Emergence of Chronic Wasting Disease as a Disease of Concern to Managers of Captive and Free-Ranging Cervids
Chronic wasting disease (CWD) of cervids has been recognized as a clinical syndrome for approximately 30 years and as a transmissible spongiform encephalopathy for 20 years. During most of that time CWD was primarily of interest to local wildlife managers in Colorado and Wyoming and a handful of researchers studying the few animal and human spongiform encephalopathies then known. However, since the identification of bovine spongiform encephalopathy (BSE) in the United Kingdom and especially since BSE was linked to a new variant of human Creutzfeldt-Jacob disease in 1996, considerable attention has been focused on all of the transmissible spongiform encephalopathies, including CWD. These diseases are thought to be caused by abnormal isoforms of normal cellular proteins called “prions.”
Chronic wasting disease is a fatal chronic central nervous system disease of captive and free-ranging mule deer (Odocoileus hemionus), white-tailed deer (Odocoileus virginianus), and Rocky Mountain elk (Cervus elaphus nelsoni). Chronic wasting disease occurs in captive research cervids in southeastern Wyoming and northcentral Colorado, and in ranched elk in two South Dakota facilities and one Nebraska facility. A few cases were diagnosed in zoological gardens and a game farm in the past, but CWD apparently did not persist in these locations. In free-ranging cervids, CWD is only known from limited contiguous areas in eastern Wyoming and Colorado. The recent diagnosis of CWD in commercial elk in South Dakota and Nebraska underscores the importance that veterinarians and owners of facilities housing cervids should be aware of the clinical presentation of this disease, epizootiology, and methods of diagnosis.
Studies of the epizootiology of CWD indicate lateral and possibly maternal transmission; the possibility of environmental contamination by the agent is being studied. Affected animals are older than approximately 18 months of age, with the majority of cases in the 3- to 6-year-old age range; however, the age of development of clinical disease probably depends on when the animal was exposed and animals 12 years of age and older have developed CWD. A minimum of about 18 months incubation appears to be required for development of clinical signs. Onset of signs is subtle, often only recognized initially by those familiar with the individual animal. Early clinical signs include behavioral changes and weight loss. As the disease progresses, animals may develop a variety of signs including polydipsia, polyuria, hypersalivation, difficulties in swallowing, head tremor, ataxia, and hyperexcitability. Not every animal will display all signs; the disease in some elk may be quite subtle. Aspiration pneumonia often occurs as a terminal event, thus CWD should be considered a differential in cases of pneumonia in prime-aged cervids.
Currently, diagnosis of CWD requires examination of the brain, at a minimum the obex region of the medulla oblongata, by histopathology. Confirmatory tests include immunohistochemistry, western blotting, and negative stain electron microscopy. Considerable research is currently underway to define the distribution, prevalence, host range, pathogenesis, epizootiology, and approaches to diagnosis of CWD.