Global wildlife trade raising risk of animal diseases spilling over to humans

The global wildlife trade is doing more than threatening endangered species — it is quietly accelerating the spread of infectious diseases from animals to humans, according to a new Yale University study published in Science.

The research shows that the longer animals circulate through wildlife markets and trade networks, the more opportunities viruses, bacteria, and other pathogens have to jump into people, increasing the risk of future outbreaks of disease.

For every 10 years an animal spends in the wildlife trade, it shares an average of one new pathogen with humans, the study found. The risk is even higher in the illegal wildlife trade and live animal markets, where animals from different regions and species are packed together under stressful, often unsanitary conditions.

“Microbes move fast, but this is just staggering,” said Dr. Colin Carlson, PhD, assistant professor of epidemiology at the Yale School of Public Health and a co-author of the paper. “Wildlife trade has been affecting our health much faster and for much longer than we thought.”

Carlson is executive director of the Viral Emergence Research Initiative (Verena), a national, multi-institutional research program at Yale University that is applying the latest scientific tools to better identify emerging viral threats and prevent the next global pandemic. Verena is supported by a Biology Integration Institute grant from the National Science Foundation. Data from Verena’s VIRION database — an open atlas of more than 9,000 vertebrate viruses — was the foundation for the analysis.

While scientists have long known that wildlife trade contributes to outbreaks — from SARS to COVID-19 — this study is the first to quantify that risk at a global scale. Researchers analyzed an unprecedented dataset of confiscated wildlife alongside four decades of information on viruses known to infect animals and humans.

“It’s hard to make the leap from stories to scientific evidence,” Carlson said. “For decades, we’ve seen outbreak after outbreak starting in the wildlife trade, but so far, that hasn’t been enough to build a case for action. Now we can say with confidence, wildlife trade is always risky for human health.”

For the study, the researchers analyzed data from more than 6,400 mammal species worldwide. About one-quarter of those species have appeared in the global wildlife trade, either as live animals or as animal products such as fur, meat, or traditional medicines.

Among traded mammals, 41% were known to share at least one pathogen with humans. By contrast, only about 6% of mammals that are not traded share diseases with people.

“We have to be careful not to jump to conclusions,” Carlson said. “We know a lot about the viruses in our pets because we spend so much time around them — but that doesn’t automatically mean they pose a risk to human health. The same could be true for other animals we interact with frequently. Until recently, it’s been hard to tell whether we’re uncovering real risks or simply finding microbes because we’re looking for them.”

By using carefully designed statistical models, the authors showed that mammals involved in wildlife trade were about 50% more likely to share diseases with humans, even after accounting for other factors such as habitat, diet, and how closely animals live near people.

The research also looked at different types of trade. Species that are sold alive — such as animals destined for pet markets, research facilities, or live food markets — were especially likely to share diseases with humans.

“The conditions that sparked the COVID-19 pandemic exist all over the world,” Carlson said. “Animals are packed together in poor health, in combinations that don’t exist in nature, spending hours a day with people who might not even wear a mask or gloves to reduce potential disease transmission.”

Illegal wildlife trade also played a role, the researchers found. While its overall impact varied depending on how risk was measured, species involved in illegal trade tended to share more pathogens with humans than those traded through legal channels.

To reduce growing risks, the authors said that governments should be tracking coronaviruses and other high-risk diseases on wildlife farms, in wet markets, and especially, in the humans who work in the wildlife trade.

“We can’t save biodiversity unless we reduce wildlife trade, but there will always be a market for furs and exotic pets,” Carlson said. “The most important thing is that we’re ready to spot the next SARS the minute it jumps from animals to people.”

Scientists from the University of Lausanne contributed to the study.