In 2003, the first mpox outbreak in the United States was linked to a shipment of exotic rodents delivered to a pet store in Illinois. Gambian giant rats, among other animals, infected prairie dogs, which then passed the virus to nearly 100 humans who interacted with them.
Ebola outbreaks frequently occur after humans come into contact with bats, sometimes consumed as food or used in traditional medicine.
And, of course, there are suggestions that the COVID-19 pandemic began at the Huanan Seafood Wholesale Market, where various wild animals—raccoon dogs, civets, and Himalayan marmots—were kept in cramped conditions.
Such anecdotes illustrate the potential dangers of the wildlife trade, whether it’s buying wild animals for consumption or capturing them for pets, as they can facilitate the transmission of pathogens from animals to humans. A single interaction could lead to widespread illness and potentially millions of deaths.
“There has long been agreement that wildlife trade poses a threat to human health,” says Colin Carlson, a disease ecologist at Yale. “However, much of our understanding is based on anecdotes.”
This patchy understanding complicates the assessment of the actual risk posed by wildlife trading compared to other factors driving the increase in infectious diseases, such as climate change and deforestation. While it seems logical that traded animals might carry pathogens more frequently, the absence of concrete data has made definitive conclusions elusive.
Now, Carlson and his colleagues have provided some clarity. Their research indicates that mammals involved in the wildlife trade are about 1.5 times more likely to harbor pathogens that infect humans compared to non-traded species. Notably, the duration of human interaction with a species correlates with an increasing number of shared viruses, particularly in cases of illegal animals and live markets.
“This study supports our existing beliefs,” notes Kevin Olival, a disease ecologist at the University of Hawai’i who wasn’t part of the research. “The wildlife trade indeed contributes to zoonotic disease risks. It has played a role in past outbreaks, possibly including COVID-19, and to prevent future ones, we must adopt a global perspective.”
An atlas of pathogens
Carlson notes that five years ago, conducting a study like this wouldn’t have been feasible. “The relevant data regarding animals and viruses simply wasn’t available,” he says, until now.
His team developed databases capable of cataloging newly identified viruses. By aligning this pathogen atlas with wildlife trade data—specifying which mammals are traded and how long they’ve been in that situation—they could identify which species share the most pathogens with humans.
The findings, while not entirely surprising, were notable. Among over 2,000 traded species, 41% shared at least one pathogen with humans, in contrast to merely 6.4% of non-traded species.
While the shared pathogens don’t reveal who is responsible for transmission, it’s likely that most of these instances involve viruses jumping from animals to humans, rather than the reverse, emphasizes Carlson. “Humans are everywhere. We make contact with numerous species, and we tend to acquire far more than we give back.”
The researchers identified specific wildlife trade aspects that appear particularly prone to facilitating viral transmission.
“Live animal markets pose a significant risk,” states Carlson.
“We’re dealing with animals that are often unhealthy, crowded, and mixed with various species,” he adds. “Viruses are actively evolving in these markets as they move across species. And workers in these settings usually lack adequate protective gear.”
The illegal wildlife trade, which includes endangered animals like pangolins, also reflects a heightened risk of pathogen spillover. This may be due to these species harboring more viruses or subpar hygiene practices in illicit markets, posits Carlson.
The study also highlights the importance of time. For every decade a species spends in the wildlife trade, another new pathogen is more likely to infect humans. “That’s significant,” Olival remarks, although he questions whether this trend may be influenced by advancements in pathogen detection.
There are countless species that have been traded for many years or hunted for generations. “It feels like we’ve opened Pandora’s box,” he continues, suggesting that these animal viruses are now integral to our world.
Risk reduction
These findings indicate that proactive measures could help mitigate risks, asserts Sagan Friant, a disease ecologist at Penn State who wasn’t involved in the study. “This research directs our focus to potential strategies for breaking transmission avenues from animals to humans. If we successfully manage those routes, many pathogens could be intercepted.”
On a global scale, this might require governments to intensify efforts against the illegal wildlife trade, such as enhancing security measures at airports.
Yet, Carlson warns that such actions could force more of the trade underground, complicating detection efforts for potential spillovers. “We must choose between making trade illegal and pushing it into the shadows or finding ways to implement public health measures in these environments.”
Another approach could involve targeting the demand for these exotic species, which generates significant revenue.
“Even if it feels like you’re not part of the wildlife trade, consider this: every citizen actually plays a role in it,” Olival explains. The midwest mpox outbreaks in 2003 were the result of people purchasing animals. So, next time you see that adorable exotic pet in a store, maybe take a moment to reconsider.”
