• Largest-ever deployment of Wolbachia-infected mosquitoes yields ‘encouraging’ results in three densely populated cities.

Bogotá (Agencies): Three cities in Colombia saw a dramatic fall in the incidence of dengue in the years following the introduction of mosquitoes carrying Wolbachia, a bacterium that prevents the insect from transmitting viruses. In neighbourhoods where the Wolbachia mosquitoes were well established, dengue incidence dropped by 94–97%.

The Aedes aegypti mosquitoes were released by the World Mosquito Program (WMP), a non-profit organization that has been conducting similar experiments in Australia, Brazil, Indonesia and Vietnam, among other countries. In Colombia, the modified mosquitoes were released in one of the country’s most populous regions. “That’s the largest continuous releases of Wolbachia [mosquitoes] globally so far, in terms of the population covered and the area,” says Katie Anders, an epidemiologist at the WMP and Monash University in Melbourne, Australia.

Anders presented the results on 22 October at the annual meeting of the American Society of Tropical Medicine and Hygiene in Chicago, Illinois.

  • ‘Real-world effects’

When infected with Wolbachia, the mosquitoes are much less likely to transmit diseases such as dengue and Zika, because the bacteria compete with these viruses. The insects also pass the bacteria on to their offspring. Researchers hope that the modified mosquitoes will interbreed with the wild population wherever they are released, and that the number of mosquitoes with Wolbachia will eventually surpass that of mosquitoes without it.

The WMP first deployed modified mosquitoes in the Aburrá Valley in Colombia in 2015 and gradually expanded the releases until late 2020. Eventually, the cities of Bello, Medellín and Itagüí, with a combined area of 135 square kilometers and home to 3.3 million people, were completely covered. WMP researchers consider an area to be “fully treated” when more than 60% of the local mosquitoes carry Wolbachia. This goal was achieved in Bello and Itagüí. In Medellín, about half of the city’s territory remained below that level.

When the scientists compared the incidence of dengue in fully treated areas with that in the same regions in the ten years before the intervention, they found that it had dropped by 95% in Bello and Medellín and by 97% in Itagüí. Since the project started, there hasn’t been a large outbreak of dengue in the region. “They’ve had six years now with a sustained suppression of dengue,” says Anders. “We’re starting to see the real-world effect of Wolbachia.”

  • Measurement challenge

It can be tricky to measure the effects of such an intervention on the incidence of dengue because the incidence oscillates naturally over the years. “A reduction in cases following the introduction of an intervention could be a simple coincidence,” says Alex Perkins, an epidemiologist at the University of Notre Dame in Indiana. But he adds that the latest results are “encouraging” and says the longer these patterns are observed, the more likely they are to be a result of the mosquitoes.

The gold standard for evaluating the efficacy of a public-health intervention is randomized controlled trials, Perkin notes. The WMP has conducted one such study in Yogyakarta, Indonesia, in which mosquitoes were released in some areas of a city and the incidence of dengue was compared with that in areas that did not receive the insects. The results suggested that the technology could reduce the incidence of dengue by 77%1. The organization is now conducting a similar one in Belo Horizonte, Brazil.

Despite the positive results, Wolbachia mosquitoes have not yet been officially endorsed by the World Health Organization (WHO). The technology awaits an evaluation by the WHO’s Vector Control Advisory Group.

  • Expansion plans

The WMP intends to scale up the project. Earlier this year, the organization announced plans to build a factory in Brazil to produce modified mosquitoes to be released in many of the country’s urban areas over the next ten years. They still face challenges — for example, the strategy to deploy mosquitoes must be tailored to fit the geographical and social circumstances of each region. Some areas are harder to access than others, and community engagement with the project can vary.

“In Brazil, we work in partnership with local health authorities, and there are cities that don’t have the capacity to help with deployment, so we need to find other partners or even hire people to do the service,” says Luciano Moreira, a specialist in mosquito-borne diseases at the Oswaldo Cruz Foundation in Belo Horizonte and one of the WMP’s collaborators. “When we are planning, we have to see the reality in each location and adapt accordingly.”

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