Chronic wasting disease has always been considered an animal problem — a brain-wasting condition that kills deer, elk, and other cervids, causing visible deterioration that hunters and wildlife managers could observe and use to identify infected animals. A study published June 16, 2026, in Science Advances challenges that assumption at its foundation: infectious prions can be present and transmissible in animals that appear completely healthy, with no clinical signs of disease.
The research, conducted by scientists at the University of Calgary Faculty of Veterinary Medicine and international collaborators and covered by ScienceDaily on June 16, also confirmed limited but measurable transmission of CWD prions to nonhuman primates in laboratory settings — the most controlled form of primate testing available, and the most direct evidence yet that the species barrier between CWD and primates (the animal group most closely related to humans) is not absolute.
"These findings show that even without obvious clinical signs, infectious prions can still be present and transmissible," said Dr. Samia Hannaoui, PhD, researcher and assistant professor at the University of Calgary Faculty of Veterinary Medicine and first author on the study, in a statement to EurekAlert.
What the Science Advances Study Found
The full study, led by Hannaoui and senior author Hermann Schätzl at the University of Calgary, used 18 female cynomolgus macaques inoculated with CWD preparations from elk, mule deer, and white-tailed deer from diverse U.S. and Canadian geographic regions. Three additional macaques served as mock controls.
The animals were observed for 4 to 10 years post-inoculation. The key findings:
- Most macaques remained asymptomatic — they showed no clinical signs of prion disease throughout the observation period
- Using sensitive in vitro prion amplification assays (seeded protein misfolding cyclic amplification, or sPMCA), researchers detected low levels of prions in the tissues of asymptomatic macaques
- When tissue from these apparently healthy macaques was inoculated into transgenic mice and bank voles — smaller animal models that are highly sensitive to prion infection — it induced prion disease with 100% transmission rates upon serial passage
The implications of that final finding are significant: the asymptomatic macaques were carrying prions capable of transmitting disease to subsequent hosts, even though those macaques themselves showed no signs of illness. The prions had adapted during their time in primate tissues and could propagate efficiently in subsequent animal models.
According to the published Science Advances abstract: "One interpretation of these findings is that CWD prions retain infectivity across species and that primate infection may manifest atypically while still enabling transmission. Our results challenge earlier conclusions that minimize the zoonotic risk of CWD and underscore the need for continued surveillance."
| CWD Science Advances Study Key Data | Detail |
| Published in | Science Advances, Volume 12, Issue 22, 2026 |
| DOI | 10.1126/sciadv.aeb7613 |
| ScienceDaily coverage | June 16, 2026 |
| Lead author | Dr. Samia Hannaoui, University of Calgary Faculty of Veterinary Medicine |
| Senior author | Hermann Schätzl, University of Calgary |
| Nonhuman primate model used | Cynomolgus macaques (18 inoculated; 3 mock controls) |
| Observation period | 4–10 years post-inoculation |
| Clinical outcome in most macaques | Asymptomatic (no signs of prion disease) |
| Prion detection in asymptomatic macaques | Low levels detected via in vitro amplification assays |
| Transmission from macaque tissue to mice/bank voles | 100% upon serial passage |
| Confirmed human CWD cases | 0 |
| CWD-confirmed U.S. states (as of Aug. 2025) | 36 states |
| CWD-confirmed Canadian provinces | 5 |
The Silent Spread Problem — Why Symptom Observation Isn't Enough
The defining challenge that this study amplifies is what the CWD research community calls the "silent carrier" problem. CWD prions are shed in the urine, feces, and saliva of infected cervids for months — and potentially years — before clinical signs develop. An animal can be infectious without being visibly sick. Hunters who rely on visual inspection to identify "healthy-looking" deer for harvest are therefore operating under a false assumption of safety.
As senior author Schätzl explained to ScienceDaily: "By the time you see clinical signs, the animal has often been infectious for a long time. That's what makes this disease particularly challenging to control."
This is not new information for wildlife managers — the long preclinical infectious period has been known for years. But the new study adds a different dimension: even in a nonhuman primate host (a species closer to humans than mice or deer), infection may manifest asymptomatically while still producing and harboring infectious prions. This "silent infection" model makes the disease harder to detect, harder to contain, and harder to define in any meaningful way using clinical signs.
