Chronic wasting disease (CWD) has recently emerged in North America as an 
important prion disease of captive and free-ranging cervids (species in the deer 
family). CWD is the only recognized transmissible spongiform encephalopathy 
(TSE) affecting free-ranging species. Three cervid species, mule deer 
(Odocoileus hemionus), white-tailed deer (O. virginianus), and Rocky Mountain 
elk (Cervus elaphus nelsoni), are the only known natural hosts of CWD. Endemic 
CWD is well established in southern Wyoming and northern Colorado, and has been 
present in this 'core area' for two decades or more. Apparently CWD has also 
infected farmed cervids in numerous jurisdictions, and has probably been endemic 
in North America's farmed deer and elk for well over a decade. Several 
free-ranging foci distant to the Colorado-Wyoming core area have been discovered 
since 2000, and new or intensified surveillance may well identify even more foci 
of infection. Whether all of the identified captive and free-ranging foci are 
connected via a common original exposure source remains undetermined. Some of 
this recently observed 'spread' may be attributable to improved detection or 
natural movements of infected deer and elk, but more distant range extensions 
are more likely caused by movements of infected captive deer and elk in 
commerce, or by some yet unidentified exposure risk factor. Research on CWD over 
the last 5 years has resulted in a more complete understanding of its 
pathogenesis and epidemiology. CWD is infectious, transmitting horizontally from 
infected to susceptible cervids. Early accumulation of PrP(CWD) in alimentary 
tract-associated lymphoid tissues during incubation suggests agent shedding in 
feces or saliva as plausible transmission routes. Residual infectivity in 
contaminated environments also appears to be important in sustaining epidemics. 
Improved tests allow CWD to be reliably diagnosed long before clinical signs 
appear. Implications of CWD are not entirely clear at this time. Natural 
transmission to humans or traditional domestic livestock seems relatively 
unlikely, but the possibility still evokes public concerns; impacts on wildlife 
resources have not been determined. Consequently, where CWD is not known to 
occur surveillance programs and regulations that prevent or reduce the 
likelihood that CWD will be introduced into these jurisdictions should be 
encouraged. Where CWD is known to occur, affected jurisdictions are conducting 
surveillance to estimate and monitor trends in geographic distribution and 
prevalence, managing deer and elk populations in attempts to limit spread, and 
developing and evaluating techniques for further controlling and perhaps 
eradicating CWD. Programs for addressing the challenges of CWD management will 
require interagency cooperation, commitment of funds and personnel, and applied 
research. Chronic wasting disease (CWD) is perhaps the most enigmatic of the 
naturally occurring prion diseases. Although recognized as a transmissible 
spongiform encephalopathy (TSE) since the late 1970s (Williams and Young 1980, 
1982), interest in and concern about CWD has only recently emerged. CWD most 
closely resembles scrapie in sheep in most respects, but recent media and public 
reaction to CWD has been more reminiscent of that afforded to bovine spongiform 
encephalopathy (BSE) less than a decade ago. Yet, with the exception of 
transmissible mink encephalopathy (TME), CWD is the rarest of the known animal 
TSEs: fewer than 1,000 cases have been diagnosed worldwide, and all but two of 
these occurred in North America. CWD is unique among the TSEs in that it affects 
free-living species (Spraker et al. 1997; Miller et al. 2000). The three natural 
host species for CWD, mule deer (Odocoileus hemionus), white-tailed deer (O. 
virginianus), and Rocky Mountain elk (Cervus elaphus nelsoni), are all in the 
family Cervidae and native to North America. Like scrapie, CWD is contagious: 
epidemics are self-sustaining in both captive and free-ranging cervid 
populations (Miller et al. 1998, 2000). The geographic extent of endemic CWD in 
free-ranging wildlife was initially thought to be quite limited and its natural 
rate of expansion slow; however, recent investigations have revealed that CWD 
has been inadvertently spread much more widely via market-driven movements of 
infected, farmed elk and deer. Both the ecological and economic consequences of 
CWD and its spread remain to be determined; moreover, public health implications 
remain a question of intense interest. Here, we review current understanding of 
CWD, its implications, and its management.