Chronic Wasting Disease - CWD

By: Wildlife Futures Team Date: Oct 21, 2021

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 disease 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 state¹. CWD is always fatal².

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 extirpated³. In turn, CWD has had a major impact on hunting and cervid farming in affected states and provinces⁴. 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 completely control or eradicate the disease².

Species Affected

CWD is known to affect free-ranging and captive white-tailed deer, mule deer, black-tailed deer, Rocky Mountain elk, moose, reindeer, Sika deer, and red deer, though it is likely that other cervid species are also susceptible¹. 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 could be 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 detected⁵.

Carnivores and scavengers have been shown to be able to pass infectious prions in their feces without becoming infected themselves⁶.

Distribution

CWD was originally found in a captive mule deer herd in Colorado and was reported in Wyoming soon after. Over the past 54 years the disease has spread to all bordering states as well as the Midwest and the Mid-Atlantic. As of October 2021, CWD has been reported in 26 states and 3 Canadian provinces⁷. It was found in 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)⁸. 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, and saliva, and indirectly through infected carcasses or environments². 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 5 months to 5 years in an experimental setting⁹. 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 indefinitely¹⁰.

Clinical Signs

CWD targets the brain and other neurological tissues as well as the lymph (immune) system in the body, resulting in abnormal behavior and progressive loss of normal bodily functions¹.

Reported clinical signs include loss of body condition (wasting), depression, grinding of teeth and excessive drinking, urination, and salivation (due to difficulty swallowing). 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 of becoming ill².

In elk and moose, the clinical signs are often more subtle and prolonged¹.

Diagnosis

It is not possible to diagnose cervids with CWD based on clinical signs alone because many other diseases and conditions cause similar changes. In addition, a CWD affected animal may have other concurrent infections such as pneumonia or EHD, further complicating the clinical signs¹. 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 tissue¹¹.

Treatment

There is currently no treatment or vaccine for CWD¹.

Management

Management of chronic wasting disease has largely proven impossible and eradication of this disease where it currently exists is unlikely². 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 populations⁴.

Furthermore, most 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 tissues⁴.

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 cervids⁴. 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”¹².

CWD is not currently believed to cause disease in humans¹³, though hunters are encouraged to take routine precautions when handling all 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 equipment thoroughly after field dressing. Hunters should also minimize handling of brain and spinal cord as CWD is most prevalent in these tissues⁵.

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 lures¹⁴ and other practices such as feeding and baiting that congregate animals², even when it is permitted.

Literature Cited

Williams, E.S. and Barker, I.K. eds., 2008. Infectious Diseases of Wild Mammals. John Wiley & Sons.
Conover, M.R. and Vail, R.M., 2014. Human Diseases from Wildlife. CRC Press.
DeVivo, M.T., Edmunds, D.R., Kauffman, M.J., Schumaker, B.A., Binfet, J., Kreeger, T.J., Richards, B.J., Schätzl, H.M. and Cornish, T.E., 2017. Endemic chronic wasting disease causes mule deer population decline in Wyoming. PLoS One, 12(10), p.e0186512.
Leiss, W., Westphal, M., Tyshenko, M.G., Croteau, M.C., Oraby, T., Adamowicz, W., Goddard, E., Cashman, N.R., Darshan, S. and Krewski, D., 2017. Challenges in managing the risks of chronic wasting disease. International Journal of Global Environmental Issues, 16(4), pp.277-302.
Centers for Disease Control and Prevention, 2020. Chronic Wasting Disease (CWD) [online]. Available from: Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases (NCEZID), Division of High-Consequence Pathogens and Pathology (DHCPP) (Accessed 29 March 2021).
Jennelle, C.S., Samuel, M.D., Nolden, C.A. and Berkley, E.A., 2009. Deer carcass decomposition and potential scavenger exposure to chronic wasting disease. The Journal of Wildlife Management, 73(5), pp.655-662
USGS National Wildlife Health Center, updated February 24, 2021. Distribution of Chronic Wasting Disease in North America, Available from: www.usgs.gov/media/images/distribution-chronic-wasting-disease-north-america-0. (Accessed: 29 March 2021)
Vikøren, T., Våge, J., Madslien, K.I., Røed, K.H., Rolandsen, C.M., Tran, L., Hopp, P., Veiberg, V., Heum, M., Moldal, T. and Neves, C.G.D., 2019. First detection of chronic wasting disease in a wild red deer (Cervus elaphus) in Europe. Journal of wildlife diseases, 55(4), pp.970-972.
Williams, ES and MW Miller. 2002. Chronic wasting disease in deer and elk in North America. Revue Scientifique tech. Off. int. Epiz. 21 (2), 305-316
Mysterud, A. and Edmunds, D.R., 2019. A review of chronic wasting disease in North America with implications for Europe. European Journal of Wildlife Research, 65(2), pp.1-13.
Haley, N.J. and Richt, J.A., 2017. Evolution of diagnostic tests for chronic wasting disease, a naturally occurring prion disease of cervids. Pathogens, 6(3), p.35.
U.S. Department of Agriculture, Animal and Plant Health Inspection Service (2019) Chronic Wasting Disease Program Standards. Available at:https://www.aphis.usda.gov/animal_health/animal_diseases/cwd/downloads/cwd-program-standards.pdf (Accessed: 29 March 2021)
Waddell, L., Greig, J., Mascarenhas, M., Otten, A., Corrin, T. and Hierlihy, K., 2018. Current evidence on the transmissibility of chronic wasting disease prions to humans—a systematic review. Transboundary and emerging diseases, 65(1), pp.37-49.
Song, H., McComas, K.A. and Schuler, K.L., 2019. Hunters’ responses to urine‐based scent bans tackling chronic wasting disease. The Journal of Wildlife Management, 83(2), pp.457-466.