Ximena Solorzano, Staff Photographer
A stand of pines envelops a lonely log cabin. A tropical forest provides a dappled canopy of light for a remote village. A wide grassy valley offers a priceless view for a mountainside community.
These regions where wilderness meets human architecture are not only aesthetically pleasing but offer a sense of closeness to the planet. Like the Earth’s beauty is palpable. But, it can also be cause for concern.
Over 3.5 billion people live in the wildland-urban interface, or WUI — the zone where cities meet and intermingle with unoccupied nature. Today, the WUI covers five percent of the planet and is growing as a result of urbanization. Some people reside in the WUI for leisure, vacationing in picturesque mountainside homes, and others, like the over one billion people who live in slums and informal settlements, have no other choice.
A Yale study published in Global Change Biology found that zoonotic, or animal-borne, disease transmission in these regions is on the rise.
“Our paper highlights a neglected risk factor for the emergence of zoonotic diseases,” Rohan Simkin ENV ’24, the lead author of the paper, wrote to the News. “These [WUI] regions are predicted to undergo rapid urban population growth in the coming decades, which will increase the physical footprint of cities, potentially expanding the risks associated with exposure to zoonotic diseases.”
Simkin conducted the study as a doctoral student with Karen Seto, the professor of geography and urbanization science.
As the human population grows, urbanization is expanding into wildlands. Seto’s research explores how this affects the planet.
“This is an exciting study as it’s the first one to examine how urbanization and the growth of the wildland-urban interface will shape zoonotic disease exposure,” Seto told the News.
The complexity of these landscapes means that urban populations live with or near livestock and wild animals, creating ideal conditions for diseases to jump from animals to humans, known as spillover.
Spillover can occur in hunting populations that sell bushmeat products in urban markets. Or it may occur where livestock become infected via interaction with wildlife hosts — like with the recent avian flu epidemic — and then pass pathogens to humans, either via direct contact or through consumption of animal products.
The WUI also puts humans in closer proximity with insect vectors like mosquitos and ticks that can bring pathogens from wild animals to humans.
“There are many different pathways, but the key thing to remember here is that WUIs are sites where urban populations are concentrated into areas with high land-use complexity, creating opportunities for novel and frequent human-animal interactions that could lead to spillover,” Simkin wrote.
While populations living in poverty or higher-density slums are most at risk, a zoonotic disease can spread across populations and pose a pandemic threat.
“Based on the weather change and the busy global traveling, it is almost undoubted that there will be more zoonotic disease in the near future,” Dr. Yu-Min Chuang, who was not involved in the study, said.
As a researcher in the School of Medicine Infectious Disease Department, Chuang studies the interaction between human hosts, mosquitos and malaria parasites. Chuang is concerned that, with the rise in zoonotic diseases capable of causing death, we won’t have the capacity to deal with them.
The COVID-19 pandemic, Chuang noted, was only dealt with due to modern biomedical genetic techniques and the novel mRNA vaccine.
“The recent [COVID-19] pandemic demonstrated the risk of new viral diseases,” Simkin said. “So there is an ongoing risk that a new animal borne virus will emerge, such as COVID, SARS etc.”
Simkin listed a few diseases of concern, including Ebola, which is clearly linked to human-wildland interactions, and yellow fever, which is transmitted by mosquitoes from wild primates into urban human populations.
“We do need the continuation of investment to help us combat unknown infectious disease,” Chuang said. “[With] global warming and climate change, we do need to pay attention to zoonotic or other infectious diseases. They will soon or later affect all of us.”
Mosquitos cause 700,000 deaths annually.