What Is Chronic Wasting Disease?
Chronic Wasting Disease (CWD) is an always fatal, contagious, neurological disease affecting deer species (including reindeer), elk, and moose. It causes a characteristic spongy degeneration of the brains of infected animals resulting in emaciation, abnormal behavior, loss of bodily functions and death.
CWD belongs to a group of diseases known as transmissible spongiform encephalopathies (TSEs). Within the TSE family, there are several diseases that affect domestic animals including scrapie, which has been identified in domestic sheep and goats for more than 200 years; bovine spongiform encephalopathy (BSE) in cattle (also known as “mad cow disease”); and transmissible mink encephalopathy in farmed mink.
Several rare human diseases are TSEs. Creutzfeldt-Jakob disease (CJD) occurs naturally in about one out of every one million people worldwide. Variant Creutzfeldt-Jakob disease (v-CJD) has been associated with contaminated beef consumption during the large-scale BSE outbreak in cattle herds in Great Britain.
What Causes CWD?
The most widely accepted theory is that the CWD agent is a misfolded prion, an abnormal form of cellular protein that is most commonly found in the central nervous system and lymphoid tissue. The abnormal prion “infects” the host animal by promoting conversion of normal cellular prion protein to the abnormal form. As a protein, the CWD agent does not contain any genetic material (DNA or RNA) as occurs with bacterial, viral, fungal, and parasitic disease agents.
The CWD infectious agent is smaller than most viral particles and does not evoke any detectable immune response or inflammatory reaction in the host animal. Like the abnormal prions that cause other TSEs, the CWD agent is highly resistant to enzymes, chemicals, heat and other materials or procedures that typically are used to inactivate disease agents.
Which Wildlife Species Are Affected by CWD
Several species of the deer family (cervids) are known to be naturally susceptible to CWD: elk, mule deer, white-tailed deer, reindeer (caribou), red deer, and moose. Captive Sika deer, captive red deer, and captive Sitka/red deer hybrids in South Korea have reportedly contracted CWD from infected captive elk imported from Saskatchewan. Susceptibility of additional cervids and other wildlife species is not entirely known, although ongoing research is further exploring this question. Surveillance in areas with heavily infected wild cervid populations has failed to detect CWD in any mammalian predators or scavengers.
Can Humans Get CWD?
Although humans can develop a prion disease called Creutzfeldt-Jakob disease, current research shows that there is a significant barrier that makes CWD transmission to humans unlikely. Given the untreatable and always-fatal nature of prion diseases, animal models (i.e., non-human primates and genetically altered mice, etc.) have been used in laboratories to investigate the potential susceptibility of humans to CWD. In general, the results of these animal model studies indicate that animal to human transmission of CWD is unlikely. However, one well-publicized study using macaques found that this species, which is closely related to humans, was susceptible to CWD when fed meat from deer with CWD. Conversely, another macaque study completed by the National Institutes of Health found no evidence of CWD transmission to this species. In addition to experimental work, epidemiological investigations in areas where CWD is well established in wild deer and elk have failed to detect any association between CWD and human neurological disease.
Public health officials currently advise that there is no strong evidence of CWD transmission to humans; however, they recommend that human exposure to the CWD infectious agent be avoided as they continue to evaluate any potential health risk.
What Can Hunters Do to Avoid Exposure to the CWD Agent?
The CDC recommends that hunters who harvest cervids in CWD-affected areas strongly consider having their animal tested for CWD. However, it should be noted that the CWD test is used primarily for disease surveillance and is NOT A FOOD SAFETY TEST.
Public health and wildlife officials advise hunters to take the following precautions when pursuing or handling deer and elk that may have been exposed to CWD:
- Avoid consuming the meat from any animal that tests positive for CWD or appears unhealthy no matter what the cause.
- Do not shoot, handle, or consume any animal that is acting abnormally or appears to be sick. Notify your state fish and wildlife agency if you see or harvest an animal that appears sick.
- Wear latex or rubber gloves when field dressing your deer, elk, or moose.
- Bone out the meat from your animal. Do not saw through bone and avoid cutting through the brain or spinal column (backbone).
- Minimize the handling of brain and spinal tissues.
- Wash hands and instruments thoroughly after field dressing is completed.
- Soaking clean (i.e., absolutely no solid tissue/foreign material present) instruments for 5 minutes in a 40% solution of household bleach has been shown to inactivate prions on stainless steel.
- Avoid consuming brain, spinal cord, eyes, spleen, tonsils and lymph nodes of harvested animals. (Normal field dressing coupled with boning out a carcass will remove most, if not all, of these body parts. Cutting away all fatty tissue will remove remaining lymph nodes.)
- If you have your deer or elk commercially processed, request that your animal be processed individually without meat from other animals being added to meat from your animal.
Can Livestock Get CWD?
