Mosquitoes infected with bacteria reduce the incidence of dengue and chikungunya in RJ

In August 2017, the Sheet reported the release of millions of mosquitoes Aedes aegypti with the bacteria Wolbachia in Rio de Janeiro as a way of trying to reduce the transmission of diseases in the city. Now, an article published in the scientific journal The Lancet Infectious Diseases shows that the initiative has had an effect. According to the researchers, the release of insects is associated with an average reduction of 38% in the incidence of dengue and 10% in the incidence of chikungunya.

From August 2017 to December 2019, 67 million mosquitoes infected with Wolbachia in 28,489 locations, covering an area of ​​86.8 km² in the state capital, where around 890,000 people live. The release was made every 50 meters, with about 100 infected mosquitoes in each of the 658,179 releases.

During and after the release period, the project team deployed 1,168 traps in the drop zones and checked how many of the Aedes aegypti captured had the bacteria. Approximately 33.8% of insects collected between 1 and 29 months after initial release tested positive for the microorganism.

The researchers cross-referenced the release and capture information with the detailed geographic record of dengue and chikungunya cases in the city before, during and after the releases and applied mathematical models to relate cases and locations, thus managing to estimate the effects of the project.

They observed that mosquitoes with Wolbachia were not established uniformly in all five release zones and this influenced the incidence of diseases. In places where the prevalence of mosquitoes with Wolbachia in the population of Aedes aegypti was less than or equal to 10%, the reduction was 7% in the incidence of dengue and 2% in the incidence of chikungunya, while in those with a prevalence greater than 60%, the decrease in cases of the diseases was 71% and 23%, respectively. In other words, the higher the prevalence, the greater the protection.

Scientists do not know, however, why the establishment of mosquitoes occurred differently and why the effects were better in dengue cases than in chikungunya cases. These aspects must continue to be investigated.

“We are very pleased with the result. Our partners in Rio de Janeiro shared the data on the geolocation of arboviruses with researchers from the University of Cambridge and they were able to carry out robust analyzes of climate, temperature, geolocation and intervention of the areas with Wolbachia compared to adjacent areas”, says Fiocruz researcher Luciano Moreira, co-author of the work and leader of the WMP (World Mosquito Program) Brazil.

With actions in 12 countries and funding from the Bill & Melinda Gates Foundation and the European Research Council, WMP has projects in five Brazilian cities: Rio de Janeiro, Niterói, Campo Grande, Belo Horizonte and Petrolina. Here, the program has the support of the Ministry of Health and is led by Fiocruz.

Moreira explains that in all partner municipalities there is a long work process. The first step is to engage the population. The project team and local agents explain to the population that viruses that cause diseases such as dengue cannot establish themselves in mosquitoes with Wolbachia. They also say that it is a natural bacterium, found in insects such as the fruit fly, and that it passes from generation to generation among infected animals.

“We no longer introduce the bacteria into eggs. This happened in 2009 and 2010, in Australia. In 2011, we brought inoculated insects and crossed them with mosquitoes from Brazil. Since then, we have a local population that contains Wolbachia and just do large-scale production. Today, our production is around 10 million eggs per week”, says Moreira.

The engagement can last from four to six months, depending on the location, and the group conducts interviews and applies questionnaires to verify that the population gives the approval for the release of mosquitoes. Meanwhile, large-scale production takes place.

The next phase is the release phase, which takes four to five months. There are weekly releases of males and females that are interbred to increase the natural prevalence of mosquitoes with the bacterium. Finally, monitoring with traps takes place to check if the Wolbachia is present in the eggs and animals collected and the analysis of data on cases of dengue, zika or chikungunya.

“The Wolbachia method is complementary and throughout the program we ask the population not to change their routine. Vector control actions in the municipality continue and people continue to do their homework to reduce breeding sites”, he highlights.

As the scale of the process varies according to the city, the facilities and the number of employees assigned by the city halls, Moreira says that it is difficult to estimate the current cost —a few years ago, it was just over R$20 per inhabitant — but he compares the initiative to infrastructure projects, in which the benefits for society remain even over the years. “We have areas where the release took place seven years ago and the mosquitoes with Wolbachia stay there”.

According to the researcher, more than 30 cities have already shown interest in adopting the program, but it is still not possible to serve them. “It’s a slow process, it takes time. It would be a case of covering the areas little by little and following the technical, epidemiological point of view, to prioritize locations with a history of arboviruses and have a greater impact”, he says.

good mosquitoes

Another initiative in the country is the Aedes do Bem project, in which people, companies and municipalities can purchase boxes with Aedes aegypti with a self-limiting characteristic that prevents the proliferation of females, responsible for the transmission of diseases.

Director of Oxitec do Brasil, responsible for the product, Natalia Verza Ferreira says that pilots were carried out in Bahia and Minas Gerais and, more recently, in the São Paulo cities of Piracicaba and Indaiatuba. It estimates sales growth of 300% in 2022 compared to 2021.