In 2003, a shipment of exotic African rodents to a pet store in Illinois sparked the United States' first mpox outbreak. Gambian giant rats and other rodents infected prairie dogs, which in turn infected nearly 100 people who handled the animals.
Ebola outbreaks are often triggered after contact with bats, which are sometimes eaten or used for traditional medicine.
And more famously, a string of scientific papers suggest the COVID-19 pandemic originated at the Huanan Seafood Wholesale Market, where scores of live wild animals — racoon dogs, civets, Himalayan marmots — were housed in cramped quarters.
Anecdotes like these are examples of how the wildlife trade — anything from purchasing wild animals for food to trapping them for pets — can open an avenue for pathogens to jump from animals to humans. A single fateful encounter can lead to hundreds, thousands, even millions of deaths.
"There's been a consensus for a long time that the wildlife trade is a risk to human health," says Colin Carlson, a disease ecologist at Yale University. "But a lot of what we know is from anecdotes."
That patchy view makes it hard to understand how risky the wildlife trade is compared to other causes of the uptick of infectious diseases, says Carlson, like climate change or deforestation. While it makes sense that traded species would infect humans more often than non-traded species, scientists couldn't definitively answer the question without more data.
Now, Carlson and his colleagues offer an answer. Traded mammals are about 1.5 times as likely to be sources of human diseases than non-traded animals, the researchers report in Science. Crucially, the longer humans have been interacting with a species, the more viruses we have in common — especially when dealing with illegal animals and live markets.
"It's a really strong paper that reinforces what we've already kind of known," says Kevin Olival, a disease ecologist at the University of Hawai'i who wasn't involved in the study. "The wildlife trade is indeed a risk for zoonotic diseases. It's driven past outbreaks, including likely COVID-19, and if we want to prevent the next one, we need to think about this globally.
An atlas of pathogens
Five years ago, a study like this wasn't really possible, says Carlson.
"The data that we have here, on animals and their viruses [and other pathogens] didn't exist," he says, at least not in a way that researchers could easily analyze. Carlson and his colleagues changed that.
They built databases to which they can add every newly discovered virus. By lining up this pathogen atlas with data on the wildlife trade — which mammals are traded and how long a species has been involved — the team could see which mammals share the most pathogens with humans.
The results, while not a total surprise, were striking. Out of more than 2,000 traded species, 41% shared at least one pathogen with humans, compared to just 6.4% of non-traded species.
Showing that pathogens are shared doesn't say anything about who did the sharing, but it's likely that the vast majority of these cases stem from pathogens jumping from animals to humans, and not the other way around, says Carlson. "Humans are ubiquitous. We are in contact with everything, and we are picking up a lot more stuff than we're putting down."
Certain parts of the wildlife trade researchers already see as risky seem especially conducive to picking up viruses, the researchers found.
"Live animal markets are a major risk factor," says Carlson.
"We're talking about animals in poor health, crowded conditions, weird combinations of species," he says. "We know that viruses are evolving in real time in these markets as they move between species." The people who work in these markets often don't have the kind of protective gear that could stop pathogens.
The illegal wildlife trade, which includes endangered or protected animals like pangolins and squirrel monkeys, was also associated with an elevated risk of spillover. That could be because such species house more viruses or illegal markets could be even more lax in hygiene, says Carlson.
Finally, the team found that time matters. For every ten years a species spends in the wildlife trade, another new pathogen is jumping over into humans, the study found. "That's significant," says Olival, and makes sense, though he wonders whether that trend could be driven by improved pathogen detection.
Altogether, there are hundreds of species that have been traded for decades or hunted for millennia. "That's a one-way ticket. In a lot of ways, the toothpaste is out of the tube here," he says, and these animal viruses are with us to stay.
Risk reduction
The results also highlight that taking action now can reduce risk, says Sagan Friant, a disease ecologist at Penn State who wasn't involved in the study. "This paper points our attention [toward] a way of blocking major routes that diseases are transmitted from animals to humans. If you can block those routes, then you're going to block a lot of pathogens."
Globally, that could mean governments cracking down on the illegal wildlife trade – beefing up surveillance at airports, for instance.
But Carlson notes that could ultimately push more of the trade underground, making it harder to detect spillovers. "We have to choose between criminalizing and pushing trade underground, or finding a way to do public health in [these] settings."
Another option is to address demand for these exotic species, which generates billions of dollars a year.
"Even though it seems like I'm not involved in the wildlife trade, as a normal citizen, you actually are," says Olival. The 2003 mpox outbreaks in the Midwest "were because people were buying animals. So that cute, little furry [exotic] animal in your pet store, maybe think twice about it."
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