Although there are livestock-specific prion diseases such as scrapie in sheep and bovine spongiform encephalopathy in cattle, there are no reported cases of natural transmission of CWD from infected cervids to domestic livestock. A research study conducted by the University of Wyoming’s Wyoming State Veterinary Laboratory found no evidence of CWD in cattle co-housed and exposed to CWD-infected deer for a period of ten years. However, the disease has been reproduced experimentally in cattle by the direct injection of the infectious agent into their brains. Additionally, one experimental study found that the CWD agent was detected in lymph nodes of the gastrointestinal tract of swine as early as four months after they were orally inoculated, although they only rarely developed clinical disease. Several investigations are ongoing to further examine this question.
Where Does CWD Occur?
The CWD distribution map in North America shows general CWD areas where the disease has been detected in the wild and in captive cervids. As of July 2020, CWD has been detected in 26 states; in wild cervids in 24 states and in two Canadian provinces and in captive cervid herds in 17 states and four Canadian provinces. Wild reindeer, red deer, and/or moose with CWD have been found in Finland, Norway, and Sweden while CWD has been detected in captive cervids in South Korea.
Where and How Did CWD Originate?
The origin of CWD is unknown, and it may never be possible to definitively determine how or when CWD arose. It was first recognized as a syndrome in captive mule deer held in wildlife research facilities in Colorado in the late 1960s, but it was not identified as a TSE until the late 1970s. Computer modeling suggests the disease may have been present in free-ranging populations of mule deer since at least the 1960’s.
Scrapie, a TSE of domestic sheep, has been recognized in the United States since 1947 and it is possible that CWD was derived from scrapie. It has been hypothesized, though not proven, that deer came into contact with scrapie-infected sheep either on shared pastures or in captivity somewhere along the front range of the Rocky Mountains where high levels of sheep grazing occurred in the early 1900s. This may have occurred in other areas as well. For example, the region in Norway where CWD was found in a wild reindeer population also is an area where significant sheep grazing occurred. This raises the possibility that CWD has originated in more than one location rather than exclusively spreading from one state or region.
It also may be possible that CWD is a spontaneous prion disease that arose in deer in the wild or in captivity and has biological features promoting transmission to other deer and elk.
How is CWD Transmitted?
The mechanisms of CWD transmission are not completely understood. Most transmission appears to be via the oral route although aerosol transmission has been demonstrated under experimental conditions. Transmission is thought primarily to be lateral (from animal to animal). Although vertical transmission (from mother to fetus) may occur, it appears to be relatively insignificant.
The CWD agent can be present in many organs and tissues within the body and can be shed by an infected animal in feces, urine, and saliva prior to onset of clinical disease. Because CWD infectious agents can be found in feces, saliva, urine, and decomposing carcasses and are extremely resistant to decomposition in the environment, transmission of CWD may occur indirectly via exposure to a contaminated environment as well as directly from animal to animal. Thus, artificially concentrating deer and elk in captivity or by providing supplemental feed, bait, minerals, or other materials to wild animals are all human behaviors that increase opportunities for direct and indirect transmission of CWD. Contaminated pastures may have served as sources of infection in some CWD outbreaks since experimental studies completed by Colorado Parks and Wildlife showed that CWD-free deer developed the disease when placed in paddocks in which CWD-affected carcasses had decomposed more than one year earlier. The persistence of the CWD agent in contaminated environments and a lack of proven decontamination procedures represent a significant obstacle to the eradication of CWD from captive and free-ranging cervid populations.
How Does CWD Spread Geographically?
The natural and human-facilitated movement of captive and wild animals is one of the greatest risk factors in introducing CWD into new areas. The natural movements of wild deer and elk contribute to the spread of CWD once it becomes established. However, human-aided transportation of captive and wild animals greatly exacerbates this risk and the spread of CWD among captive herds via live animal movement is well documented
The apparent spread of CWD between captive and wild cervids is a matter of hot debate. Although CWD apparently has spread from captive to wild cervids in some instances, it is an inaccurate oversimplification to believe that all CWD outbreaks in wild cervids are due to spillover from captive deer or elk. It is likely that the disease has been passed from captive to wild animals as well as from wild to captive animals, and CWD transmission between wild and captive cervids should be regarded as a two-way street.
Another risk factor for the spread of CWD to new areas is via the carcasses or carcass parts of dead, infected cervids. Most states restrict the importation of intact carcasses and high-risk tissues from states known to have CWD or from all states. In addition, some states prohibit movement (intrastate and interstate) of carcasses and certain parts out of CWD- affected areas. Please visit the Hunter FAQ and Recommendations for Hunters pages of this website for more information on how to handle and transport carcass parts.
What Are the Signs That an Animal Has CWD?
Most animals infected with CWD appear clinically normal. This is because the minimal incubation period between infection and the onset of signs of clinical disease is approximately 16 months. The maximal incubation period is unknown, as is the point at which shedding of the CWD agent in feces, saliva, and urine begins during the prolonged incubation period. While yearlings and fawns can contract the disease, late-stage signs of CWD are usually only visible in adult animals because of the disease’s prolonged incubation period. In most cases, animals with CWD show no visible signs of the disease for all but the last few months of the disease’s cycle. Thus, the majority of infected animals appear healthy and are impossible to distinguish from uninfected animals. Because CWD affects the central nervous system of infected animals, other causes of mortality (predators, vehicle collisions, other diseases) often remove infected animals from the population before outward signs of the disease become apparent. This is one reason that sick or dead animals with typical signs of CWD are found only occasionally by hunters, wildlife watchers, and the public.
