This chronic wasting disease review originally was presented at the 67th North American Wildlife and Natural Resources Conference by the late Elizabeth S. Williams (Williams et al, 2002). The review has been updated and is current as of July 2019. Primary editor and contributing author is John R. Fischer, DVM, PhD.
Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy (TSE) of members of the deer family (cervids). Natural infections have been found in mule deer, white-tailed deer, elk, moose, and reindeer among native North American species as well as in red deer, Sika deer, and their crosses, primarily in South Korea. Muntjac deer (native to southern Asia) have been experimentally infected via oral inoculation of brain from CWD-positive white-tailed deer, and Eurasian fallow deer were found to be susceptible to CWD when inoculated directly into the brain. Like all TSEs, CWD is uniformly fatal.
The TSEs have similar clinical features, pathology, and causative agents, which are believed to be abnormal prion proteins (misfolded prions that do not contain genetic material and do not propagate or degrade like other infectious disease agents). There are theories regarding alternate causes of CWD, including bacteria, viruses, and trace mineral imbalances; however, the preponderance of scientific information supports prions as the cause of TSEs and the vast majority of the scientific community accepts this theory.
Other TSEs include bovine spongiform encephalopathy (BSE or “Mad Cow Disease”), which affects cattle, and scrapie, which affects sheep and goats. Among the TSEs, the scrapie and CWD agents are unique in that they can persist in the environment and remain infectious for several years. There are several rare and fatal TSEs of humans, including Creutzfeldt-Jakob disease (CJD) and variant Creutzfeldt-Jakob disease, which is associated with consumption of the BSE agent. Since the 1996 announcement of an apparent relationship between BSE and variant CJD, there has been considerable animal and human health agency and media interest in TSEs. Consequently, CWD is a disease of increasing concern as it continues to be detected in cervids in new areas of North America and northern Europe.
Many biological aspects of CWD pose significant challenges for controlling or eradicating the disease. However, even greater challenges are associated with balancing the complex and often competing or conflicting interests of the general public, hunters, the captive cervid industry, traditional livestock industries, and many state and federal wildlife, animal health and public health agencies. This is a review of the biological features of CWD and strategies for its management.
History of Chronic Wasting Disease
(Key events in CWD chronology can be found in the CWD Timeline)
Chronic wasting disease first was observed as a clinical syndrome of mule deer in research facilities more than 50 years ago. Modeling suggests that the disease may have been present in free-ranging mule deer populations for more than 60 years. The origin of CWD is unknown and it may never be possible to determine how or when CWD arose. 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 is possible, although never proven, that deer were exposed to the scrapie agent 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. It also may be possible that CWD is a spontaneous TSE that arose in deer in the wild or in captivity and has biological features promoting transmission to other cervids.
The overall duration of CWD infection (time from exposure to end-stage clinical disease) has been difficult to determine in natural cases because the time of exposure is unknown. In experimental settings, minimum incubation (time from exposure to onset of clinical disease) was about 15 months and mean time from oral infection to death was about 23 months in mule deer. The incubation period ranged from 12-34 months in orally infected elk. The maximum disease course is unknown, but can exceed 25 months in experimentally infected deer and 34 months in elk. The youngest animal diagnosed with clinical CWD was 17 months old at death, suggesting 16-17 months may be the minimum natural incubation period. Clinically normal fawns as young as 5-6 months have tested positive for CWD. It is unknown when an animal may become infectious to other animals (Williams and Miller, 2002).
Following the prolonged incubation period, CWD-affected animals show loss of body condition and exhibit several behavioral changes. Clinical disease often is more subtle and prolonged in elk than in deer. Affected animals may walk repetitive courses; they may show subtle incoordination and wide-based stance; they may display subtle head tremors; they may be found near water sources; they may have periods of somnolence; and they may carry their ears and/or head lowered. These animals continue to eat but consume reduced amounts of feed, leading to gradual loss of body condition. Excessive drinking, urination, salivation, and drooling are common in the terminal stages.
