Chronic wasting disease (CWD), is a transmissible spongiform encephalopathy (TSE) naturally affecting members of the deer family. TSEs are a family of diseases caused by misfolded proteins called prions and include similar diseases such as mad cow disease in cattle, Creutzfeldt–Jakob disease in humans, and scrapie in sheep. In the United States, CWD affects mule deer, white-tailed deer, red deer, sika deer, elk, caribou, and moose. The transmission of CWD to other species such as squirrel monkeys and humanized mice has been observed in experimental settings.
In 1967, CWD was first identified in mule deer at a government research facility in northern Colorado, United States. It was initially recognized as a clinical "wasting" syndrome and then in 1978, it was identified more specifically as a TSE disease. Since then, CWD has been found in free-ranging and captive animal populations in 33 US states and five Canadian provinces. In addition, CWD has been found in one Minnesota red deer farm, one wild reindeer herd in Norway (March 2016) as well as in wild moose. Single cases of CWD in moose have been found in Finland (March 2018) and in Sweden (March and May 2019, September 2020). CWD was found in South Korea in some deer imported from Canada. CWD is typified by chronic weight loss and clinical signs compatible with brain lesions, aggravated over time, always leading to death.
Although reports in the popular press have been made of humans being affected by CWD, a 2004 study for the US Centers for Disease Control and Prevention concluded that "more epidemiologic and laboratory studies are needed to monitor the possibility of such transmissions". A 2019 study added that "the potential exists for transmission to humans and subsequent human disease". The epidemiological study further concluded, "as a precaution, hunters should avoid eating deer and elk tissues known to harbor the CWD agent (e.g., brain, spinal cord, eyes, spleen, tonsils, lymph nodes) from areas where CWD has been identified". In April 2024, it was revealed that two men from the same hunting group contracted Creutzfeldt–Jakob, prompting medical researchers to speculate transmission had occurred from consuming CWD-positive venison.
Signs and symptoms Due to CWD taking 18–24 months after initial exposure to onset, most cases of CWD occur in adult animals; the youngest animal to exhibit clinical symptoms of the disease was 15 months. The disease is progressive and always fatal. The first signs are difficulties in movement. The most obvious and consistent clinical sign of CWD is weight loss over time. Behavioral changes also occur in the majority of cases, including decreased interactions with other animals, listlessness, lowering of the head, tremors, repetitive walking in set patterns, and nervousness. Excessive salivation and grinding of the teeth also are observed. Most deer show increased drinking and urination; the increased drinking and salivation may contribute to the spread of the disease. Loss of fear of humans and appearance of confusion are also common. The Animal and Plant Health Inspection Service summarized it as:
Cause The cause of CWD (like other TSEs, such as scrapie and bovine spongiform encephalopathy) is a prion, a misfolded form of a normal protein, known as prion protein (PrP), that is most commonly found in the central nervous system and peripheral nervous system. The misfolded form has been shown to be capable of converting normally folded prion protein, PrPC ("C" for cellular) into an abnormal form, PrPSc ("Sc" for scrapie), thus leading to a chain reaction; CWD is thought to be transmitted by this mechanism. The abnormality in PrP has its genetic basis in a particular variant of the protein-coding gene PRNP that is highly conserved among mammals and has been found and sequenced in deer. The build-up of PrPSc in the brain is associated with widespread neurodegeneration.
Genetics The allele which encodes leucine, codon 132 in the family of Elks, is either homozygous LL, homozygous MM, or heterozygous ML. Individuals with the first encoding seem to resist clinical signs of CWD, whereas individuals with either of the other two encodings have much shorter incubation periods.
In white-tailed deer, polymorphisms at codons 95 (Q->H) and 96 (G->S) dramatically affect CWD progression and prion strain specification. Deer containing the 96S allele have delayed progression to clinical disease onset.
When deer with the 95H polymorphism become infected, they select for altered strains of CWD.
Spread Researchers in July 2019 stated that "with all the research on the malignity of prions, and the permanence of prions in the wider environment, and their resistance to destruction and degradation, it is necessary to reduce the potential sources of exposure to CWD." In fact an APHIS scientist observed that, while the longevity of CWD prion is unknown, the scrapie prion has been measured to endure for 16 years. PrPCWD is insoluble in all but the strongest solvents, and highly resistant to digestion by proteases. PrPCWD converts the normal protein PrPC into more of itself upon contact, and binds together forming aggregates. Prusiner noted in 2001 that:
CWD prions are transmissible within the Cervidae family. The disease has not been confirmed being transmissible to humans or livestock.
Nevertheless, a 2024 study highlighted a potential link between CWD and human prion disease following the death of two hunters who consumed meat from a CWD-infected deer population. In 2022, a 72-year-old man died within a month of presenting with confusion and aggression after his friend had previously died of Creutzfeldt–Jakob. While the diagnosis was confirmed as sporadic Creutzfeldt–Jakob, researchers emphasized that cross-species transmission is plausible in animal models and could not be ruled out in this cluster, emphasizing the need for further surveillance.
