Pathogens along for the ride

Alex Eben Meyer

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For decades, public health experts assumed that the booming international wildlife trade has increased the risk of pathogens moving from animals to humans. But they had no proof. 

Now they do. New research by a Yale scientist provides clear evidence that the wildlife trade is significantly increasing the risks that dangerous microbes will spread from animals to us. Such jumps can lead to deadly global pandemics, as happened with Covid.

In a paper published this spring in Science, Colin Carlson, an assistant professor of epidemiology at the Yale School of Public Health, found that more than 40 percent of mammalian species entwined in the wildlife trade can host a pathogen that can also infect humans. “Because of wildlife trade, infectious diseases are moving incredibly quickly from animals to humans,” Carlson says. “I think it’s almost a certainty that we will have more pandemics because of this.”

Carlson and his colleagues looked at more than 2,000 commonly traded mammals, including primates, rodents, and bats, and synthesized this with data about thousands of pathogens. They showed that on average, a wild mammal species shares one additional pathogen with humans for every ten years it has been involved in the global wildlife trade.

Carlson’s analysis included both the legal and the illegal wildlife trade. According to Carlson, the illicit trade tends to be a center of animal-to-human pathogen transfer because it tends to be less regulated and less hygienic. Nevertheless, he argues that trying to eliminate the illegal market is counterproductive, because this only drives it further underground, where it is even less controlled. Instead, he says, governments and public health groups should work to help those involved in the trade find other ways to make a living, and should also increase the surveillance of pathogens and infectious diseases in traded animals. “We can’t stop it,” he says. “But we can try to manage it.”

His next project is to examine whether the risk of global pandemics is growing. Just as with pathogen jumping, researchers have assumed that this risk is increasing. But they don’t know for sure. To answer that question, Carlson will use big data tools to analyze large databases of disease outbreaks over the last century.

“I strongly suspect that the time between outbreaks is getting smaller,” he says. “But we need to run the numbers.”