GLOBAL WILDLIFE MARKETS — Mammal species involved in global wildlife trade acquire on average one additional human-infecting pathogen for every decade they remain in trade, according to a new study led by researchers at the Yale School of Public Health. The study, which drew on 40 years of global wildlife trade records and pathogen data, found that 41% of mammal species in trade carry pathogens that can infect humans, compared to 6.4% of species not in trade.

Traded mammal species are about 1.5 times as likely to be sources of human diseases as non-traded species. "That time-in-trade effect is the smoking gun. We wouldn't see that unless pathogens were jumping from animals to humans," Colin Carlson, an epidemiologist at Yale, said.

At each stage of wildlife trade — hunting, breeding, transport, storage, and sale — animals are handled, confined, and often brought into close proximity with humans and other species. These conditions can facilitate the spread of viruses, bacteria, and parasites. Repeated contact creates more opportunities for pathogens to circulate, adapt, and potentially spill over into human populations.

Carlson and his colleagues built databases to track every newly discovered virus and aligned this pathogen atlas with wildlife trade data. "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. Right now, we're flying blind, especially in places where we've criminalized wildlife trade and driven it underground," he said.

Species involved in illegal wildlife trade are more likely to share pathogens with humans than legally traded species, but the study found that legal trade also poses risks. "Focusing on illegal wildlife trade is not enough. Pathogen transmission is a consequence of general and diverse uses of wildlife by people. This includes illegal and legal trade," Meredith Gore, a conservation criminologist at the University of Maryland, said. She added: "There are clear and currently unmet opportunities for more directly including zoonotic disease risk consideration into current regulations."

Few countries systematically track which species cross their borders, and even fewer conduct routine pathogen screening of traded wildlife. "Risk is accumulating in a way that current surveillance isn't capturing," Evan Eskew, a disease ecologist at the University of Idaho, said. Eskew said expanding surveillance for species known to carry zoonotic pathogens could help identify risks earlier and prevent outbreaks from spreading.

In 2003, a shipment of exotic African rodents to a pet store in Illinois led to 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.