Dengue rates plunged after the release of lab-altered mosquitoes

Mosquitoes infected with Wolbachia bacteria may be associated with a 97% drop in dengue infections in three cities in Colombia's Aburra Valley. 

Researchers for the non-profit World Mosquito Programme released the results at the annual meeting of the American Society of Tropical Medicine and Hygiene in late October. 

The World Mosquito Programme is looking to stop the spread of several 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 Medellíin and Itagua. Although researchers have conducted experiments like this worldwide, these releases marked the programme's largest yet.

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

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

They found that introducing the infected mosquitoes into local mosquito populations was "associated with a significant reduction" in dengue of up to 97% in each city compared to 10 years before 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% drop in dengue in neighbourhoods 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 Programme 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 programme's method by 77%, while in Brazil, the disease burden was reduced by 38% (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 Programme'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, 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

That sounds like good news, and it could be. There are some caveats, however: one is that the World Mosquito Programme'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 Programme 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.

Clare Roth is an editor and reporter focused on science and migration in Berlin. She has worked for Bloomberg, Reuters and local newspapers.

Disclaimer: This article first appeared on Deutsche Welle and is published by a special syndication arrangement.