🦟 Bacteria in mosquitoes reduced dengue cases by 70 percent in Singapore

• In a two-year experiment in Singapore, the number of wild mosquitoes decreased by 77 percent in areas where Wolbachia-infected male mosquitoes were released.
• Residents in treated neighborhoods had approximately 70 percent lower risk of developing symptomatic dengue.
• The results from the randomized study are published in The New England Journal of Medicine.

Mosquitoes fighting mosquitoes

Dengue is a mosquito-borne virus that affects millions of people every year. Symptoms can range from flu-like illness to severe bleeding and organ failure. Now a large-scale experiment in Singapore shows that a naturally occurring bacteria can be used to significantly reduce the spread of the disease.

Within Project Wolbachia–Singapore, researchers have used the bacterium Wolbachia, which occurs naturally in many insects, to render disease-carrying mosquitoes harmless. The method involves releasing male mosquitoes of the species Aedes aegypti that carry Wolbachia. Male mosquitoes do not bite humans, but when they mate with wild female mosquitoes that lack the bacteria, the eggs do not hatch. Over time, repeated releases lead to a decline in the mosquito population.

How the study was conducted

The researchers selected 15 large residential areas in Singapore and randomly divided them into two groups. Eight areas received releases of Wolbachia-infected sterile male mosquitoes twice a week. Seven areas served as a control group with no releases. The experiment lasted 24 months, from mid-2022 to late 2024.

To measure the effect, the researchers used two methods. They captured and counted wild female mosquitoes using specialized equipment. They also analyzed national health databases to identify residents who sought medical care and tested positive for dengue.

Sharp decline in mosquitoes and disease cases

In the areas where the mosquitoes were released, the number of wild female mosquitoes decreased by approximately 77 percent – from 0.18 to 0.041 mosquitoes per trap. The decline in mosquitoes was followed by fewer dengue infections. After six months or more, only 6 percent of residents in the treated areas had tested positive for dengue, compared with 21 percent in the control areas.

Overall, exposure to Wolbachia-carrying mosquitoes reduced the risk of dengue by roughly 71 to 72 percent over a period of three to twelve months.

Why new methods are needed

Traditional methods of mosquito control, such as removing breeding sites and using mosquito repellents, can work in the short term but often fail to deliver lasting reductions in dengue cases. Chemical mosquito repellents also pose risks as they can affect human health, harm biodiversity, and disrupt the surrounding environment.

The Wolbachia method works differently. The bacteria prevents the dengue virus from replicating inside the mosquitoes, making them far less capable of spreading the disease. The technique is called Wolbachia-mediated incompatible insect technique combined with sterile insect technique (IIT-SIT). The researchers assess that the method can complement both traditional mosquito control and vaccines to further control the spread of dengue and other diseases carried by Aedes mosquitoes.

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