U of I scientist tracks diseases spread by wildlife trade

As a child, Evan Eskew had a pet tarantula spider that he handled with thick gardening gloves, and a pet snake he fed goldfish.  

Because it was venomous, the spider worried him. The snake not so much.  

“I was very afraid of the tarantula,” Eskew said. “The garter snake was definitely better to handle.”  

The keeping of semi-exotic animals made Eskew, like many Americans who purchase animals in pet stores — chinchillas, cockatoos, scorpions, gators, bearded dragons or box turtles — part of a global wildlife trade.  

These days, Eskew is a data scientist at University of Idaho’s Institute for Interdisciplinary Data Science, a university research hub that provides the computing infrastructure and expert support needed to advance modern data science, genomics and bioinformatics. He uses statistical methods to understand issues in conservation biology and disease ecology.  

The wildlife trade and the diseases associated with it are among his research specialties. A recent study published in the journal Science used a large data set to illustrate how the commercial trade of wild animals from around the globe drives animal-to-human pathogen transmission.  

“The wildlife trade contributes to wildlife disease, and then there is the wildlife trade contributing to human disease,” Eskew said. “When you look at the animal species that are traded versus the ones that are not traded, the traded species have more pathogens that can be transmitted to humans.”  

Pathogens that can be passed from animals to humans are called zoonotic pathogens.  

“The longer a species is traded the more likely they are to accumulate zoonotic pathogens,” he said.  

Eskew and his colleagues examined trade data for mammal species from the past 40 years and showed that the longer a species had been legally traded, the greater the likelihood that humans and the species will share a pathogen ranging from a virus to a bacterium, fungus or parasite. High-profile infectious disease outbreaks in humans that are connected to wildlife use and trade include the emergence of HIV, the 2013-2016 West African Ebola epidemic, and a 2003 mpox virus outbreak in the United States.  

Wildlife commerce is often associated with animals that are caught, warehoused, sometimes for several generations, and transported globally to be sold as pets in stores or online. The trade also includes animals used for food, religious practices, or clothing, Eskew said.  

“It’s an important form of human-animal interaction that affects around a quarter of all mammal species,” he said. “There are a lot of opportunities for cross-species pathogen transmission.”  

As a child growing up in rural South Carolina, Eskew’s early adventures led to an interest in reptiles and amphibians. Salamanders, toads, snakes and frogs were more accessible than big mammals. He followed his interest to college earning advanced degrees pursuing amphibian ecology and conservation.  

That’s why another pathogen spread through the wildlife trade and deadly to amphibians is of particular interest to him.  

In the 1970s, biologists around the world started noticing a decline in frogs and salamanders.  

“Some scientists thought, no, it’s a one-off,” Eskew said. “Amphibians can have boom-and-bust population cycles.”  

Some amphibian species in Central America and Australia disappeared. By the 1990s, the continual decrease in frog and salamander populations was undeniable. Scientists later learned that a group of fungi, informally known as chytrids, was responsible for the steady decline or extinction of hundreds of species of amphibians.  

“This pathogen was widely dispersed for decades. Its spread probably accelerated in the latter half of the 20th century, when animals were more commonly traded and transported internationally via ships and airplanes,” Eskew said.  

Museum specimens of amphibians collected in the late 1800s and early 1900s in the United States have been sampled and found to have chytrid.  

“We think, ancestrally, the chytrids affecting amphibians are from Asia,” he said. “They were moved out of Asia and are now established on every continent occupied by amphibians. The fungus has been around in North America for over a century. That doesn’t mean it was widely established, but it has been spreading for quite some time.”  

An article Eskew published in Communications Earth & Environment deals with a strain of the chytrid fungus that doesn’t yet appear on the North American continent, but, if introduced, would be deadly to North American salamanders. Eskew and colleagues crunched reams of wildlife trade data to show that a 2016 United States trade policy reduced the import of salamanders that could harbor the disease. 

“Unfortunately, the amphibian trade into the United States continues to risk introducing the pathogen because other species that may carry it are still traded in large quantities,” he said.  

Although most wildlife commerce is legal, few countries collect and compile trade data, which makes it difficult for researchers like Eskew to get a handle on what is being traded and where it’s going.  

The United States is one of the world’s largest legal wildlife importers and the only country to collate and release comprehensive wildlife trade data. The data show that for a 22-year period beginning in 2000, almost 30,000 species and over 2.85 billion individuals were traded in the country. About half of those are declared to originate from the wild. 

The lack of a global database for animal trade makes identifying trends, or accurately assessing the impact on biodiversity and conservation, a guessing game, he said.  

Ballpark figures as to the scope of the trade have been conservative.  

“The latest studies show that the size and scope of the legal wildlife trade far exceed previous assessments,” Eskew said.