Chronic wasting disease: A transmissible spongiform encephalopathy (TSE) of North American deer and elk, a progressive neurodegenerative disorder that produces spongiform changes in the brain and chronic weight loss leading to the death of these animals. There is no known relationship between chronic wasting disease (CWD) and any other TSE of animals or people.

There are three main theories on the nature of the agent that causes CWD: (1) the agent is a prion, 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 by promoting the conversion of normal cellular protein to the abnormal form; (2) the agent is an unconventional virus; (3) the agent is a virino, or "incomplete" virus composed of nucleic acid protected by host proteins. The CWD agent is smaller than most viral particles and does not evoke any detectable immune response or inflammatory reaction in the host animal.

Most cases of CWD occur in adult animals. The disease is progressive and always fatal. The most obvious and consistent clinical sign of CWD is weight loss over time. Behavioral changes also occur in the majority of cases, including decreased interactions with other animals in the pen, listlessness, lowering of the head, blank facial expression, and repetitive walking in set patterns within the pen. In elk, behavioral changes may also include hyperexcitability and nervousness. Affected animals continue to eat grain but may show decreased interest in hay. Excessive salivation and grinding of the teeth are seen. Most deer show increased drinking and urination.

Diagnosis is confirmed by necropsy examination and testing. On microscopic examination, lesions of CWD in the central nervous system resemble those of other spongiform encephalopathies. In addition, using a technique called immunohistochemistry, scientists test brain tissues for the presence of the abnormal prion protein.

Species that have been affected with CWD include Rocky Mountain elk, mule deer, white-tailed deer, and black-tailed deer. Other ruminant species, including wild ruminants and domestic cattle, sheep, and goats, have been housed in wildlife facilities in direct or indirect contact with CWD-affected deer and elk. No cases of CWD or other TSE's have been detected in these other ruminant species.

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