28.7 Chronic wasting disease

Chronic wasting disease (CWD) is a neurodegenerative disease, caused by a prion that affects cervids including deer, elk and moose. Until recently, CWD was an obscure illness that seemed to be found only in a small geographic area in northeastern Colorado and southeastern Wyoming. However, this disease has now been detected in wild and farmed cervids in many other U.S. states and Canada. Korea reported outbreaks in captive cervids imported from Canada, as well as in their offspring. Chronic wasting disease can be devastating in farmed herds. This disease is always fatal once the symptoms appear, and most or all of the herd can eventually become infected. It is one of the most difficult prion diseases to control: CWD prions are transmitted from animal to animal, as well as from contaminated environments for two years or more. This disease is also endemic in wild populations. More than 12,000 captive elk and thousands of wild deer and elk have been killed in the U.S. and Canada in control efforts. In addition, there are concerns about the possible impact of CWD on humans. Cooking does not destroy prions, and ingestion of another prion, the agent that causes bovine spongiform encephalopathy (BSE), has been linked to a fatal human neurologic disease. CWD prions have been found in muscle (meat), as well as other tissues of cervids, and could enter the food supply. Although the evidence so far suggests that CWD probably does not affect humans, the possibility that it could be zoonotic has not been ruled out. Etiology CWD is a member of the transmissible spongiform encephalopathies (TSEs), a group of neurodegenerative disorders caused by unconventional disease agents. These agents are resistant to the treatments that ordinarily destroy bacteria, spores, viruses and fungi. They are generally thought to be prions, although a minority opinion suggests that TSEs may be caused by virinos or retroviruses. Prions are infectious proteins that appear to replicate by converting a normal cellular protein into copies of the prion. The cellular protein, which is called PrPc, is found on the surface of neurons. Pathogenic isoforms of PrPc are designated PrPres; PrP CWD or PrP TSE are other names for this protein. Prions that cause different diseases (e.g. CWD, scrapie or BSE) are considered to be different strains of PrPres. There may be more than one variant of the CWD prion.

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