Chronic wasting disease continues to spread in cervids

Recent research addresses questions about CWD, human prion disease

Chronic wasting disease (CWD) continues to threaten deer and elk populations as it has been detected in free-ranging cervids in 34 states and four provinces and in captive cervid facilities in 19 states and three provinces. Most recently, the California Department of Fish and Wildlife announced May 7 that it had been detected in the state’s deer and elk.

Multiple jurisdictions have documented more than 50% positive rates in their adult male samples in more heavily affected areas, according to the U.S. Geological Survey (USGS). Some of the same jurisdictions have documented over 30% positive rates in adult females.

“CWD is clearly spreading geographically, and increasing in prevalence in locations where it is established,” said Bryan Richards, emerging disease coordinator at the USGS National Wildlife Health Center.

GIF shows the distribution of CWD throughout a map of North America
Changes in the documented distribution of chronic wasting disease in North America from 2000-2023. (Courtesy of the USGS National Wildlife Health Center)

Aggressive responses to CWD have been limited to a subset of affected states, Richards said, while other states are assuming more of a monitoring stance, “and monitoring alone will not alter disease outcomes.”

That said, wildlife agency responses to CWD in free-ranging cervids have evolved over the years, according to an article published in the Wildlife Society Bulletin in March 2023.

“Hands-off or delayed management was more common in early years … whereas quick, localized management, often in the form of deer removal and harvest, carcass transportation, and wildlife feeding regulations, has been frequently implemented in recent years,” such as Minnesota and New York, the authors wrote.

Protecting cervids from disease

APHIS provides millions of dollars each year to state and tribal governments, universities, and others to control CWD in wild and farmed cervids. The majority of these funds continue to be allocated to basic research and monitoring, with a smaller portion going towards applied research and active management, Richards said.

While the search for better, more acceptable tools may be appropriate, harvest-based tools, as documented in the Association of Fish and Wildlife Agencies (AFWA) Best Management Practices for the Prevention, Surveillance, and Management of Chronic Wasting Disease, remain underutilized, Richards added.

Cooperators and veterinarians can reduce the risk of CWD spreading to and infecting a farmed cervid herd by maintaining strict biosecurity and raising cervid species that are resistant or less susceptible to CWD.

A herd of deer in a grassy field looking at the camera
Scientists think chronic wasting disease (CWD) spreads between animals through contact with saliva, blood, urine, or feces of an infected animal. They suspect it can also spread indirectly through the environment.

“We continue to see CWD spread among captive facilities, with new positive facilities being detected every year,” Richards said. “How infectious material is being introduced into some of these facilities remains an open question.”

Human-caused introduction of infectious material remains high on the differential list for many facilities with CWD. Everything from artificial insemination practices to biopsy tools for antemortem testing could contribute to the spread, so appropriate and thorough disinfection is critical, Richards said.

Recent CWD research

As CWD cases increase, researchers continue to investigate if the disease could be transmissible to humans.

“There is currently not enough research to determine whether cross-species prion transmission is possible,” said Tanya Espinosa, public affairs specialist for the United States Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS).

Current evidence does not show that CWD can be spread to humans by eating the meat of an infected animal, encountering infected wildlife, or touching or consuming contaminated soil or water, but a recent article challenges that long-held assumption.

Researchers at the National Institutes of Health’s (NIH) Rocky Mountain Laboratories have been testing whether CWD could infect human neural tissue. They did so by exposing human cerebral organoids with two different prion genotypes—one of which has previously been associated with susceptibility to zoonotic prion disease—to high concentrations of CWD from three different sources for seven days.

Their findings, published online May 17 in the journal Emerging Infectious Diseases, showed that no new CWD propagation or deposition of protease-resistant forms of human prions were seen in the CWD-exposed organoids, which remained uninfected for up to six months. This suggest that a substantial species barrier prevents CWD from being transmitted to humans.

Additional studies are under way to identify if any prion diseases could be occurring at a higher rate in people who are at increased risk for contact with potentially CWD-infected deer or elk meat, the CDC website states. Because of the long time it takes before any symptoms of disease appear, scientists expect the study to take many years before they will determine what the risk, if any, of CWD is to people.

“As with any hunter-harvested meat, hunters should ensure they use personal protective equipment and maintain biosecurity while prepping any meat from ungulates,” Espinosa said.

The AVMA paper “Disease precautions for hunters” includes guidelines for hunters and hunting dogs about risks they may encounter. More information can also be found on the U.S. Department of Agriculture's Animal and Plant Health Inspection Service website.