Chronic Wasting Disease (CWD) is classified as a prion disease, otherwise known as a transmissible spongiform (sponge-like) encephalopathy (TSE). It is a rare, progressive neurodegenerative disorder that affects both animals and humans. Characteristically, TSEs have a long incubation period, spongiform changes associated with nerve cell loss, and a failure to induce an inflammatory response. The causative agent of TSEs is not well understood but is believed to be a prion, which is an agent capable of causing abnormal folding of normal cellular prion proteins in the brain leading to brain damage and the signs and symptoms of the disease.
Bovine Spongiform Encephalopathy (BSE), or Mad Cow Disease, is classified as a TSE first identified in cows in the United Kingdom during the 1970s. It is thought to be the result of feeding cattle a meat-and-bone meal containing BSE or scrapie-infected sheep products. Scrapie is a prion disease found in sheep. BSE reached epidemic proportions in the United Kingdom that peaked in January 1993 at almost 1,000 new cases per week.Through 2008, more than 184,500 cases of BSE had been confirmed in the U.K. alone in more than 35,000 herds.
Through May 2009, there have been 20 identified cases of BSE in North America: 17 in Canada, and three in the United States. The U.S. cases were identified in cows in Washington (2003), Texas (2005), and Alabama (2006).
CWD is known to affect North American hoofed, ruminant mammals such as mule deer, white-tailed deer, elk, and moose. It was first identified as a fatal wasting syndrome in captive mule deer in Colorado in the 1960s, and in the wild in 1981. By the mid-1990s, CWD had been diagnosed among free-ranging deer and elk in Colorado and Wyoming, where the disease is now endemic. Currently, the geographic range of diseased wild animals includes 11 U.S. states: CO, IL, KS, NE, NM, NY, SD, UT, WV, WI, and WY.
Variant Creutzfeldt-Jacob Disease (vCJD), is a human prion disease first described in the United Kingdom in 1996. There is now strong scientific evidence that the agent responsible for the outbreak of BSE in cows is the
same agent responsible for the outbreak of vCJD in humans. Both are fatal brain diseases with long incubation periods measured in years, and both are caused by an unconventional transmissible agent. The human risk, however, is thought to be low even after consumption of contaminated meat.
To date, no strong evidence of CWD transmission to humans has been reported, but several human cases of prion disease have been considered suspect. A study published by the CDC (Vol. 10, No. 6, June 2004) titled Chronic Wasting Disease and Potential Transmission to Humans concluded that “The lack of evidence of a link between CWD transmission and unusual cases of CJD, despite several epidemiologic investigations, and the absence of an increase in CJD incidence in Colorado and Wyoming suggest that the risk, if any, of transmission of CWD to humans is low.” It goes on to state that though studies indicate the possibility of transmission to humans, “no human cases of prion disease with strong evidence of a link with CWD have been identified. However, the transmission of BSE to humans and the resulting vCJD indicate that, provided sufficient exposure, the species barrier may not completely protect humans from animal prion diseases.”
According to the CDC, “Specific studies have begun that focus on identifying human prion disease in a population that is at increased risk for exposure to potentially CWD-infected deer or elk meat. Because of the long time between exposure to CWD and the development of disease, many years of continued follow-up are required to be able to say what the risk, if any, of CWD is to humans.”
The median duration of illness for CJD is 4-5 months, and the symptoms include dementia and early neurologic abnormalities. The median duration of illness for vCJD is 13-14 months, and the symptoms include prominent psychiatric/behavioral problems, a painful sense of touch, and delayed neurologic abnormalities. Prion diseases are usually rapidly progressive and always fatal.
The CDC recommends that hunters consult with state wildlife agencies to identify areas where CWD occurs. They should avoid eating the meat of deer, mule deer, moose, and elk that look sick, and may wish to have susceptible animals harvested from known CWD-positive areas tested for CWD before consuming the meat. Gloves should be worn when field-dressing carcasses, the meat should be boned-out, and handling of the brain and spinal cord tissues should be minimized.
(Sources: http://www.cdc.gov, http://www.cwd-info.org/index.php/fuseaction/recommendations.gmu , and the Chronic Wasting Disease Alliance www.cwd-info.org .