced cases of dengue.

The researchers said results showed a 47 per cent drop in dengue in neighborhoods where the mosquitoes had been released. They added that this was the largest contiguous implementation of these infected mosquito releases. The positive results “highlight the operational feasibility and real-world effectiveness of… deployment in large urban settings, and the reproducibility of the public health benefit across different ecological settings.”

One and done?

Although the Colombia releases mark the largest yet conducted, World Mosquito Program researchers have been facilitating similar experiments around the world. Earlier studies found that in Yogyakarta in Indonesia, for example, dengue cases were reduced through the program’s method by 77 per cent, while in Brazil, the disease burden was reduced by 38 per cent (so far).

Scientists are currently testing many ways to eliminate vector-borne diseases like dengue. Experts say the “one-and-done” nature of the World Mosquito Program’s method is a plus.

“Once you introduce the Wolbachia mosquitoes into the native mosquito populations, they stay there. You don’t have to release more mosquitoes,” Biologist Rafael Maciel de Freitas, who works at the Brazilian Oswaldo Cruz Foundation and the Bernhard Nocht Institute for Tropical Medicine, told DW in April 2023.

However, Freitas told DW that there is concern the method won’t work forever, given the high possibility the dengue pathogen will find a way to adapt to—and circumvent—the Wolbachia bacteria. “The virus will probably find a way to overcome the Wolbachia effect,” Freitas said. “I wouldn’t say the Wolbachia method is the solution to dengue, but I think we have a better answer to the disease this way,” Freitas added.

More work to be done

Sounds like good news, and it could be. There are some caveats, however: one is that the World Mosquito Program’s methods are expensive to implement. And at this point, it is still unclear whether the dengue declines detected in Colombia and elsewhere can be attributed solely to the Wolbachia-infected mosquitoes. Dengue comes in waves—sometimes a city in a dengue-prone area will go years without seeing an outbreak.

Finally, there are certain areas where the Wolbachia-infected mosquitoes don’t seem to cause a decline in dengue cases, or cause a much smaller decline than in other areas. Scientists still aren’t sure what makes some areas appear resistant to the approach and others not.

The World Mosquito Program is looking to scale up operations in the coming decade—it announced plans earlier this year to build a factory in Brazil that would infect some 5 billion mosquitoes with Wolbachia annually.