Chronic wasting disease (CWD) is a contagious disease of captive and free-ranging deer and elk that causes progressive, fatal CNS disease in adult animals. It is a member of the transmissible spongiform encephalopathy (TSE) family of diseases that includes bovine spongiform encephalopathy; scrapie of sheep and goats; transmissible mink encephalopathy; and kuru, Creutzfeldt-Jakob disease (CJD), and variant CJD of humans. CWD was first identified as a clinical syndrome in the late 1960s among captive mule deer in Colorado; a decade later, it was recognized to be a spongiform encephalopathy with characteristics similar to those of scrapie. It is found in free-ranging populations of mule deer, white-tailed deer, and elk (wapiti) in southeastern and south-central Wyoming, northern Colorado, northwestern Nebraska, and southwestern South Dakota and in a population of white-tailed deer in south-central Wisconsin. Small numbers of free-ranging deer in other states have been identified by intensive testing; it is not yet known if these represent endemic foci. CWD has also been found in farmed elk and white-tailed deer in a number of western states and Canadian provinces and a few midwestern states. CWD has been identified outside of North America only once; a few elk imported into Korea from Canada had CWD. Many states and provinces have developed regulations for control and management of CWD in farmed populations, and federal regulations are in place in Canada and the USA. It is a reportable disease in most jurisdictions.
The etiologic agent of CWD is a protein with abnormal conformation (ie, protease-resistant prion protein). These disease-associated proteins are derived from normal host cell surface glycoproteins (designated cellular prion protein or PrPC). Disease-associated proteins cause PrPC to assume an abnormal conformation that is highly resistant to the cellular processes that break down normal proteins. Thus the abnormal PrP accumulates in host tissues.
CWD is known to naturally affect only mule deer, white-tailed deer, and elk. Experimentally, CWD can be transmitted by intra-cerebral inoculation to cattle, sheep, goats, domestic ferrets, mink, mice, hamsters, and squirrel monkeys. Transmission following oral inoculation of other cervid species has been reported. A large study to investigate susceptibility of cattle by the more natural route of oral or contact exposure began in 1997, and there has been evidence of CWD in the experimentally exposed cattle. Differences in species susceptibility to CWD probably relate to sequence differences among normal host PrP proteins.
Transmission, Epidemiology, and Pathogenesis
CWD is transmitted horizontally. Contact with either infected animals or carcasses of animals with CWD results in transmission of disease to other susceptible cervids. While maternal transmission cannot be ruled out, it does not appear to be important. The agent probably enters a susceptible host via ingestion and is taken up by lymphoid tissues associated with the alimentary tract. The agent can be detected in lymphoid, nervous tissues, blood, saliva, and muscle tissue. Prions can be detected in the blood and saliva of infected animals as soon as 3 mo after infection. The agent most likely arrives in the brain by retrograde movement up the vagus nerve to the dorsal motor nucleus of the vagus at the obex region of the medulla oblongata. Spongiform lesions in the brain develop first in the vagal nucleus at about the time of onset of clinical disease. This occurs naturally with an incubation period of ∼1.5–3 yr, although maximum incubation periods have yet to be determined. Prevalence in captive herds of deer and elk may reach nearly 100% in heavily contaminated facilities; prevalence in free-ranging cervids is extremely variable from <1% to ∼30%. Because prions are highly resistant to environmental and chemical inactivation, they may accumulate in the environment and thus be available to infect susceptible cervids. Consequently, close confinement of farm-raised cervids will likely potentiate the spread of CWD. Likewise, winter feeding of deer and elk will concentrate cervid populations and will likely potentiate horizontal transmission. Decomposing carcasses may provide a source of infectivity to susceptible animals. The historical loss of natural predator and scavenger populations (wolf, bear, mountain lion, etc) may have reduced the reservoir of infection and limited widespread environmental contamination with the infectious agent.
The movement of CWD in populations of free-ranging deer and elk follows natural migration routes, often along waterways and natural corridors. In the past, movement of CWD in farmed deer and elk in commerce was through human-facilitated transportation of animals incubating CWD. Now that programs and regulations are in place in most jurisdictions, movement of CWD in live animals should be curtailed. Surveillance, however, should continue.
