Chronic wasting disease (CWD) has been known as a clinical syndrome of mule deer (Odocoileus hemionus) for more than 30 years; modeling suggests the disease may have been present in free-ranging populations of mule deer for more than 40 years. Only four species of the family Cervidae are known to be naturally susceptible to CWD: mule deer, white-tailed deer (Odocoileus virginianus), Rocky Mountain elk (Cervus elaphus nelsoni), and moose (Alces alces), though it is very likely that other subspecies of C. elaphus are susceptible to CWD. Susceptibility of other cervids to CWD is not known. Cattle and other domestic livestock appear to be resistant to natural infection; to date, only three of 13 cattle have become infected with the CWD agent following experimental intracerebral inoculation, although this and other experimental studies begun in 1997 are not yet completed.

The origin of CWD is not known and it may never be possible to definitively determine how or when CWD arose. Though of academic interest, determining the origin is probably not very important from a management perspective; nonetheless, speculation continues. Scrapie, a TSE of domestic sheep, has been recognized in the United States since 1947, and it is possible that CWD was derived from scrapie. Arguments can be made both for and against this hypothesis. It is possible, though never proven, that deer came into contact with scrapie agent either on shared pastures or in captivity somewhere along the front range of the Rocky Mountains, where high levels of sheep grazing occurred in the early 1900s. In addition, in vitro models suggest there is less of a species barrier to interspecies TSE transmission between deer, elk, and sheep than between these cervids and either cattle or humans. However, CWD has never been identified in other areas of North America or other parts of the world where cervids and domestic sheep with scrapie must have co-mingled. Strain typing experiments determined that CWD is not like known scrapie strains, though direct comparisons with North American scrapie strains has not been conducted. Experimental transmission of CWD to a domestic goat by intracerebral inoculation had a prolonged incubation; shorter incubation would be expected with scrapie strains in goats. Experimental scrapie in cattle and lesions of CWD in cattle are quite different.

It may be possible that CWD is a spontaneous TSE that arose in deer in the wild or in captivity and has biological features promoting transmission to other deer and elk. The majority of human CJD cases are thought to be spontaneous and associated with conformational change in a normal cellular protein (PrPC) to the abnormal disease associated protease resistant protein (PrPres) considered by many to be infectious agents of the TSEs. Occurrence of spontaneous CJD is approximately 1 per 1 million population per year. Spontaneous CWD may have happened in deer though it is difficult to see how this could be proven. © Chronic Wasting Disease Alliance