What is chronic wasting disease (CWD)?
Chronic wasting disease (CWD) is an invariably fatal neurological disease that affects deer, elk, and moose in North America. The disease was first described in the late 1960s in captive animals in Colorado. As of 2006, CWD-infected animals—from either wild or captive populations—had been found in Colorado, Illinois, Kansas, Montana, Nebraska, New Mexico, New York, Oklahoma, South Dakota, Utah, West Virginia, Wisconsin, and Wyoming in the United States and in the provinces of Alberta and Saskatchewan in Canada. In the late stages of the disease, infected animals show progressive weight loss, listlessness, increased salivation and urination, increased water consumption, depression, and death.
The brains of dead animals show characteristic vacuoles, or holes, that give the brain the appearance of a sponge. This pathology is characteristic of all the transmissible spongiform encephalopathies (TSEs), including bovine spongiform encephalopathy (BSE) of cows (so-called mad cow disease), scrapie of sheep, and Creutzfeldt-Jakob disease (CJD) of humans. Although CWD is similar to these other diseases, it is not identical to them. These diseases appear to be caused by the misfolding of a small protein (prion) normally located in the membranes of neurons and other cells. The normal function of this protein is unknown, but when misfolded, it accumulates in the brain, resulting in the death of neurons. The misfolded, pathogenic prion appears to be able to direct the misfolding of any normal prion present in a cell, thus replicating its aberrant structure.
Prion diseases can be either inherited or transmitted. Humans can inherit CJD or contract it from contaminated blood or brain tissue. Evidence suggests that variant CJD in humans can result from consuming beef contaminated with the BSE prion. There is concern about whether the CWD prion in deer and elk can also “jump” the species barrier to humans in a manner similar to that of BSE. Research using monkeys suggests that there is a species barrier to humans, making it unlikely that CWD will be transmitted to humans despite contact with contaminated blood or tissues.
CWD is believed to be transmitted from animal to animal through contact with body fluids or feces. Research published in 2006 indicated that the disease is most easily transmitted via saliva and blood, although other modes of transmission have not been ruled out. Two recent studies found no strong clinical or epidemiological evidence that the disease can be transmitted to humans via the consumption of contaminated meat. Laboratory studies indicate that such transmission is possible, however. Because of the long incubation period for the disease, however, it is difficult to prove or disprove transmission from deer, elk, or moose to humans.
Many states where CWD is prevalent have banned the baiting of deer and are thinning the deer population. Deer and elk in crowded populations, particularly captive herds, appear to be more susceptible to CWD.
Research efforts are focused on developing a sensitive field diagnostic test. Definitive diagnosis is made by autopsy, but tests based on tonsil biopsy and antibodies to detect prions in body fluids are also being developed. Transgenic mouse models of each of the known TSEs are also being created in order to learn more about transmission and pathology.
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