CWD prions also persist in the environment — in soil, on vegetation, in water — for extended periods after being shed by infected animals. This environmental persistence contributes to ongoing transmission within deer populations and makes decontamination of affected land essentially impossible.
The Human Risk Question — What the CDC Says and Why Vigilance Is Warranted
No human cases of CWD have ever been confirmed. This fact is important and must be stated clearly. The CDC's current guidance reflects this: the CDC's About CWD page states that while there is "no strong evidence" of CWD transmission to humans, "some animal studies suggest CWD poses a risk to certain types of monkeys that are more closely related to people than deer or elk."
The biological concern is rooted in the history of prion diseases and cross-species transmission. Bovine spongiform encephalopathy (BSE, "Mad Cow Disease") was dismissed as a human risk for years before variant Creutzfeldt-Jakob disease (vCJD) emerged in the UK in the 1990s, killing at least 178 people who had consumed contaminated beef. The species barrier between cattle BSE and humans turned out to be permissive, not absolute. The question of whether CWD's species barrier to humans is permissive or absolute has not been definitively answered, and the new Science Advances data suggest it is not absolute in primate tissues.
The CDC's current guidance for hunters recommends:
- Do not harvest or consume any deer, elk, or moose that appears sick or behaves abnormally
- Have deer and elk tested for CWD before eating the meat, especially in areas where CWD is known to exist
- If the test results are positive, do not eat the meat
- Avoid eating the brain, spinal cord, eyes, spleen, tonsils, and lymph nodes of any deer or elk harvested in an area where CWD occurs
The June 2026 study adds a specific, evidence-based urgency to this last point: the asymptomatic carrier finding means that visual health assessment is insufficient to identify safe animals. Testing — not inspection — is the appropriate risk mitigation strategy for hunters in CWD-affected states.
What Wildlife Managers Are Doing — and What Still Needs to Change
CWD is confirmed in 36 U.S. states as of August 2025 and continues its geographic expansion. State wildlife agencies conduct CWD surveillance programs, and many states have mandatory or voluntary testing programs for harvested deer and elk. Regulations vary significantly: some states prohibit the transport of carcasses or certain carcass parts from CWD-positive areas, while others require the testing of harvested animals from high-prevalence zones.
The June 2026 study's authors emphasize that continued surveillance is not optional: "As CWD continues to spread, researchers say understanding how prion diseases behave, including the potential for silent or atypical infections, remains critical to protecting both wildlife and public health." The finding that silent carriers can transmit infection to subsequent hosts strengthens the case for expanded pre-harvest testing, stricter carcass transport regulations, and continued environmental monitoring.
Frequently Asked Questions
What did the Science Advances CWD study find?
University of Calgary researchers inoculated 18 cynomolgus macaques with CWD preparations. Most macaques remained asymptomatic, but sensitive laboratory testing detected low levels of prions in their tissues. When those tissues were passed to transgenic mice and bank voles, 100% transmission rates were achieved on serial passage — confirming that apparently healthy primate tissue harbored infectious, transmissible prions.
Has CWD ever infected a human?
No confirmed human cases of CWD have ever been reported. The CDC notes there is no strong evidence of human CWD transmission. However, the CDC also advises caution given the disease's continued geographic spread and the biological parallels to BSE (Mad Cow Disease), which did make the cross-species jump to humans.
If a deer looks healthy, is it safe to eat?
Not necessarily. The June 2026 study reinforces that infectious prions can be present in tissues of animals that show no clinical signs. Visual inspection alone is insufficient to determine safety. The CDC recommends testing deer and elk for CWD before eating meat in areas where the disease is known to exist.
Which states have confirmed CWD?
As of August 2025, CWD has been confirmed in at least 36 U.S. states. It is most prevalent in the Midwest, West, and Mid-Atlantic, but its range is expanding. Contact your state wildlife agency for the most current prevalence map and testing availability.
What parts of a deer should not be eaten even if the animal tests negative?
The CDC recommends avoiding the consumption of brain, spinal cord, eyes, spleen, tonsils, and lymph nodes — the tissues with the highest prion concentrations — even from animals in CWD-positive areas, as a precautionary measure, regardless of test results.
