The Global Wildlife Trade Drives Animal-To-Human Disease Transmission

Many epidemiological investigations have pointed towards the global trade in wildlife as a source for transmission of pathogens from animals to people. These have included SARS, COVID-19, Ebola, and mpox, all known to cause serious and even deadly disease in humans.

The current study sought to quantify how much the wildlife trade increases the odds of sharing at least one pathogen between any given mammal species and humans. The authors also tested illegal trade and live animal markets for their effect on transmission risk, and assessed whether the number of pathogens shared with humans depends on the amount of time a mammal species has spent in the trade. The study included over 6,000 different mammals, of which just over 2,000 were traded.

The authors relied on four datasets:

• The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), a global database of trade in listed species;
• The Law Enforcement Management Information System, a database of records of live animals and animal products imported into the U.S.;
• The Dataset of Seized Wildlife and Their Intended Uses, a compilation of international illegal trade records; and
• CLOVER, a database of mammal pathogens.

To isolate the effect of the wildlife trade, the authors had to control for other factors thought to influence the likelihood of zoonotic pathogen transmission. These included:

• Where a species is found;
• Whether a species has adapted to benefit from human environments (known as synanthropy);
• How closely related a species is to other zoonotic host species; and
• Whether a species is consumed as food.

Another very important factor is the amount of research effort devoted to uncovering such a pathogen in the first place. Without this scientific work, the pathogen would remain unknown.

For the analyses, it was necessary to develop an indicator of the amount of scientific research that exists for each particular species. This “index of research effort” combined two key variables: the total number of scientific publications and the proportion of those publications focused on pathogens and diseases. The authors also created a “time in trade” variable to represent the number of years each particular species has spent in the trade, using CITES data covering a 40-year period from 1980 to 2019.

The results showed that:

• Traded mammals are 50% more likely to share pathogens with humans than non-traded species.
• Synanthropic species are 20% more likely to share pathogens with humans compared to mammals who don’t live in or near human-modified environments.
• Species used for wild meat are 10% more likely to share pathogens with humans versus those not consumed, though this finding was only marginally significant and should be used with caution.

Species traded alive are not only 34% more likely to share pathogens with humans, they also share on average 50% more pathogens than those traded solely as products. And while illegal trade doesn’t increase transmission risk, species present in the illegal trade still share 40% more pathogens with humans than those only traded legally. In other words, illegal trade might not determine whether pathogens initially cross to humans, but it may create conditions that subsequently allow more of them to do so. Together, these results support the view that illegal trade and live animal markets promote pathogen spillover. However, the authors conclude that traded status (as a binary yes/no) remains the most important factor for zoonotic disease transmission, more so than specific aspects of the trade.

Lastly, the analysis confirmed that time in trade increases the number of pathogens shared with humans: on average, a wild mammal species shares one additional pathogen with humans for every 10 years spent in the trade.

There are some limitations to note. The study used global trade data, and therefore doesn’t capture the effects of local wild animal markets, which are widespread. The results indicated that trade status impacts the amount of research done on a particular species. This further underlines the critical role of the authors’ “index of research effort” variable, which attempted to control for this bias. They also conducted sensitivity analyses to test the robustness of their methods.

While statistical modeling can’t fully uncover the precise mechanisms of pathogen transmission, the authors argue that animal-to-human transmission is the most likely driver of pathogen exchange in the wildlife trade. They call for greater biosurveillance and support potential reforms to CITES that would increase international focus on preventing pathogen spillover. The authors also argue that reducing the overall number of animals in the wildlife trade will be necessary to reduce the risk of future zoonotic pathogen emergence. This offers an opportunity for advocates to push for the wildlife trade to be both curtailed and regulated — not just for its animal welfare concerns, but for the immense risks it poses to human health.