[Reading Science] Billions of 'Sterile Mosquitoes' Released to Control Dengue Fever

Global Mosquito Program Pilot Project in Brazil
Large-scale Release of Bacteria-infected Sterile Male Mosquitoes
Reducing Total Mosquito Population to Control Dengue and Other Diseases

There is a saying, "An eye for an eye, a tooth for a tooth." Scientists have completed the development of a technology to eradicate disease-carrying mosquitoes such as those spreading dengue fever by using 'sterile mosquitoes' infected with a specific bacterium, and have begun large-scale pilot control in Brazil, attracting attention.

According to the international academic journal Nature on the 17th, the non-profit foundation 'World Mosquito Program (WMP)' recently announced plans to release billions of sterile male mosquitoes infected with a specific bacterium annually in urban areas of Brazil for ten years starting next year to prevent dengue fever. Previously, WMP conducted research over several years in Brazil, Australia, Colombia, Indonesia, Vietnam, and other countries to control Aedes aegypti mosquitoes that transmit tropical infectious diseases such as dengue fever using the Wolbachia bacterium. Male mosquitoes infected with this bacterium become sterile, so even if they mate with female mosquitoes and lay eggs, the eggs do not hatch. Naturally, as the number of such sterile mosquitoes increases, the overall mosquito population decreases, reducing the transmission of infectious diseases. In 2016, a county government in California, USA, together with a private company, developed this method and achieved a 95% reduction in the Aedes aegypti mosquito population. Subsequently, Verily, the life sciences division of Alphabet, Google's parent company, joined and demonstrated successful experimental results in several locations including Australia.

WMP, together with its Brazilian partner, the Oswaldo Cruz Foundation, will conduct a large-scale pilot mosquito control project scaling up these research results starting in 2024. They plan to produce and release up to 5 billion Wolbachia-infected sterile male mosquitoes annually. Brazil is one of the countries with the highest incidence of dengue fever worldwide, with more than 2 million reported cases each year.

However, scientists note that despite the successful results so far, this large-scale release experiment faces several challenges. Although this bacterium can infect half of all insect species, Aedes aegypti mosquitoes are generally not easily infected. However, after confirming that male Aedes aegypti mosquitoes infected with Wolbachia become sterile and reduce mosquito populations, thereby lowering disease incidence, a separate development was made.

Overcoming regional disparities is also a challenge. In Indonesia, which has extensive tropical rainforests, dengue fever incidence decreased by 77%. In Brazil, experiments were conducted in five cities; in the Niter?i area, dengue incidence dropped by 69%, while in Rio de Janeiro it decreased by 38%. This is presumed to be due to environmental factors. For example, the greater the wild mosquito population, the more effective the release of Wolbachia-infected male mosquitoes was. Conversely, in large urban areas like Rio de Janeiro, distrust among local residents hindered mosquito releases, resulting in somewhat lower effectiveness.

Accordingly, WMP plans to focus on releasing as many Wolbachia-infected male mosquitoes as quickly as possible using drones, motorcycles, and cars. However, the impact of mosquito population reduction on the general natural ecosystem remains unknown.

Brazilian health authorities have also approved this bacterium-infected sterile mosquito control technology. However, the World Health Organization (WHO) has not yet approved it. The approval will be discussed at the Vector Control Advisory Group meeting scheduled for the end of this month. Luciano Moreira, chief researcher at the Oswaldo Cruz Foundation, pointed out, "Even if this bacterium-infected mosquito control is successful, other public health measures such as vaccines must continue to be used," adding, "Sterile mosquito control is only a complementary measure and integrated control policies must be mobilized."