Chronic wasting disease in deer and elk in North America.

Chronic wasting disease (CWD) has emerged as an important disease of wildlife in North America. The disease is a unique member of the transmissible spongiform encephalopathies (TSEs) or prion diseases, which naturally affect only a few species. Of the TSEs, CWD is the only one found in free-ranging species. However, interest in CWD has recently grown, by association with the better-known TSEs such as variant Creutzfeldt-Jakob disease of humans and bovine spongiform encephalopathy. Knowledge of the geographic distribution of CWD, though still limited, has greatly improved since the mid-1990s as a result of surveillance in free-ranging deer and elk and in commercially owned Rocky Mountain elk (Cervus elaphus nelsoni), and the disease has now been found in multiple areas of the plains and Rocky Mountain foothills of western North America. Studies of the biology and natural history of CWD over recent years have resulted in a better understanding of the pathogenesis and epidemiology of the disease. Early involvement of the lymphoid tissues of the alimentary tract during the incubation period of CWD suggests plausible routes for agent exit from an infected individual, such as in faeces or saliva. Chronic wasting disease is laterally transmitted and environmental contamination may play an important role in local maintenance of the disease. Studies on the epidemiology of CWD have led to the development of models to help explain the history of CWD and to simulate future impacts on deer and elk populations. Diagnostic tests have been improved, allowing diagnosis early in the incubation period, long before the appearance of clinical disease. Surveillance techniques and programmes have been developed and instituted by wildlife management agencies for free-ranging deer and elk and by state and federal agricultural agencies for privately-owned elk. During the 1990s, perceptions of TSEs have altered dramatically; perhaps most remarkably, the goal of global eradication of all prion diseases is now being discussed.