As forests disappear, mosquitoes are not vanishing with them. Instead, they are adapting in ways that bring them closer to people. A new study published in Frontiers in Ecology and Evolution shows that in Brazil’s rapidly shrinking Atlantic Forest, many mosquito species are increasingly feeding on humans rather than on wild animals. Scientists warn that this quiet behavioural shift could significantly raise the risk of diseases such as dengue, Zika and yellow fever, especially for communities living near forest edges. The findings highlight an often overlooked consequence of deforestation: it can reshape disease dynamics long before outbreaks become visible.

Mosquitoes are adapting to life in a shrinking forest

Stretching along Brazil’s coastline, the Atlantic Forest was once one of the most biodiverse regions on Earth. Today, only about a third of it remains intact. Decades of urban expansion, agriculture and infrastructure development have fragmented the landscape, pushing wildlife into smaller and more isolated patches. While many animals decline or disappear under these conditions, mosquitoes often persist and adapt, exploiting new environments created by human activity.To understand how mosquitoes are responding to these changes, scientists carried out fieldwork in two forest remnants in the state of Rio de Janeiro: the Guapiaçu Ecological Reserve and Sítio Recanto Preservar. These areas represent landscapes where protected forest patches exist alongside human settlements, a common pattern across much of the Atlantic Forest today.The team captured mosquitoes using light traps and focused on female mosquitoes that had recently taken a blood meal, as these meals reveal which hosts the insects are feeding on.In total, the researchers collected more than 1,700 mosquitoes from over 50 species. Among the mosquitoes that had fed recently, DNA analysis revealed a striking pattern: most identifiable blood meals came from humans, not from wildlife. Only a small number of meals were traced to birds, amphibians, rodents or other animals.“This is crucial,” said Jeronimo Alencar of the Oswaldo Cruz Institute, the study’s senior author. In a forest environment that still contains a variety of potential animal hosts, a strong preference for humans greatly increases the chances that mosquitoes will transmit pathogens to people.

Why deforestation pushes mosquitoes toward people

Mosquitoes are highly sensitive to host availability. As forests are cleared and wildlife declines, traditional blood sources become scarcer. Humans, meanwhile, become more abundant and more accessible, especially in newly settled or fragmented landscapes.“With fewer natural hosts available, mosquitoes are forced to seek alternative blood sources,” explained Sergio Machado from the Federal University of Rio de Janeiro, a co-author of the study. “They end up feeding more on humans simply because we are the most prevalent hosts in these areas.”

Rising disease risks at the forest edge

The Atlantic Forest region is home to mosquitoes capable of transmitting a range of viruses, including dengue, Zika, yellow fever, chikungunya and Mayaro. When mosquitoes feed on both wildlife and humans, they can act as bridges, moving pathogens between ecosystems and people.A stronger tendency to bite humans increases the likelihood of outbreaks, particularly in communities living near forest remnants where contact between people and mosquitoes is frequent. Larger studies will be needed to refine the estimates. Even so, the pattern observed is consistent with ecological theory and with findings from other regions experiencing rapid habitat loss.“Knowing that mosquitoes in an area have a strong preference for humans serves as an early warning,” Machado said. It allows public health officials to focus surveillance and prevention efforts where risks are likely to be highest.The study adds to growing evidence that deforestation is not only an environmental issue but also a public health concern. By altering who mosquitoes bite, forest loss can quietly increase disease risk without any immediate or obvious signs.Protecting and restoring forests, researchers argue, may therefore play an important role in reducing future outbreaks. The message is clear: when ecosystems are destabilised, the consequences often reach far beyond the forest, landing directly on human health.