A pioneering public health program in a Brazilian city is successfully suppressing dengue fever, offering a beacon of hope as the nation and the wider world grapple with an explosive outbreak of the mosquito-borne illness. While neighboring Rio de Janeiro declared a public health emergency with tens of thousands of cases, the city of Niterói has maintained dramatically low infection rates. This success has prompted Brazil’s government to adopt the strategy as a national public policy, with plans for a massive expansion.
The innovative method involves releasing large numbers of Aedes aegypti mosquitoes that have been intentionally infected with a bacterium called Wolbachia. This naturally occurring microbe essentially renders the mosquitoes incapable of transmitting the dengue virus to humans. As the modified insects, nicknamed “wolbitos,” breed with the wild population, the bacterium spreads, creating a sustainable, long-term defense against the disease that also proves effective against other viruses like Zika and chikungunya.
A Tale of Two Cities
The contrast between Niterói and its famous neighbor, Rio de Janeiro, just across the Guanabara Bay, is stark. Amid a recent outbreak, Rio recorded an incidence rate of more than 700 confirmed dengue cases per 100,000 people, totaling over 42,000 infections and forcing the city to open dedicated dengue health centers. In Niterói, a city of half a million people, the incidence rate was just 69 per 100,000, with around 400 suspected cases during the same period. This remarkable difference is attributed to the city’s early and comprehensive adoption of the Wolbachia method.
Niterói was the site of a pilot project with the World Mosquito Program (WMP) that began in 2015 and became the first Brazilian city to have the protective mosquito population fully established across its territory. A 2021 study had already demonstrated the program’s impact, associating it with a 69% reduction in dengue cases. The city’s mayor, Axel Grael, sought out the partnership after a severe epidemic in 2012, and he now credits the technique for the city’s “much better results” and the sustained absence of major outbreaks.
The Microbe Behind the Method
The key to the program’s success is Wolbachia, a bacterium that is naturally present in about half of all insect species, though not typically in the Aedes aegypti mosquito, the primary vector for dengue. Scientists discovered that when the bacterium is introduced into this mosquito species, it competes with viruses like dengue, Zika, and chikungunya, making it much harder for the viruses to replicate inside the mosquito. This effectively blocks the transmission pathway to humans.
From Lab to Neighborhood
The process is managed in Brazil by the renowned Fiocruz Institute in partnership with the WMP. In a laboratory, scientists inject Wolbachia into mosquito eggs. These eggs are then hatched, and the resulting adult mosquitoes carry the bacterium. These modified mosquitoes are released in targeted neighborhoods, where they breed with the local wild mosquitoes. Because an infected female passes the bacterium to all her offspring, the percentage of Wolbachia-carrying mosquitoes in the local population steadily increases. Over time, the vast majority of mosquitoes carry the bacterium, providing lasting protection for the human residents. Before releases begin, public health officials engage in community outreach to explain that residents should expect to see more mosquitoes temporarily and that local insecticide spraying must be halted for the method to work.
A National Strategy Emerges
Given the overwhelming success in Niterói and other trial locations, Brazil’s Ministry of Health has announced it will be the first country to adopt the Wolbachia method as a public policy tool. The strategy is seen as a crucial medium- to long-term solution to curb the escalating crisis of mosquito-borne diseases. The government is now funding a significant expansion of the program, which has already been introduced in five cities, protecting over 3 million people.
Building a Mosquito Factory
To support this nationwide deployment, a new, large-scale biofactory has been inaugurated in Belo Horizonte, operated by Fiocruz. Beginning in 2024, the facility is projected to produce five billion Wolbachia-infected mosquitoes per year, eventually ramping up to 100 million per week. This industrial scale is essential for the planned expansion, which will begin with releases in six new municipalities across five states in mid-2024, with another 22 municipalities slated to join the following year. The expansion aims to protect an additional 1.7 million people initially.
Global Implications of a Local Success
The World Health Organization reports a dramatic rise in dengue cases since 2000, and it is estimated that half of the world’s population is now at risk. The success in Brazil is therefore being watched closely around the globe. The Wolbachia strategy was first pioneered by the WMP in Australia in 2011 and has since been tested in over a dozen countries. A randomized controlled trial in Yogyakarta, Indonesia, demonstrated a 77% reduction in dengue incidence, providing further robust evidence of the method’s effectiveness.
Experts praise the method as a safe, sustainable, and effective tool for controlling viral transmission. While challenges remain, including logistical hurdles in deploying the mosquitoes in hard-to-reach urban areas like violence-plagued favelas, the low-cost and self-sustaining nature of the intervention is highly appealing to public health authorities worldwide. As climate change expands the geographical range of Aedes mosquitoes, the lessons learned in Niterói may become a critical blueprint for protecting millions more from this persistent global threat.