- Another study has shown that the worldwide trade of wild animals increases the spread of disease between wildlife and humans. The new research focused on mammal species.
- Any sale of wild animals, their meat or products increases risk the that contagious pathogens may jump the species barrier and infect humans.
- Researchers found that mammals sold in the global wildlife trade are 50% more likely to share pathogens with humans than those that aren’t bought and sold. They also found that repeated and prolonged human contact may create more opportunities for spillover.
- Contrary to conventional wisdom, illegally traded species were no more likely to carry these zoonotic pathogens than those imported and sold legally, often as exotic pets. The study highlights the need for stronger biosurveillance, better information sharing and a “One Health” approach to wildlife trade that considers risks to both animals and humans.
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Pandemics and novel diseases are perennial threats to human survival. People, wildlife and livestock carry a wide range of viruses, bacteria, fungi and parasites. Close contact creates opportunities for pathogens to jump between species.
To assess public health risks of massive legal and illegal trade in wildlife, an interdisciplinary team delved into trade records on thousands of species spanning the last 40 years. They focused on mammals.
The researchers found that worldwide trade in wild mammals, as well as their parts and products, creates more opportunities for pathogens to mutate and jump from animals into humans over time — and poses a serious public health threat, conclusions they recently published in the journal Science.
For decades, scientists and virologists have warned that the incidence of spillover is rising in a more crowded, interconnected world shaped by travel and trade. Many of the most dangerous or deadly outbreaks of contagious disease in recent history originated in animals, including mpox (1958), Marburg virus (1967), Ebola (1976), HIV/AIDS (first clinical evidence 1981) and COVID-19 (2020).
A rescued chimpanzee receives veterinary care in Freetown, Sierra Leone, after being confiscated from the illegal pet trade there. As close human relatives, apes share a number of diseases with humans. Image by Cate Twining-Ward.
Animals and pathogens shipped worldwide
The global wildlife trade creates repeated opportunities for animals, pathogens and people to come into close contact and share germs. Animals are legally shipped around the world for food. They’re sold as pets. Their parts are used in fashion, home décor and religious practices and they’re also used in research. In 2022, the legal trade was valued at $220 billion a year by CITES, the global wildlife trade treaty.
The illegal trade is also huge, worth up to $23 billion, with poachers, hunters and breeders moving millions of animals and their parts across the planet each year. They’re often confined in crowded, stressful conditions that lower immunity and their susceptibility to pathogens — and increase opportunities for disease to spread.
To assess the human health risks of this massive wildlife trade, researchers analyzed several large data sets, including trade records from CITES; LEMIS, a U.S. Fish and Wildlife Service database; the data set of seized wildlife and their intended uses, which includes records from roughly 5,000 seizures; and CLOVER, a database cataloging mammal-pathogen associations.
The team, composed of ecologists, wildlife trade experts and epidemiologists, chose to focus on mammals, as they are the most widely represented in existing data sets. They used a “One Health” approach in their analysis. As defined by the U.S. Centers for Disease Control and Prevention, this approach “recognizes that the health of people is closely connected to the health of animals and our shared environment.”
A leopard skin displayed in a public market in the Middle East, illustrating the illegal trade of wild mammal parts. Image by Jonathan Kolby.
Cow carcasses for sale at a wet market in Shandong, China. Many outdoor markets sell animals live, dead and in parts. Bringing species together from all over the world increases risk of diseases jumping between them — and infecting humans. Image by Felicity Zhang.
Of the 2,079 mammal species the team studied, 41% could transmit at least one disease to humans; just 6.4% of those that aren’t sold carried zoonotic (species-jumping) pathogens.
Even after accounting for factors such as how closely related the mammals were to one another, where they lived, how often they came into contact with people or were consumed as food, traded mammals were still 50% more likely to host zoonotic pathogens than those that aren’t bought and sold.
The study’s findings challenge common assumptions about risk: Illegally traded species were no more likely to carry zoonotic pathogens than animals traded and sold legally.
Many people think of wildlife trade mainly as illegal trafficking, said Evan Eskew, a study author who analyzed the data. But “the legal trade is likely to be just as risky for zoonotic disease transmission as the illegal trade.” Eskew noted that it can include far more ordinary forms of commerce than many people realize, such as the caged bird you brought home as a pet.
