Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy (TSE) of deer and elk.  TSEs are neurological diseases characterized by microscopic empty spaces in the brain matter, creating a "spongy" appearance. First recognized as a clinical "wasting" syndrome in 1967 in mule deer in a wildlife research facility in northern Colorado, CWD was identified as a TSE in 1978.   Other TSEs are: scrapie in sheep, bovine spongiform encephalopathy in cattle, Creutzfeldt-Jakob Disease and New Variant Creutzfeldt-Jakob Disease in humans. However, there is no known relationship between CWD and any other TSEs of animals or people.

To date, CWD has been found only in cervids (members of the deer family) in North America.  Most cases of CWD occur in adult animals.  CWD is a progressive disease typified by chronic weight loss leading to death.  Behavioral changes also occur in the majority of cases, including decreased interactions with other animals, listlessness, lowering of the head, blank facial expression, and repetitive walking in set patterns.  Excessive salivation and grinding of the teeth also are observed.  Most deer show increased drinking and urination.

CWD has been diagnosed in captive elk herds since 1978 in several states and in two Canadian provinces.  The locations of captive elk herds with known CWD infection are: South Dakota, Nebraska, Colorado, Oklahoma, Montana, Kansas, Alberta and Saskatchewan.  Transportation of elk from these herds is prohibited.

In the mid-1980s, CWD was detected in free-ranging deer and elk in contiguous portions of northeastern Colorado and southeastern Wyoming.  In 2001, CWD was also found in free-ranging deer in the southwestern corner of Nebraska (adjacent to Colorado and Wyoming) and on February 2002 in Wisconsin.  The areas of northern Colorado, southern Wyoming, and southwestern Nebraska and southeastern Wisconsin in which free-ranging deer and/or elk positive for CWD have been found are referred to as the endemic areas. The is no transportation of cervids from infected captive heards and wild cervids have not been translocated from the endemic areas, either.  

Species prone to CWD include Rocky Mountain elk, mule deer, white-tailed deer, and black-tailed deer.  Researchers have placed CWD infected deer and elk in close contact with wild ruminants and domestic cattle, sheep, and goats to test for disease transmission. So far there has been no evidence of disease transmission.  Research is ongoing to further explore the possibility of transmission of CWD to other species.

The agent responsible for CWD (and other animal TSEs) has not been completely characterized.  The theory with the most supporting evidence is that the CWD agent of infection is a prion. A prion is an abnormal form of a normal protein, known as cellular prion protein, most commonly found in the central nervous system.  The abnormal prion protein "infects" the host animal by promoting conversion of normal cellular prion protein to the abnormal form.  The CWD agent is smaller than most viral particles and does not evoke any detectable immune response in the host animal.  Based on experience with other TSE agents, the CWD agent is assumed to be resistant to enzymes and chemicals that normally break down proteins, as well as resistant to heat and normal disinfection procedures.  

Stanley Prusiner, a neurologist at the University of California's School of Medicine won the Nobel Prize for Medicine in 1997 for his more than twenty years of work investigating the pathogens responsible for TSEs. He deduced that proteins that lack DNA called "protease resistant prions" are the agents that transform other normal prions into their own image, though scientists aren't sure if or how protease resistant prions are naturally transmitted in a particular species.

Researchers are developing live-animal diagnostic tests for CWD.  Until such time as a live test is available, definitive diagnosis is based on postmortem examination (necropsy) and testing. Scientists use a technique called immunohistochemistry to test brain tissue for the presence of the abnormal prion protein to diagnose CWD. Currently, only the immunohistochemistry test can confirm CWD. Aspiration pneumonia, which may be the actual cause of death, also is a common finding in animals affected with CWD.  On microscopic examination, lesions of CWD in the central nervous system resemble those of other TSEs. 

Information provided by:

USDA Animal and Plant Health Inspection Service
Colorado Division of Wildlife

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