Chronic wasting disease (CWD) spreads through direct contact between infected deer, elk, and moose, and through contaminated environments where prions persist in soil and water for years. What makes CWD so difficult to control is that infected animals begin shedding infectious prions in their saliva, urine, and feces as early as six months after infection, potentially years before they show any visible symptoms.
Shedding Through Saliva, Urine, and Feces
CWD is caused by misfolded proteins called prions that replicate in an animal’s tissues and eventually spill into bodily fluids. In a longitudinal study of white-tailed deer, researchers detected prions in feces as early as six months after infection, in saliva by nine months, and in urine by 18 months. Feces appear to carry the highest prion concentrations of the three, though saliva and urine contain enough to initiate infection when other deer ingest them.
The long silent period is what makes CWD so contagious. Deer that look perfectly healthy can contaminate shared spaces for a year or more before anyone would suspect they’re sick. In studies of white-tailed deer with certain genetic profiles, some animals remained asymptomatic for four to five years after infection, shedding prions the entire time. Genetics play a role in how consistently an animal sheds: deer with the most common genetic variant (96GG) shed prions more frequently and reliably than deer with rarer variants.
Nose-to-Nose and Social Contact
Direct transmission happens when deer interact in ways that expose them to each other’s bodily fluids. Grooming, nuzzling, sharing food, and even just occupying the same bedding areas all create opportunities. Deer are social animals, and behaviors like mutual grooming and nose-to-nose contact during breeding season put healthy animals in direct contact with prion-laden saliva. Mothers can also expose fawns during nursing and grooming.
Prions Persist in Soil for Years
Once an infected animal deposits prions in the environment through its waste or saliva, or when a carcass decomposes on the landscape, those prions bind to soil particles and remain infectious for years. Unlike bacteria or viruses, prions are not living organisms. They’re misfolded proteins that resist ultraviolet light, heat, and most disinfectants, which means natural weathering does almost nothing to neutralize them.
Soil type matters significantly. Research on mule deer herds in Colorado found that for every 1% increase in clay content within a deer’s home range, the odds of CWD infection rose by up to 8.9%. Clay minerals like montmorillonite and kaolinite bind tightly to prions, and this binding actually enhances their infectivity rather than neutralizing it. Areas with clay-rich soils essentially become long-term reservoirs that keep recycling the disease back into deer populations.
Plants Can Carry Prions From Soil
One of the more surprising transmission pathways involves plants. Several crop and wild plant species can absorb prions through their roots from contaminated soil and move them into stems and leaves. In laboratory experiments, mice that ate above-ground tissues of alfalfa grown in prion-contaminated soil developed clinical prion disease at a significant rate. Barley grown in similar conditions caused subclinical infections, meaning prions accumulated in the animals’ brains and spleens even without obvious symptoms.
This means deer grazing on vegetation in contaminated areas may be ingesting prions even without direct contact with soil, feces, or other deer. The finding has broad implications for how CWD sustains itself in a landscape once established.
Mineral Licks and Feeding Sites as Hot Spots
Any location where deer gather repeatedly creates a concentration point for prion contamination. Natural mineral licks are a prime example. Deer visit these sites to consume mineral-rich soil and water, depositing saliva, urine, and feces each time. Researchers confirmed that CWD-infected white-tailed deer leave detectable prions at mineral licks they visit, and the continuous traffic of both healthy and infected animals turns these spots into transmission hot spots.
The same logic applies to artificial feeding stations, bait piles, salt blocks, and winter feeding areas. This is why many state wildlife agencies ban or restrict supplemental feeding and baiting in areas where CWD has been detected. Even species that avoid direct contact with deer may use the same mineral lick sites at different times, raising the possibility of cross-species exposure.
Scavengers Spread Prions Across the Landscape
Coyotes, crows, and other scavengers that feed on CWD-infected carcasses can pass infectious prions through their digestive tracts and deposit them in new locations. In controlled studies, coyote feces remained infectious for at least three days after the animals consumed prion-contaminated brain tissue. Since transient coyotes range over areas exceeding 100 square kilometers, they can potentially introduce prions to previously uncontaminated territory far from the original infection site.
Crows have also been documented passing infectious prion material in their feces after scavenging. Neither coyotes nor crows appear to develop prion disease themselves, but they serve as mobile vehicles that redistribute prions across the landscape.
Hunters and Carcass Transport
Human activity has likely contributed to CWD’s geographic spread across North America. When hunters harvest a deer in an infected area and transport the carcass, particularly the brain and spinal cord (where prion concentrations are highest), they can introduce prions to new locations. Disposing of carcass remains in a backyard, landfill, or wooded area hundreds of miles from where the animal was killed deposits prions in soil that local deer may later encounter.
This is why most states with known CWD infections restrict the transport of certain carcass parts, typically the brain, spinal column, and head, out of designated management zones. Boned-out meat, cleaned skull caps, and finished taxidermy mounts are generally considered lower risk.
Why Decontamination Is So Difficult
Prions are extraordinarily resistant to destruction. A 40% household bleach solution can decontaminate hard surfaces like stainless steel after a five-minute soak, according to NIH researchers. But bleach fails to penetrate tissue. In the same study, pieces of CWD-infected brain retained prion activity even after soaking in full-strength bleach for 30 minutes. Standard cooking temperatures, freezing, and burial do not eliminate prions either.
This resistance is central to why CWD is considered virtually impossible to eradicate once it’s established in a region. The prions accumulate in soil, bind to clay, get taken up by plants, and cycle through scavengers, creating a web of environmental contamination that persists independently of the deer population itself. Reducing deer density can slow transmission rates, but it cannot address prions already embedded in the landscape.
Risk to Humans
Despite CWD’s aggressive spread among cervids, current evidence supports a strong species barrier between CWD prions and humans. Mice engineered to express human prion proteins have not developed disease after CWD exposure. Cynomolgus macaques, which are genetically closer to humans than any other lab model tested, showed no signs of prion infection even 13 years after inoculation. And human brain organoids exposed to CWD prions in the lab did not propagate the disease.
That said, the CDC considers the zoonotic potential “unknown” rather than zero, particularly given the high likelihood that CWD-contaminated venison enters the human food supply. Testing harvested deer before consuming the meat remains the most practical precaution for hunters in affected areas.