CWD spillover worries prion disease experts

POWELL — Prion disease experts are concerned about spillover of Chronic Wasting Disease into other species of wildlife, livestock and even humans from direct contact with or environmental contamination from cervids (like deer, elk and moose) that commonly carry the disease.

CWD could already be transmitting to humans, according to experts who met to discuss the quickly spreading disease during a Thursday Center for Infectious Disease Research and Policy seminar on CWD spillover, preparedness and response.

They spoke of concerns about the evolution of the disease, which already has different strains.

In 2000, five U.S. states and one Canadian province had confirmed cases of CWD in cervids.

By 2021, 36 U.S. states and four Canadian provinces had reported cases of the fatal prion disease.

A prion is an abnormally shaped protein that can corrupt normal proteins in the brain. The result is brain damage and progressive, fatal neurological diseases.

“We know that there are some animal prion diseases that have spilled over to humans and some that have not, so we don’t know the exact risk (of CWD spillover), but we know that it’s not zero,” said Brian Appleby, director of the National Prion Disease Pathology Surveillance Center.

Appleby is just one of dozens of the top CWD experts across the country who are participating in a project to study the evolution of the prion disease and to prepare for spillover into other species.

There are already known individual strain differences, possibly within the U.S., but definitely across the world, and those strains might have different abilities to transmit to humans or other wildlife and livestock that may be more likely to transmit to humans.

Unfortunately, prion diseases have a very long incubation period, so even the human-to-human transmission incubation period is measured in years.

The only prion disease known to infect humans to date has been bovine spongiform encephalopathy, or mad cow disease, a fatal brain disease in cows that is caused by a prion and caused a major outbreak in the 1980-90s. Cases were reported in at least 24 countries.

Most cases occurred in the United Kingdom, but many other countries also reported cases.

The disease proved capable of spreading to humans through beef contaminated with brain or spinal cord tissue from an infected cow or blood transfusions from an infected person.

The human form of the disease is called variant Creutzfeldt-Jakob disease, a rare, fatal brain disease caused by abnormal prions.

Symptoms often include psychiatric problems, like anxiety and depression, followed by neurological issues like dementia and poor coordination. It’s always fatal, usually within a few years of diagnosis.

“The only zoonotic illness that we know with variant CJD, the mean incubation period was about 10 years. So it could be that chronic wasting disease is transmitting to humans already. We just don’t know it, because the incubation period is so long,” Appleby said.

There is a precedent for prion disease spillover, he said, and there is great concern that chronic wasting disease could spill over to other species of wildlife, livestock and humans — especially as CWD strains evolve. Wild hogs in Texas have already been found positive for the disease.

“There are multiple factors right now that have contributed to the need for this project. First of all, as you all are so well aware, there’s a continued spread of CWD and wild cervids. It’s been remarkable. The potential of evolving CWD prion strains is a reality, and so combine that with the continued spread, there’s more and more chances for this to happen,” said Director of CIDRAP Mike Osterholm.

Because researchers don’t have a good handle yet on how long CWD prions can exist in the environment, it’s very difficult to know how to deal with this issue.

The binding and persistence of prions is dependent on climate, soil type, probably on prion strain. Although there isn’t a lot of evidence for strain variants at this point, just the knowledge that diseases evolve necessitates preparations for the day that CWD is found to spill over.

The top findings of CIDRAP, as reported in a webinar last week entitled “Chronic Wasting Disease preparedness and response” includes the fact that it is unclear how CWD might spill over to humans or any other non cervid species; there’s no CWD specific surveillance in non cervid animals; there’s no single test capable of identifying a spillover and no standardized surveillance of CWD strains. Furthermore, lab capacity is limited and difficult to expand at this point.

In many states, testing success is due to engaging hunters and requesting samples from them.

However, “disease fatigue” in hunters has already challenged some states seeking to engage hunters about CWD testing.

But not so in Wyoming, where the Wyoming Game and Fish Department tested more than 5,250 samples from cervids last year — about 170 more than the previous year. That said, testing for CWD in livestock and humans is “something our agency is not working on,” a spokesperson for the agency said Monday.

This story was published on April 15, 2025.