The clinical stage of CWD varies from a few days to approximately a year, with most animals surviving from a few weeks to several months after the onset of the clinical signs described above. While a protracted clinical course is typical, acute death occasionally may occur; this may be more common in the wild than in the relative security of captivity. Aspiration pneumonia is a common finding at postmortem examination of terminal CWD cases and may confuse the diagnosis if the brain is not examined. Aspiration pneumonia likely is due to difficulty swallowing, excessive salivation, and inhalation of foreign material. Thus, the brain should be examined for evidence of CWD on every prime age cervid that dies with pneumonia.
Clinical signs of CWD alone are not diagnostic as several other diseases cause similar symptoms. Diagnosis can be confirmed upon postmortem examination of the brain for spongiform lesions and/or accumulation of the CWD-associated prion in brain and/or lymphoid tissues. The correct portion of the brain must be examined for a meaningful test.
The CWD prions are detectable long before animals appear sick and the majority of animals that test positive for CWD look and act healthy because samples are obtained in greatest numbers from hunter-harvested cervids. Hunters rarely harvest sick-looking animals, so these infected animals are in the pre-clinical stage of infection.
Lymph nodes near the base of the skull and a specific portion of the brain stem are the samples collected from dead cervids, whether they were killed by hunters or otherwise, and tested. In addition, CWD-associated prion material can be detected in biopsies of lymph nodes, tonsil, and rectal mucosa from live animals. However, testing these samples from live animals is not as sensitive as post-mortem testing, may not detect some infected animals, may result in injury or death of the animal being tested, and is simply not practical on a large-scale basis in wild cervids. Research efforts continue to focus on developing more sensitive tests and some now are available that can detect CWD prions in environmental materials such as soil and water. However, as with samples from live animals, testing these materials is difficult and less reliable than post-mortem testing.
Research and field observations have increased our understanding of CWD, but some details of its transmission remain unknown. In contrast to BSE, CWD is not a foodborne disease associated with rendered ruminant meat and bone meal. Instead, field observations and experimental work provide strong evidence of animal-to-animal (lateral) transmission, which is similar to scrapie. Doe to fawn transmission in utero also may occur but appears to be relatively rare. Some interspecies transmission probably occurs among natural host species with suspected transmission from mule deer to elk, mule deer to white-tailed deer, and elk to mule deer and white-tailed deer.
The CWD agent has been detected in numerous tissues within the body and is shed in saliva, feces, and urine. Shedding of the CWD agent begins prior to development of clinical signs. In one study, CWD prions were detectable by three months (saliva) or six months (urine) post-exposure and remained so throughout the disease course (Henderson et al, 2015).
Prions that cause CWD and other TSEs are extremely resistant in the environment. Consequently, transmission may occur indirectly from a contaminated environment as well as directly from one animal to another. Certain soil elements, such as clay, appear to enhance the persistence and infectivity of CWD prions. Concentrating deer and elk in captivity or by providing feed, bait, or mineral supplements in the wild increases the likelihood of direct and indirect transmission. In fact, a field study in Wisconsin detected CWD prions in soil, water, and/or deer feces at several mineral licks in a CWD-affected area and the researchers concluded that mineral licks can serve as reservoirs for CWD prions and facilitate disease transmission (Plummer et al, 2018). Although CWD prions have not been detected in vegetation in natural environments, experimental studies have shown that plants can bind the prions superficially as well as uptake them from soil with subsequent distribution to leaves and stems. The persistence of CWD prions in contaminated environments represents a significant obstacle to eradication of CWD from captive and free-ranging cervid populations.