Animals with clinical CWD are >16 mo of age and show a spectrum of signs. The earliest and most difficult to appreciate are subtle changes in behavior and weight loss. These changes are often detectable only by animal caregivers familiar with the individual animal. As the disease progresses, behavioral changes may include alterations in how the animal interacts with herdmates and caregivers, loss of wariness, somnolence, persistent walking, polydipsia and polyuria, and hyperexcitability when handled. Affected animals may show variable locomotor signs including ataxia (especially posterior ataxia) and head tremors. Late in the disease, animals may have a low head carriage, drooped ears, and fixed staring gaze; they may hypersalivate and grind their teeth. Death following routine chemical immobilization has been noted. Aspiration pneumonia may be the only presenting clinical sign; CWD should be suspected in any adult cervid with aspiration pneumonia. Weight loss is progressive throughout the course of disease, even when adequate feed is present, but it is important to recognize that CWD may be present in cervids that are not emaciated. Death of CWD-affected animals may be precipitated by cold weather or other acute stressors. Affected cervids are more susceptible to hunting, predation, vehicle collisions, and other forms of death by misadventure.
Some animals may die of CWD without gross lesions. When present, gross lesions are nonspecific and reflect the clinical signs. The most common are poor body condition, watery rumen contents, and dilute urine. Aspiration pneumonia in farmed cervids is often the proximal cause of death. Samples should be, and in many jurisdictions are required to be, submitted to certified laboratories to be tested for evidence of CWD. It is good practice either to send the carcass to the diagnostic laboratory or to collect a wide variety of samples so that if diseases other than CWD are present they will be identified. At a minimum, samples for CWD testing should include brain and retropharyngeal lymph nodes. Many laboratories accept whole heads from cervids for testing. Surveillance programs for free-ranging cervids vary depending on the jurisdiction and are usually conducted by the local wildlife management agency, which should be consulted if CWD is suspected in a free-ranging deer or elk. Carcasses and offal should be disposed of in a manner that limits the exposure of farm-raised and free-ranging cervids to any potentially infectious material.
Diagnosis of CWD is by detecting disease-associated PrP in CNS or lymphoid tissues. Detection of spongiform encephalopathy by routine histopathology has been supplanted by more sensitive and specific tests, which include immunohisto-chemistry or ELISA on brain and/or lymph nodes. In the USA, these tests are run only at USDA-certified laboratories. In mule deer and white-tailed deer, the CWD PrP accumulates in the retropharyngeal lymph node prior to arriving in the brain; thus, it is considered to be the most important tissue to collect for testing. Both brain and lymph node samples should be collected from elk. The correct portion of the brain (ie, the obex, at the caudal end of the fourth ventricle below the cerebellum) must be collected for a meaningful test. The laboratory should be consulted to determine if the samples must be fixed in 10% buffered formalin, chilled or frozen, or if portions of the samples should be sent both fixed and frozen. ELISA is used as a screening test, and immunohistochemistry, which is considered the preferred test, is used to confirm positive ELISA.
Differential diagnoses for animals suspected of CWD include brain abscesses, traumatic injuries, meningitis, encephalitis, peritonitis, pneumonia, arthritis, starvation and nutritional deficiencies, dental attrition, and anesthetic deaths.
Treatment and Control
There are no treatments available for any TSE. Control in farmed cervids is by depopulation with indemnity and development of herd plans. These plans typically require 5 yr of monitoring to achieve the highest status. The bases for CWD control programs in the farmed cervid industry are individual animal identification, CWD testing in all animals in the herd that die over a certain age, and limiting new herd additions to animals from herds of comparable or higher CWD status.
Control of CWD in free-ranging populations is extremely difficult and varies depending on the location. All jurisdictions have banned movement of live cervids from endemic areas for translocations, and many have regulations on movement of parts of hunted deer and elk. In areas where CWD occurs, attempts at control have included population reduction, test and removal, and intensified surveillance.
Only a few disinfectants and methods of disposal inactivate prions. Fresh household bleach at 50% concentration for 30–60 min will inactivate the agent and is inexpensive and readily available, but it may be corrosive to some surfaces and instruments. Additional disinfectants are being considered for general use but are not yet approved. Incineration in a medical incinerator, alkaline digestion in specially designed equipment, and disposal in municipal landfills are used for disposal of tissues and carcasses of animals with CWD.
Although CWD has been present in hunted populations of deer and elk for >30 yr, no case of human CWD has been identified. The risk to humans appears to be minimal. The CWD agent has been detected in the muscle tissue of infected cervids. Public health authorities and wildlife management agencies suggest the following precautions for hunters and people handling cervids in the areas where CWD is found to further reduce risk of human exposure: do not harvest deer or elk that appear to be sick or abnormal; wear rubber, plastic, or latex gloves when dressing the carcass; avoid contact with brain, spinal cord, and lymphoid tissues; debone the meat when processing; disinfect knives, saws, and tables with 50% bleach; and have the animal tested for CWD. All pubic health authorities recommend that animals positive for any TSE not be consumed by humans or other animals.
Last full review/revision July 2011 by Jeffrey W. Tyler, DVM, MPVM, PhD,Deceased