Chronic Wasting Disease

This fact sheet was published on March, 2020. 

Cause

Chronic wasting disease (CWD) is caused by a prion, which is a protein that can behave like an infectious agent. These infectious proteins cause a degenerative neurological disease of domestic and wild members of the deer family (deer, elk, reindeer, and moose) otherwise known as cervids. It is categorized as a transmissible spongiform encephalopathy (TSE) or infectious diseases of the brain where cells are lost, leaving vacant areas. Other known TSEs include Scrapie in domestic sheep, Bovine Spongiform Encephalopathy (mad cow disease) in domestic cattle, and Creutzfeldt-Jakob disease (CJD) in humans. CWD was fi­rst reported in captive mule deer in 1967 in Colorado, but was not identified in the wild until 1981 in an affected elk from the same state. CWD is always fatal.

Significance

Chronic wasting disease has been shown to cause mule deer populations to decline and long- term predictions suggest that these and other affected species could be locally extirpated. In turn, CWD has had a major impact on hunting and cervid farming in affected states and provinces. Because of a long shedding and incubation period, and the persistence of the prion responsible for CWD in the environment, it is considered impossible to control or eradicate the disease.

Species Affected

CWD is known to affect free-ranging and captive white-tailed deer, mule deer, black-tailed deer, Rocky Mountain elk, moose, reindeer, and red deer, though it is likely that other cervid species are also susceptible. This disease is not known to infect humans, however the Centers for Disease Control and Prevention (CDC) states that, although there are no reported human cases, animal studies in non-human primates suggest that CWD is transmissible via ingestion of CWD infected meat, as well as contact with brain or body fluids from infected animals. CDC is therefore concerned that CWD may pose a risk to people. Along with the World Health Organization they recommend testing all harvested cervids and not consuming the meat if CWD is detected.

 Carnivores and scavengers have been shown to be able to pass infectious prions in their feces without become infected themselves.

Distribution

CWD was originally found in a captive mule deer herd in Colorado and was reported in Wyoming soon after. Over the past 52 years the disease has spread to all bordering states as well as the Midwest and the Mid-Atlantic. As of March 2020, CWD has been reported in 26 states and 3 Canadian provinces (see map). It was exported to South Korea following importation of infected elk from Canada. CWD was identified in 2016 in Norway (reindeer, moose, red deer), in 2018 in Finland (moose), and in 2019 in Sweden (moose). It is possible that CWD may also occur in other states, provinces and countries without strong animal surveillance systems where it remains undetected.

Transmission

CWD is transmitted both directly via contact between infected animals through prion contaminated feces, urine, saliva, and indirectly through infected carcasses or environments. Though it is still uncertain, current research suggests that the minimum incubation period from time of infection to the presentation of clinical signs is around 16 months, but can range from 42 days to 5 years. Animals can directly transmit the disease to other animals during this time. Importantly, the CWD prion has been detected in animals’ feces for at least 11 months before they start exhibiting overt signs of disease. The prions that cause CWD are extremely resistant to heat and disinfectants, and can remain infective in soil for many years, perhaps indefinitely.

Clinical Signs

CWD targets the brain and other neurological tissues as well as the lymph or immune system in the body resulting in progressive loss of normal bodily functions and abnormal behavior.

Reported clinical signs include loss of body condition (wasting), depression, grinding of teeth and excessive salivation (due to difficulty swallowing), drinking, and urination. One or more behavioral changes including depression, listlessness, stumbling, lowering of the head and drooping of the ears may be seen. Infected animals also lack fear of humans and predators. The course of clinical disease can extend to a year, but most animals will die within several months.

In elk and moose, the clinical signs are often more subtle and prolonged.

Diagnosis

It is not possible to diagnose cervids with CWD based on clinical signs alone because many other diseases and conditions cause similar signs. In addition, other conditions like pneumonia can be present in a CWD affected animal. Therefore, post mortem examination of suspected animals is required for diagnosis. Special laboratory techniques are used to diagnose CWD from samples of brain (more specifically the obex region), lymph node (retropharyngeal lymph nodes), or tonsil tissue.

Treatment

There is currently no treatment or vaccine for CWD.

Management

Management of chronic wasting disease has largely proven impossible and eradication of this disease where it currently exists is unlikely. Many agencies are working together following a national management plan in hopes of containing CWD and preventing its further spread. Many have also established long-term surveillance programs to monitor wild cervid populations.

Furthermore, the majority of states have adopted regulations that prohibit the importation and exportation of cervid body parts that are more likely to harbor prions, including the spinal cord, the brain, and lymph tissues. In endemic regions like Colorado, additional regulation is in place banning removal of any parts of deer, elk or moose from affected areas to prevent the spread of CWD.

Other possible management actions to control the spread of CWD include quarantining and depopulating captive herds that test positive for the disease, prohibiting import of cervids from states and provinces with confi­rmed cases of CWD, and prohibiting feeding and baiting of wild cervids. A Voluntary Herd Certification Program for captive cervids has been established by the USDA AHPHIS to “control the incidence of CWD and prevent the interstate spread of CWD”.

CWD is not currently believed to cause disease in humans, though hunters are encouraged to take routine precautions when handling harvested animals. Hunters should not shoot, handle, or consume any animal that appears sick. To prevent exposure to any disease-causing agent, including the prion of CWD, hunters should wear gloves while dressing game and should wash hands and instruments thoroughly after fi­eld dressing. Hunters should also minimize handling of brain and spinal cord tissues.

Hunters and others who value this natural resource can contribute to the prevention of the introduction and spread of CWD by interacting with elected officials, observing regulated prevention measures (see above) and encouraging others to do so. They can also refrain from use of urine-based lures and other practices such as feeding and baiting that congregate animals, even when it is permitted.

Images

Distribution of Chronic Wasting Disease in North America, updated March 4, 2020.Distribution of Chronic Wasting Disease in North America, updated March 4, 2020.