New study highlights the impact of rapid urbanization on the emergence of zoonotic diseases

Nearly 3.5 billion people live in the messy transition zone between cities and wild places, where agriculture abuts homes; suburbs sprawl into the forest; and humans, wildlife, and livestock readily intermix. This wildland-urban interface (WUI) covers just 5% of Earth’s land surface, but it could provide prime habitat for the transmission of zoonotic diseases from their wildlife hosts to people, according to a new study led by the Yale School of the Environment.

The study is the first to look at how rapid urbanization into the WUI could influence the likelihood of disease spillover and highlights how important limiting animal exposure and wildland encroachment is for disease management, especially as urban populations soar, the researchers said.

The risk is particularly high in the Global South where a diverse array of potential hosts and rapid, informal growth could leave people especially vulnerable. In the next 25 years, the population in urban areas is expected to increase by 2.5 billion, with 90% of that growth predicted in Africa and Asia. The research also emphasizes the need to better understand the diversity and behavior of wildlife hosts—and the pathways through which they interact with people—especially in the tropics.

“The wildland urban interface is the perfect place for diseases to emerge because you have people, livestock, and wildlife in these tightly intermixed land use arrangements,” said Rohan Simkin, a Ph.D. candidate at YSE who led the study. “But there’s plenty of opportunity as cities grow, and this is particularly true in places like Africa and Asia where cities are going to grow rapidly over the next 20 or 30 years, to design cities that avoid a lot of these impacts.”

The study, published in Global Change Biology, mapped the distribution of almost 700 mammals associated with over 100 different diseases across the global WUI.

“Where there is a higher diversity of hosts, you have a diversity of pathogens and more pathways through which people might interact with them,” Simkin said.

The authors were taken aback by just how widespread the species known to carry zoonotic diseases are, Simkin said. For example, the tanezumi rat can spread plague and thrives across entire continents. Over 700 million people live in areas with suitable habitat for 20 or more species, and essentially every person in the WUI lives alongside at least one potential host species, the authors noted.

The researchers identified several hotspots, including areas with large populations, such as parts of China, or particularly expansive WUIs, such as the Northeastern U.S. They found that the vast majority of people living in the areas with the richest assortment of potential wildlife hosts are in lower and middle-income countries in the Global South where biodiversity generally is the highest.

These are the same areas where disease risk can be exacerbated by limited health care, poor sanitation, and informal housing—and where the bulk of future urbanization is expected to happen. However, this is also where data is the most limited, they noted.

“We have this real lack of knowledge around disease ecology in places where people are actually most vulnerable,” Simkin said.

Addressing that gap, and better understanding how—and how often—people and different hosts interact, is essential for mapping the true risk of disease spillover, the authors said.

“The trifecta of urban expansion into wildlands, increased connectivity of people worldwide, and urban growth in places with high pathogens will pose significant disease spillover risks that we are just starting to understand,” said Karen Seto, the Frederick C. Hixon Professor of Geography and Urbanization Science, who co-authored the study.