Chronic wasting disease infection among captive cervid herds can range from a single infected animal to remarkably high prevalence. In one infected research facility, more than 90% of mule deer resident for more than two years died or were euthanized as a result of suffering from CWD. Nearly 80% of captive, commercial white-tailed deer in a Wisconsin herd of 70 deer and in an Iowa herd of 350 animals tested positive for CWD at depopulation as did all eight animals in a small, affected herd in Minnesota. Among captive elk, CWD was the primary cause of adult mortality in two research herds, and high prevalence (59%) was detected in a group of 17 elk slaughtered from an infected farmed herd.
Unmanaged, CWD-affected areas expand geographically, and prevalence persistently increases in free-ranging cervid populations. To estimate prevalence in free-ranging populations, tissues from clinically normal deer and elk harvested by hunters in CWD-endemic areas are collected and tested at random. In severely affected areas in Wisconsin and Wyoming, prevalence climbed from single digits in the early 2000s to 40-50% fifteen years later. Among deer, prevalence generally is highest in adult males, often double that of adult females. Negative impacts have been documented in whitetail and in mule deer populations with high prevalence in Wyoming and an elk herd with high CWD prevalence in Colorado.
Field transmission of CWD to non-cervid wildlife species, domestic livestock, or humans has not been detected. Surveys for CWD in wild carnivores and omnivores in CWD-endemic areas have yielded no positive animals. Cattle that were cohoused with affected mule deer in a contaminated environment for ten years did not develop clinical disease or test positive for CWD. However, an experimental study demonstrated CWD transmission to domestic pigs inoculated orally or directly into the brain with material from affected deer. The researchers concluded that although the species barrier to CWD transmission in swine is relatively high, it may be possible for swine to serve as a CWD reservoir under natural conditions (Moore et al, 2017).
Currently, there is no strong evidence that CWD is transmissible to humans. However, in the absence of complete information on risk, and in light of similarities of other animal and human TSEs, public health officials and wildlife management professionals recommend that hunters harvesting deer and elk in affected areas, meat processors, and taxidermists handling cervid carcasses should take common sense measures to avoid exposure to the CWD agent and to other zoonotic pathogens.
In recent years, there has been considerable attention directed at the conflicting conclusions from studies of the CWD susceptibility of macaques, which are primates often used experimentally as animal models of human disease. In a study published in 2018, researchers found no apparent transmission of CWD to macaques that received affected cervid brain homogenate orally or directly into the brain (Race et al, 2018). However, preliminary results presented at the Prion 2017 conference in Edinburgh, Scotland indicated CWD transmission to macaques that ingested or were inoculated with muscle or brain from deer with CWD. In response to these preliminary results, the U.S. Centers for Disease Control and Prevention (CDC) revised its recommendations to hunters in 2017. They currently read: “Hunters harvesting wild deer and elk from areas with reported CWD should check state wildlife and public health guidance to see whether testing of animals is recommended or required in a given state or region. In areas where CWD is known to be present, CDC recommends that hunters strongly consider having those animals tested before eating the meat.” Cervids that test positive for CWD should not be consumed. Additional information and recommendations can be found at https://www.cdc.gov/prions/cwd/prevention.html.
The known distribution of CWD in captive and free-ranging cervids continues to increase but likely remains underestimated. Among captive cervid herds, CWD has been identified through a combination of surveillance and epidemiologic investigations. As of July 2019, CWD has been detected in 112 herds of captive, commercial deer and/or elk in 17 states, 109 herds in four Canadian provinces, and in captive elk, red deer, Sika deer and their crosses in South Korea, which imported infected captive elk from Saskatchewan in 1997. Distribution of CWD in free-ranging deer, elk, and moose has been determined primarily through testing of samples from clinically normal, hunter-harvested animals as well as from animals showing clinical signs suggestive of CWD. Chronic wasting disease in free-ranging cervids has been found in 24 states across the United States and in Alberta and Saskatchewan in Canada (see Map). The disease also has been found in wild reindeer, moose, and red deer in Norway as well as in wild moose in Finland and Sweden.