An awareness display about illegal wildlife trade at Chatuchak Market in Bangkok, Thailand. New research found that contrary to conventional wisdom, illegally traded mammals are no more likely to carry diseases that infect humans those those bought legally, sometimes as pets. Image courtesy U.S. Government Accountability Office.
The longer mammals are traded, the higher the risk
But the team’s most striking discovery was that traded mammals were more likely than nontraded mammals to spread disease to people.
Going through a list of 583 mammal species logged by CITES and sold internationally from 1980 to 2019, the researchers also found that the longer a species remained in trade, the more zoonotic pathogens it may carry. For every decade in global commerce, an animal acquired the potential to host about one more pathogen that can infect people.
But several experts cautioned against reading that figure too literally. The estimate depends on data that capture only part of the history of wildlife trade, said Jonathan Kolby, a biosecurity expert and former CITES policy specialist with the U.S. Fish and Wildlife Service who was not involved in this study. Many species, he said, were likely traded long before being logged in those records, while others may be missing altogether.
While using 40 years of data “sounds like a long period of time,” he said, those data sets don’t reflect the complex trade history of each species included.
Rabbits for sale in a market in South Jakarta, Indonesia. Most people don’t realize that wild animals bought as pets may also carry disease. Image by Krotz/ (CC BY-SA 3.0).
Students in Sierra Leone hold signs during an education event raising awareness about the illegal wildlife trade. Image by Cate Twining-Ward.
These blind spots are geographic as well as temporal. The study relied mostly on international trade data, yet domestic and regional trade networks are both widespread and often poorly documented.
Eskew called the lack of data “our biggest problem” in trying to understand zoonotic risk, adding that “you couldn’t overemphasize” how little researchers truly know about which species are traded across most countries.
The danger may lie not only in importation, but in the trade that follows. Eskew pointed to the 2003 U.S. mpox outbreak as an example. Infected African rodents, including giant pouched rats (Cricetomys sp.), dormice (Graphiurus sp.) and rope squirrels (Funisciurus sp.), were imported from Ghana to Texas and then shipped to a distributor in Illinois. There, they were housed with prairie dogs and then sold across the Midwest.
Eleven people in five states who encountered these prairie dogs spiked fevers, broke out in a rash and exhibited other symptoms that pointed to mpox, a disease that easily jumps from animals to humans.
The complex nature of wildlife trade is one reason Meredith Gore, a conservation social scientist and study co-author, said the paper’s implications extend beyond what CITES can address. She noted that the treaty is a conservation tool, with limited funding, not a compliance arm — and “it’s not a public health regulation.”
A slow loris for sale in a wildlife market in Möng La, Shan, Myanmar. All eight subspecies are on the endangered species list. Image by Soggy Benenovitch.
The study doesn’t offer a full map of wildlife trade, nor can it predict where the next disease outbreak will emerge. Reducing opportunities for zoonotic spillover, Gore said, will require closer surveillance of wildlife trade in all its forms, legal and illegal, domestic and international, as well as data sharing and cooperation between countries.
“You don’t know what you don’t know,” Gore said. “It’s not that this information is not knowable. It’s just that we’re not necessarily engaging in the biosurveillance that we need to be.”
Banner image: This red-tailed monkey was photographed at a hotel in the Democratic Republic of Congo. Apes and monkeys are widely traded legally and illegally. Image by Jonathan Kolby.
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Citations:
Gippet, J. M. W., Carlson, C. J., Klaftenberger, T., Schweizer, M., Eskew, E. A., Gore, M. L., & Bertelsmeier, C. (2026). Wildlife trade drives animal-to-human pathogen transmission over 40 years. Science, 392(6794), 178-182. doi:10.1126/science.adw5518
Morens, D., Breman, J., Claisher, C., Doherty, P. Hahn, B. … Taubenberger, J. (2020) The Origin of COVID-19 and Why It Matters. The American Journal of Tropical Medicine and Hygiene, 103(3):955–959. doi:10.4269/ajtmh.20-0849
Reed, K. D., Melski, J. W., Graham, M. B., Regnery, R. L., Sotir, M. J., Wegner, M. V., … Damon, I. K. (2004). The detection of monkeypox in humans in the Western Hemisphere. New England Journal of Medicine, 350(4), 342-350. doi:10.1056/NEJMoa032299
