Chronic wasting disease (CWD) is a fatal and highly contagious prion disease affecting farmed and wild cervids. In North America, CWD occurs in various species, including white-tailed deer, mule deer, elk, and moose. CWD has also been observed in wild reindeer, moose and red deer from the Scandinavian Peninsula. The inadvertent movement of asymptomatic infected animals has contributed to the spread of CWD across North America and resulted in CWD outbreaks in various species of farmed-raised cervids in South Korea.
First described in captive deer from Colorado and Wyoming in the late 1960's, the geographical range of CWD in North America now includes Alberta, Saskatchewan and at least twenty-five states in the U.S. Within a given captive population, the prevalence of CWD infection can be very high (79% of all animals in residence at one facility). In wild deer from Alberta, the prevalence of CWD continues to increase (1.9% in white-tailed deer and 8.2% in mule deer) (AEP, 2017).
Similar to other prion diseases, the progression of CWD is characterized by long pre-clinical incubation periods (i.e. the animal appears healthy) that can last several years. Early clinical signs include fixed stares, repetitive movements, withdrawal from herd mates, hyperexcitability when approached or reduced the fear of handlers. During the course of the disease, affected animals can display altered postures with lowered head and ears, teeth grinding, increased salivation with drooling, excessive water consumption, and urination. Difficulty in swallowing and regurgitation eventually result in deteriorated body condition (i.e. emaciation), recumbency and death.
The transmission cycle of CWD can involve direct exposure to infected cervids, their tissues or fluids. Although the disease primarily affects the central nervous system (brain), during the pre-clinical phase, the infectious agent (CWD prions) is shed in the saliva, urine, and feces of infected animals. Several other tissues throughout the body (i.e. muscle, heart, antler velvet) are also contaminated with CWD prions.
Prions can enter the environment from the secretions and excretions of infected cervids, as well as from contaminated carcasses. Bound to soil microparticles, prions are not easily degraded, remaining in the environment for decades and indirectly contributing to the transmission of CWD. In addition, soil-bound prions can be more infectious than unbound prions when ingested. Reducing the exposure of cervids to prion-contaminated environments is a challenging problem.
Although CWD is not readily transmitted to other animal species, the susceptibility of cattle, sheep, domestic cats and primates (i.e., squirrel monkeys) have been confirmed experimentally. Natural transmission of CWD into species other than cervids has not been documented, however, the existence of multiple forms of the pathogen (i.e., strains) increases the likelihood of interspecies transmission. There is, as yet, no evidence of CWD infection in humans. Caution is strongly advised when handling suspected CWD-positive animals and it is also recommended cervids are tested for CWD before consumption.
Updated April 23, 2018