There is no vaccine capable of preventing CWD infection in deer or elk. To date, experimental vaccines have only prolonged the incubation period but have not provided complete immunity. No treatment is available for animals affected with CWD; death is inevitable once clinical signs develop. It follows that controlling CWD is highly problematic. Long incubation periods, extremely subtle signs of early clinical disease, absence of a reliable and practical ante-mortem diagnostic test, environmental contamination with an extremely resistant infectious agent, and incomplete understanding of transmission combine to limit options for controlling or eradicating CWD and underscore the importance of prevention. Primary prevention strategies currently include regulation of the movement of live cervids and cervid carcasses, which are considered the two highest risks for introducing CWD into new areas.
In captive facilities, management options currently are limited to depopulation or quarantine of CWD-affected herds. Repeated attempts to eradicate CWD from cervid research facilities have failed. Whether contaminated environments can ever be completely disinfected remains questionable and current recommendations include an extended open period prior to repopulation following the depopulation of affected captive cervid facilities. Free-ranging cervids also should be excluded from affected and previously-affected captive premises. Free-ranging reservoirs of infection near infected cervid farms could severely impair attempts at eradication from captive facilities.
Difficulties in managing infected captive herds and premises underscore the need for comprehensive surveillance to prevent movement of infected animals in commerce. In 2014, the USDA fully implemented a CWD Herd Certification Program (HCP) in cooperation with state agencies regulating captive cervids. Participation in the HCP is mandatory for interstate shipping of commercial captive cervids; participation is voluntary for shooting enclosures, hobby farms, and other facilities not engaged in interstate shipping. The HCP established minimal requirements, such as testing of all on-site deaths over 12 months of age for a 5-year period, necessary to acquire and maintain herd certification, which is required for interstate shipping. Unfortunately, CWD continues to be detected in herds following their certification, and an interstate shipment of a positive deer from a certified herd has been documented. States may have requirements that are more stringent than the USDA program. Information on the USDA HCP and Cervid Health Program can be found at (https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/animal-disease-information).
Managing CWD in free-ranging animals presents even greater challenges, and goals and strategies vary among affected states and provinces. Management of CWD in wild populations must be regarded as a long-term commitment. Early detection is key to successful control and possible eradication of CWD. In areas where CWD may not yet be endemic, eradication could be considered a reasonable management goal. In fact, eradication attempts are underway in an isolated, mountainous region in Norway where CWD was detected in wild reindeer in 2016. The CWD prevalence in the herd was approximately 1%, and Norwegian authorities believe CWD was introduced (source unknown) recently, was detected early, and eradication was feasible. All wild reindeer (approximately 2,000 animals) in the herd were destroyed by February 2018; the area will remain free of wild reindeer for several years before reintroduction is attempted. The success of this eradication attempt is believed to be enhanced by the low CWD prevalence, apparently early detection, and the mountainous, physical barrier that precludes immigration of reindeer from other areas.
In endemic areas, managers have refrained from committing to eradication because it appears unattainable with the management tools currently available. Management programs in endemic areas generally focus on containing CWD geographically and reducing its prevalence. For example, in the CWD affected area of northern Illinois, sustained selective culling of deer has successfully limited prevalence and slowed geographic expansion. States conduct long-term, active surveillance programs to monitor CWD distribution and prevalence in endemic areas to determine the extent of the affected area and to evaluate the effects of disease management efforts like those in Illinois.
Affected states and provinces have employed a variety of specific strategies for managing CWD in free-ranging wildlife. Translocating and artificially congregating cervids by providing supplemental feed, minerals, and/or bait may be banned or regulated stringently to limit range expansion and decrease transmission. Selective culling of clinical suspects was practiced throughout the original endemic area of Colorado and Wyoming for a number of years, but this approach alone has proven insufficient to reduce prevalence on a herd level.
