Wild mammals that are traded are 50% more likely to share pathogens with humans when compared with mammals that are not traded, according to a new study in Science.

​Using data from 1980 to 2019, researchers in Switzerland and the United States looked at the length of time an animal had been traded. They found that, on average, a wild mammal species shares one additional pathogen with humans for every 10 years it is present in the global wildlife trade.

“This finding implies that pathogens hosted by traded species that currently do not infect humans are more likely to do so in the near future compared with those hosted by nontraded species,” write the authors.

Pathogen transmission occurs through so-called spillover events. This is when a virus, bacterium, fungus, or parasite jumps from its host to a brand-new species. Within the context of the wildlife trade, the study explains that human exposure to a zoonotic pathogen might occur through hunting, or breeding, or during transport, stockpiling, warehousing, retail, consumption, or keeping a wild animal as a pet.

Time versus volume 

The study addresses an important topic and grapples with complex data, said Jonathan E. Kolby, PhD, an ecologist specializing in biosecurity and wildlife diseases at the Smithsonian National Zoo and Conservation Biology Institute, who was not involved in the research.

He wishes, however, that researchers had examined the intensity of trade rather than simply whether a species was traded within a given year.

I’m much more worried about something that might be carrying zoonotic pathogens that we’re getting bombarded with, than something that there’s very, very, very little trade.

For example, if just a single long-tailed macaque and 1,000 white-fronted capuchin monkeys were traded in 2007, the study weighs both species equally for that year. Omitting the overall number of individual animals from the analysis oversimplifies on-the-ground dynamics, critiqued Kolby, because the greater the number of individual animals being traded, the more opportunities for pathogens to spill over.

“I’m much more worried about something that might be carrying zoonotic pathogens that we’re getting bombarded with, than something that there’s very, very, very little trade,” he said.

Doing the type of analysis that Kolby suggests adds several levels of complexity, said lead author Jérôme Gippet, PhD, an ecologist who studies human-wildlife interactions at the University of Fribourg in Switzerland.

During the initial phases of data exploration, Gippet said his team looked at the number of trade events and got “very similar results” when compared with the length of time a species was traded.

We thus chose to keep things simple and limited our analyses to time in trade, which has the advantage of being easy to interpret.

“We thus chose to keep things simple and limited our analyses to time in trade, which has the advantage of being easy to interpret,” he told CIDRAP News.

Domestic trade poses risk, too

Another “blind spot” highlighted by Kolby is that the paper examined only international trade. Kolby said this framing fails to consider domestic trade, which is just as important from a global health perspective. 

Pathogens don’t care about international borders. What matters is that commerce encourages people to interact with wild animals in ways that expose them to pathogens.

For example, HIV is of zoonotic origin. A similar virus, simian immunodeficiency virus, circulates among various non-human primates in sub-Saharan Africa. Scientists suspect the first people who were infected hunted and butchered these animals for food. This might have happened in the 1920s, if not earlier. So, while the initial spillover event, or events, that precipitated the HIV/AIDS epidemic occurred at the local level, the global implications decades later were devastating.

Wildlife trade increases the risk of that first jump by bringing people into close contact with many wild species.

Kolby emphasized that he agrees with the authors’ overall conclusion that interaction with wild animals through trade is a serious public health risk. He’s glad this topic is getting attention. But because of what Kolby sees as shortcomings in the analysis and framing, he worries the paper’s conclusion will be seen as alarmist at a time when the public is losing trust in medical science and public health.

“The details matter,” he said.

The authors of the paper acknowledge that they did not consider the role of domestic trade, noting that data on this type of commerce are currently fragmented and geographically uneven. 

Still, Gippet said, the research shows that traded species are more likely to share pathogens with humans, “Which supports the idea that wildlife trade increases the risk of that first jump by bringing people into close contact with many wild species.”