How the prions that cause CWD spread is unknown, but recent research indicates that prions can be excreted by deer and elk, and are transmitted by eating grass growing in contaminated soil. Animals born in captivity and those born in the wild have been affected with the disease. Transmission of CWD is thought to be lateral (from animal to animal). Vertical transmission may occur, although it appears to be relatively unimportant in maintaining epidemics. An infected deer's saliva is able to spread the CWD prions. Exposure between animals is associated with sharing food and water sources contaminated with CWD prions shed by diseased deer.
Direct
CWD may be directly transmitted by contact with infected animals, their bodily tissues, and their bodily fluids. Spread may result from contact with infected deer regardless of if they are symptomatic.
Ticks harbor transmission-relevant quantities of infectious prions; consumption of ticks during grooming (alogrooming), a behavior typical of interactions among female deer and females and young, may contribute to oral prion exposures and between-deer (including maternal to offspring) transmission.
Recent research on Rocky Mountain elk found that with CWD-infected females, many subclinical, a high rate (80%) of maternal-to-offspring transmission of CWD prions occurred, regardless of gestational period. While not dispositive relative to disease development in the fetus, this does suggest that maternal transmission may be yet another important route of direct CWD transmission.
Experimental transmission
In addition to the cervid species in which CWD is known to naturally occur, black-tailed deer and European red deer have been demonstrated to be naturally susceptible to CWD. Other cervid species, including caribou, are also suspected to be naturally vulnerable to this disease. Many other non-cervid mammalian species have been experimentally infected with CWD, either orally or by intracerebral inoculation. These species include monkeys, sheep, cattle, prairie voles, mice, and ferrets.
An experimental case study of oral transmission of CWD to reindeer shows certain reindeer breeds may be susceptible to CWD, while other subpopulations may be protective against CWD in free-ranging populations. None of the reindeer in the study showed symptoms of CWD, potentially signifying resistance to different CWD strains.
Experimental research has also shown that CWD can be transmitted to raccoons via intracerebral inoculation, although with limited efficiency and long incubation periods, suggesting that some non-cervid wildlife may serve as incidental hosts under specific conditions.
Indirect
Environmental transmission has been linked to contact with infected bodily fluids and tissues, as well as contact with contaminated environments. Once in the environment, CWD prions may remain infectious for many years. Thus, decomposition of diseased carcasses, infected "gut piles" from hunters who field dress their cervid harvests, and the urine, saliva, feces, and antler velvet of infected individuals that are deposited in the environment, all have the potential to create infectious environmental reservoirs of CWD.
In 2013, researchers at the National Wildlife Research Center in Fort Collins, Colorado, successfully infected white-tailed deer with the misfolded prion via the nasal passage, when the prions were mixed with clay. This was important because the prions had already been shown by 2006 to bind with sandy quartz clay minerals.
One avian scavenger, the American crow, was recently evaluated as a potential vector for CWD. As CWD prions remain viable after passing through the bird's digestive tract, crows represent a possible mechanism for the creation of environmental reservoirs of CWD. Additionally, the crows' extensive geographic range presents ample opportunities for them to come in contact with CWD. This, with the added population density and longevity of communal roosting sites in both urban and rural locations, suggests that the fecal deposits at roosting sites may represent a CWD environmental reservoir. Conservative estimates for crows' fecal deposits at one winter roosting site for one winter season ranged from 391,552 to 599,032 kg.
CWD prions bind so tightly to soil surface particles that the ground becomes a source of infection and may be a major route of transmission due to frequent ground contact when animals in the deer family graze.
Prevention
By 2012, a voluntary system of control was published by APHIS in the US Federal Register. It depended on voluntary minimum standards methodology, and herd certification programs to avoid interstate movement of the disease vector. It was based on a risk management framework. As of August 2019, APHIS law in 9 CFR Part 55 - CONTROL OF CHRONIC WASTING DISEASE dealt with this problem.
Introduced for the 2019 Minnesota hunting season, a no-cost deer carcass-incineration program was rolled out by Crow Wing County officials hoping to stem the spread of CWD in the region. CWD was found among wild deer in Crow Wing County for the first time in January 2019. The voluntary program encourages both residents and visiting hunters to bring harvested deer carcasses to the county landfill east of Brainerd, Minnesota for incineration and disposal.
In Pennsylvania, the Game Commission established a network of CWD disposal dumpsters across affected regions to help hunters properly dispose of high-risk deer parts such as heads and spinal columns. This initiative, which began in 2018, aims to reduce environmental contamination and prevent the unintentional spread of infectious prions to new areas. Educational outreach and mandatory testing zones were also implemented to increase hunter compliance and awareness.