The longer a species stays in the wildlife trade, the more dangerous it becomes. A new study explains why

Animals traded through global wildlife markets are far more likely to carry diseases that can infect humans, and the risk grows the longer those species remain in circulation, according to a new study.

The analysis, published Thursday in Science, examined decades of global wildlife trade data and found that 41% of traded mammal species share at least one pathogen with humans, compared with just 6.4% of species not involved in trade.

The researchers also found that the number of pathogens shared between animals and humans increases over time. On average, a species acquires one additional human-infecting pathogen for every decade it is present in the global wildlife trade.

The findings suggest that wildlife trade does not simply expose humans to existing disease risks, but may actively amplify them over time.

“Our study is the strongest evidence to date that reducing wildlife trade will reduce pandemic risk,” said Colin Carlson, an epidemiologist at the Yale School of Public Health and a co-author of the study.

Scientists have already long linked wildlife trade to specific outbreaks such as HIV, Ebola and COVID-19. The new research, which draws on 40 years of global trade records and pathogen data, attempts to measure the relationship on a larger scale.

The results point to a broader pattern. Repeated and prolonged contact between humans and wild animals creates more opportunities for pathogens to move between species.

“What stands out most is how clearly the findings reinforce something many of us in disease ecology have been concerned about for years: it’s not just the presence of wildlife trade, but the intensity and duration of contact that elevates risk,” said Thomas Gillespie, a professor of environmental sciences and environmental health at Emory University, who was not involved in the study.

Wildlife trade, as defined in this study, includes a wide range of activities, from hunting and breeding to transport, storage and sale. At each stage, animals are handled, confined and often brought into close proximity with both humans and other animal species. These conditions can facilitate the spread of viruses, bacteria and parasites.

Over time, those repeated interactions create more opportunities for pathogens to circulate, adapt and potentially spill over into human populations.

Carlson said one of the most striking findings was how strongly time in trade predicted pathogen sharing.

“That time-in-trade effect is the smoking gun,” he said. “We wouldn’t see that unless pathogens were jumping from animals to humans.”

He added that the findings suggest wildlife trade should be considered one of the major drivers of disease emergence, alongside deforestation, agriculture and climate change.

The study also found that certain forms of trade may carry higher risks. Species sold in live-animal markets were more likely to share pathogens with humans than those sold as meat or animal products. Illegally traded species also were more likely to be the cause of disease, though researchers emphasized that risk is not limited to illicit markets.

“Focusing on illegal wildlife trade is not enough,” said Meredith Gore, a conservation criminologist at the University of Maryland and a co-author of the study. “Pathogen transmission is a consequence of general and diverse uses of wildlife by people. This includes illegal and legal trade.”

Most international frameworks governing wildlife trade, including the Convention on International Trade in Endangered Species of Wild Fauna and Flora, or CITES, were designed primarily to protect species from overexploitation, according to Gore.

“There are clear and currently unmet opportunities for more directly including zoonotic disease risk consideration into current regulations,” Gore said.

In particular, the global nature of the trade complicates efforts to manage risk.

“Animals and pathogens do not care about political borders,” said Jérôme Gippet, a biologist at the University of Fribourg and the University of Lausanne in Switzerland and the study’s lead author. “Without globally coordinated efforts, I do not see how we can limit these risks efficiently.”

The researchers say their findings underscore the need for a more coordinated approach that bridges conservation, public health and trade policy and treats wildlife trade as a central driver of global health risk. The study’s findings also highlight gaps in disease surveillance systems, which often fail to detect pathogens circulating in wildlife before they reach humans.

“Risk is accumulating in a way that current surveillance isn’t capturing,” said Evan Eskew, a disease ecologist at the University of Idaho and a co-author of the study.

Few countries, he said, systematically track which species are being traded across their borders, and even fewer conduct routine pathogen screening in those animals. As a result, potential threats can go undetected until they spill over into human populations.

Eskew said expanding surveillance, particularly for species already known to carry zoonotic pathogens, could help identify risks earlier and prevent outbreaks from spreading.

“We need to be looking for the next pandemic virus on fur farms, in hunting communities, and even at border checkpoints where wildlife are imported,” Carlson said. “Right now, we’re flying blind, especially in places where we’ve criminalized wildlife trade and driven it underground.”