Although it seems intuitive that lowered herd densities should reduce transmission and likelihood of dispersal of affected animals to adjacent areas, historic migration patterns and social behaviors characteristic of some deer and elk populations may limit the effectiveness of wholesale density reduction. This approach also is likely to be unpopular with hunters, landowners, and the general public, and their support is essential for successful management. Early detection through adequate and sustained surveillance and aggressive intervention via selective culling or more generalized population reduction show the greatest promise of preventing new endemic foci from being established if prevention has failed. This may have proven effective in New York where two free-ranging deer were detected in 2005 in the vicinity of an affected captive cervid facility; no additional infected deer have been found since then despite extensive testing.
Fortunately, excellent guidance on CWD management is available to state and provincial wildlife management agencies. The Western Association of Fish and Wildlife Agencies (WAFWA) developed the Recommendations for Adaptive Management of Chronic Wasting Disease in the West. These recommendations include three primary strategies: reduce artificial points of host concentration, adjust hunting regulations with bias toward males, and develop harvest strategies using prior fall harvest information to maximize removal of infected animals. The WAFWA recommendations are designed to assess management effects and adapt accordingly. They can be found at https://wafwa.org/wpdm-package/recommendations-for-adaptive-management-of-chronic-wasting-disease-in-the-west/.
In 2018, the Association of Fish and Wildlife Agencies (AFWA) adopted the AFWA Best Management Practices for Prevention, Surveillance, and Management of Chronic Wasting Disease. These BMPs and a supporting technical summary provide recommendations to states and provinces as they address the growing threat that CWD poses to wild cervid populations. The BMPs can be found at: https://www.fishwildlife.org/application/files/5215/3729/1805/AFWA_CWD_BMPS_12_September_2018_FINAL.pdf.
Chronic wasting disease in captive and free-ranging cervids represents serious management problems. Captive herds and facilities are quarantined, thus limiting use and value of infected or exposed animals and premises. Indemnity for depopulated cervids has been made available but may not be adequate to remove infected and exposed animals from the landscape. Spillover of CWD from captive herds into local free-ranging cervid populations may have occurred in some locations; further spillover could establish more endemic foci, thereby impairing long-term viability of cervid farming and wildlife management in those areas. It should be noted that CWD transmission is a two-way street at the fence line between captive and free-ranging cervids.
Implications for free-ranging populations of deer and elk may be even more significant. Ongoing surveillance and management programs are expensive and divert limited resources from other wildlife management needs. Perhaps most important, field studies indicate that CWD could substantially impact severely infected cervid populations by lowering adult survival rates and destabilizing long-term population dynamics. Ultimately, public and agency concerns and perceptions about human health risks associated with all TSEs may erode hunting participation in endemic areas and may dramatically influence management of free-ranging cervid herds where CWD is endemic. It follows that responsible wildlife management and animal health agencies should continue working to understand and limit distribution and occurrence of CWD in free-ranging and captive cervids.
Henderson, DM, ND Deckers, CE Hoover, N Garbino, CK Mathiason, and EA Hoover. 2015. Longitudinal detection of prion shedding in saliva and urine by chronic wasting disease-infected deer by real-time quaking-induced conversion. Journal of Virology 89(18): 9338–9347.
Moore, SJ, MHW Greenlee, NMS Kondru, JD Smith, RA Kunkle, A Kanthasamy, and JJ Greenlee. 2017. Experimental transmission of the chronic wasting disease agent to swine after oral or intracranial inoculation. Journal of Virology 91:e00926-17. (https://doi.org/10.1128/JVI.00926-17)
Plummer, IH, CJ Johnson, AR Chesney, JA Pedersen, and MD Samuel. 2018. Mineral licks as environmental reservoirs of chronic wasting disease prions. PLOS. (https://doi.org/10.1371/journal.pone.0196745)
Race, B, K Williams, CD Orru, AG Hughson, L Lubke, and B Chesebro. 2018. Lack of transmission of chronic wasting disease to cynomolgous macaques. Journal of Virology (https://doi.org/10.1128/JVI.00550-18)
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