Humans have given wild animals their diseases nearly 100 times, researchers find

In a study published March 22 in Ecology Letters, the authors describe nearly one hundred different cases where diseases have undergone "spillback" from humans back into wild animals, much like how SARS-CoV-2 has been able to spread in mink farms, zoo lions and tigers, and wild white-tailed deer.

"There has understandably been an enormous amount of interest in human-to-wild animal pathogen transmission in light of the pandemic," says Gregory Albery, Ph.D., a postdoctoral fellow in the Department of Biology at Georgetown University and the study's senior author. "To help guide conversations and policy surrounding spillback of our pathogens in the future, we went digging through the literature to see how the process has manifested in the past."

In their new study, Albery and colleagues found that almost half of the incidents identified occurred in captive settings like zoos, where veterinarians keep a close eye on animals' health and are more likely to notice when a virus makes the jump. Additionally, more than half of cases they found were human-to-primate transmission, an unsurprising result both because pathogens find it easier to jump between closely-related hosts, and because wild populations of endangered great apes are so carefully monitored.

"This supports the idea that we're more likely to detect pathogens in the places we spend a lot of time and effort looking, with a disproportionate number of studies focusing on charismatic animals at zoos or in close proximity to humans" says Anna Fagre, DVM, Ph.D., MPH, a virologist and wildlife veterinarian at Colorado State University who was lead author on the study, and has also published research on the risks of SARS-CoV-2 spillback using laboratory experiments with the North American deer mouse (Peromyscus maniculatus). "It brings into question which cross-species transmission events we may be missing, and what this might mean not only for public health, but for the health and conservation of the species being infected."

Disease spillback has recently attracted substantial attention due to the spread of SARS-CoV-2, the virus that causes COVID-19, in wild white-tailed deer in the United States and Canada. Some data suggest that deer have given the virus back to humans in at least one case, and many scientists have expressed broader concerns that new animal reservoirs might give the virus extra chances to evolve new variants.

In their new study, Albery and colleagues find a sliver of good news: scientists can use artificial intelligence to anticipate which species might be at risk of contracting the virus. When the researchers compared species that have been infected with SARS-CoV-2 to predictions made by other researchers earlier in the pandemic, they found that scientists were able to guess correctly more often than not.

"It's quite satisfying to see that sequencing animal genomes and understanding their immune systems has paid off," says Colin Carlson, Ph.D., an assistant research professor in the Center for Global Health Science and Security at Georgetown University Medical Center and an author on the study. "The pandemic gave scientists a chance to test out some predictive tools, and it turns out we're more prepared than we thought."

The new study is part of a National Science Foundation-funded project called the Viral Emergence Research Initiative, or Verena. The Verena team uses data science and machine learning to study "the science of the host-virus network" -- a new field that aims to predict which viruses can infect humans, which animals host them and where, when and why they might emerge. Those insights could be critical if scientists want to understand how and why humans share their diseases with animals.

Spillover may be predictable, the authors conclude, but the biggest problem is how little we know about wildlife disease. "We're watching SARS-CoV-2 more closely than any other virus on earth, so when spillback happens, we can catch it. It's still much harder to credibly assess risk in other cases where we're not able to operate with as much information," says Carlson. As a result, it's hard to measure how severe a risk spillback poses for human health or wildlife conservation, particularly for pathogens other than SARS-CoV-2.

"Long-term monitoring helps us establish baselines for wildlife health and disease prevalence, laying important groundwork for future studies," says Fagre. "If we're watching closely, we can spot these cross-species transmission events much faster, and act accordingly."

Additional study authors also included Lily E. Cohen, Icahn School of Medicine at Mount Sinai; Evan A. Eskew, Pacific Lutheran University; Max Farrell, University of Toronto; Emma Glennon, University of Cambridge; Maxwell B. Joseph, University of Colorado Boulder; Hannah K. Frank, Tulane University; and Sadie J. Ryan, University of Florida and University of KwaZulu-Natal.

The authors are a part of the Verena consortium, co-founded by Carlson and Albery. The authors report having no personal financial interests related to the study. Support for Verena is provided by the U.S. National Science Foundation (BII 2021909).