글로벌 야생동물 거래, 동물에서 인간으로의 질병 확산 촉진

Live animal markets and the illegal sale of wildlife pose particular dangers, but any sale of wild animals or animal products poses spillover risks, a new study suggests.

The global wildlife trade is driving the spread of disease from animals to people, according to a new study of thousands of wild mammal species and 40 years of international trade records.

Species that have been sold on the international wildlife market, which includes both live animals and animal products, were 50 percent more likely than species that were not traded to share pathogens with humans, scientists found. And the longer that a species had been part of the global wildlife trade, the more pathogens it shared.

The spread of zoonotic diseases, or those that can pass from animals to humans, is an inherent part of the global wildlife trade, the researchers concluded. But live animal markets and the illegal wildlife trade appear to amplify these risks, according to the study, which was published in Science last week.

“There’s no safe trade,” said Jerome Gippet, an ecologist at the University of Fribourg in Switzerland and an author of the paper. “As long as we continue trading species, we will expose ourselves to this problem.”

The global wildlife trade is an enormous industry, encompassing the sale of meat, fur, exotic pets, lab animals and more. It also provide opportunities for a variety of animal pathogens — including Ebola, mpox, salmonella and more — to jump into humans, a process known as spillover.

Numerous human outbreaks have been linked to the wildlife trade, and evidence suggests that the virus that causes Covid-19 may have first jumped into humans at a live animal market in Wuhan, China.

“We have a lot of case studies,” said Colin Carlson, a global change biologist at Yale University and an author of the study. “But we have a really poor evidence base about what the wildlife trade is actually doing.”

In the study, the researchers consulted several large global databases that included records on legal animal imports and seizures of illegally traded animals to determine which wild mammal species were included in the global wildlife trade. They ultimately identified more than 2,000 such species, roughly one-quarter of the planet’s mammal species.

Then, the scientists turned to a database that catalogs animal pathogens and the species that host them. They found that 41 percent of the mammal species involved in the wildlife trade hosted at least one pathogen that had also been known to infect humans. In contrast, just 6 percent of species that hadn’t been traded shared any pathogens with humans.

After controlling for some potentially complicating factors — such as how extensively different species had been studied — the scientists concluded that traded species were 1.5 times as likely to harbor zoonotic pathogens as those that were not bought and sold.

For a subset of species, they also counted the total number of years, from 1980 to 2019, in which trade had been recorded. For every 10 years that a species spent in trade, it shared one additional pathogen with humans.

In some cases, people could be passing their pathogens to wild animals. (Humans have repeatedly transmitted Covid-19 to wild deer, for example.)

But given how many species are traded, and how many pathogens they carry, the scientists suspect that diseases are primarily spreading from animals into humans. In many cases, these animals might have been carrying pathogens that were already capable of infecting people; the global wildlife trade just gave them the chance to do so.

“The more species are traded, the more opportunities they will have to transmit pathogens to humans,” Dr. Gippet said.

But in other cases, animal pathogens may only evolve into human health threats because of the wildlife trade, Dr. Carlson said. At live animal markets, for example, pathogens can jump between animal species, adapting in ways that help them infect new hosts.

“What you get are steppingstones, where viruses are evolving in these markets,” Dr. Carlson said. “Maybe they’re able to adapt to humans for the first time.”

Indeed, the researchers also found that species that were traded in live markets shared more pathogens with humans than those that were not. Some evidence suggested that the same was true of species traded through illegal channels, where animals might live in dirtier conditions and never receive veterinary inspections.

“The study’s authors demonstrate through data what we’ve long suspected to be true — that human use and exploitation of wildlife actually increases the risks of spillover,” said Ann Linder, an associate director at the animal law and policy program at Harvard Law School who was not involved in the new research. “Wildlife, left alone in intact ecosystems, pose very little risk of spreading zoonotic disease to people.”

Still, Ms. Linder said, it is surprising that the researchers were able to document such a strong link to the wildlife trade, given that the data on the sale of wild animals has been notoriously poor and incomplete.

“We’re really not able to speak with any kind of certainty about the size and scale of the trade itself,” she said. “We need more studies like this, but, perhaps more fundamentally, we need more and better data to even begin to understand our own risk.”

Emily Anthes is a science reporter, writing primarily about animal health and science. She also covered the coronavirus pandemic.