If an infected animal survives to the final stage of this always-fatal disease, the most obvious and consistent clinical sign is emaciation. CWD-affected animals continue to eat but amounts of feed consumed are reduced, leading to gradual loss of body condition. Excessive drinking and urination also are common in the terminal stages.
Behavioral changes also occur in the majority of cases of late-stage CWD, including decreased interactions with other animals, listlessness, lowering of the head, blank facial expression, and repetitive walking in circles or other set patterns. In elk, behavioral changes may also include hyper-excitability and nervousness. Excessive salivation, drooling, and grinding of the teeth also are observed.
It is important to note that the above-mentioned clinical signs alone are not sufficient to definitively diagnose CWD. There are other maladies that can have signs that mimic those of CWD such as brain abscesses, brain worm in elk, trauma-related injuries, or other diseases such as epizootic hemorrhagic disease. Currently, the only conclusive diagnosis for CWD involves examination and testing of the brain, tonsils and/or lymph nodes after death.
How Is CWD Detected?
Clinical signs and visible lesions of CWD alone are not conclusive, and currently there is no practical live animal test. A definitive diagnosis is based on examination of the brain for the characteristic microscopic spongiform lesions and/or accumulation of the CWD-associated prion protein in brain and lymphoid tissues using a technique called immunohistochemistry. Gross lesions seen at necropsy reflect the clinical signs of CWD, primarily emaciation. Aspiration pneumonia, which may be the actual cause of death, also is a common finding in animals affected with CWD.
Research to develop live-animal diagnostic tests for CWD is an area of intense interest. Tests currently can be conducted on biopsy samples of lymphoid tissue from tonsils, lymph nodes, and rectal mucosa of live deer and elk, but this is not practical on a large-scale basis for free-ranging populations. In addition, these tests are currently not as accurate at detecting infected animals when compared to testing samples from dead deer and elk.
Why Are We Concerned About CWD?
CWD poses serious problems for wildlife managers, hunters, wildlife watchers, and the implications for free-ranging deer and elk are significant:
- Heavily infected populations of wild deer and elk are unlikely to thrive, and negative population-level impacts of CWD have been well documented in deer and elk.
- Where it occurs, CWD usually alters the management of wild deer and elk populations, particularly harvest regulations.
- Ongoing surveillance programs are expensive and divert resources from other wildlife management needs.
- Minimizing human exposure to CWD appears prudent.
- Ultimately, public and agency concerns and perceptions about human health risks may erode hunters’ confidence and their willingness to hunt in areas where CWD occurs.
What Is Being Done About CWD?
Efforts to address CWD are accelerating rapidly. Below are only a few of the efforts currently underway:
- The Association of Fish and Wildlife Agencies (AFWA), which represents all 50 state fish and wildlife management agencies, developed and approved best management practices for CWD in 2018. Science-based tools recommended by AFWA include, but are not limited to, regulation of live cervid and carcass movements, prohibition of activities that artificially congregate susceptible species, targeted removal, hunting, surveillance and monitoring, and public education.
- Many states have developed science-based CWD surveillance, response, and/or management plans and made them available on their websites.
- Several state wildlife agencies aggressively collect and test wild elk and deer for the presence of CWD, and have instituted surveillance programs to examine hunter-harvested deer and elk.
- Selective and localized culling of animals suspected to have been exposed to CWD is or has been practiced by some states to slow transmission of the disease.
- Some state wildlife agencies have adopted regulations that prohibit transportation of intact, hunter-harvested cervid carcasses or high-risk carcass parts out of CWD–affected areas.
- The use or sale of natural, urine-based hunting lures has been banned in some states due to concerns that these products could be contaminated with the CWD agent and/or cause unnatural congregation of wild cervids.
- Many states allow importation only of boned out meat, quarters (without spinal column or head), processed venison, cleaned skull plates and antlers, and taxidermy specimens, etc. from CWD-affected areas or from all out-of-state locations.
- Several states have implemented moratoriums on the importation of live cervids. Some states have also halted intra-state movement of live deer and elk and banned supplemental and baiting feeding programs.
- CWD surveillance of captive cervid farming operations is voluntary unless herd owners wish to ship animals interstate in which case participation in a cooperative State/Federal (USDA-APHIS-Veterinary Services) Herd Certification Program (HCP) is mandatory. Currently CWD testing of all cervids older than one year of age that die on site for five years without detection of an infected animal, as well as fencing, inventory and other requirements must be met prior to certification to ship animals interstate. When CWD is detected in a captive cervid facility, that facility typically is quarantined, and a herd plan is adopted that may or may not include destruction of all captive cervids in the facility.
- Research is being conducted to develop better techniques for prevention, detection, and management as well as the social implications of CWD management.
- Federal funding is being sought to develop or enhance CWD-related research, management, and public information/education.
Where Can I Learn More About CWD?
Additional information is available on this website, including a number of links to other websites providing credible information about CWD. The links to state wildlife agencies provide considerable in-depth information regarding CWD status in those states.