Occurrence, Transmission, and Zoonotic Potential of Chronic Wasting Disease

Samuel E. Saunders¹, Shannon L. Bartelt-Hunt, and Jason C. Bartz

Author affiliations: University of Nebraska-Lincoln, Omaha, Nebraska, USA (S.E. Saunders, S.L. Bartelt-Hunt); Creighton University, Omaha (J.C. Bartz)

Abstract

Chronic wasting disease (CWD) is a fatal, transmissible prion disease that affects captive and free-ranging deer, elk, and moose. Although the zoonotic potential of CWD is considered low, identification of multiple CWD strains and the potential for agent evolution upon serial passage hinders a definitive conclusion. Surveillance for CWD in free-ranging populations has documented a continual geographic spread of the disease throughout North America. CWD prions are shed from clinically and preclinically affected hosts, and CWD transmission is mediated at least in part by the environment, perhaps by soil. Much remains unknown, including the sites and mechanisms of prion uptake in the naive host. There are no therapeutics or effective eradication measures for CWD-endemic populations. Continued surveillance and research of CWD and its effects on cervid ecosystems is vital for controlling the long-term consequences of this emerging disease.

Chronic wasting disease (CWD) is an inevitably fatal, infectious neurodegenerative prion disease naturally affecting North American mule deer (Odocoileus hemionus), white-tailed deer (Odocoileus virginianus), elk (wapiti, Cervus canadensis), and moose (Alces alces) (1,2). Other prion diseases, or transmissible spongiform encephalopathies, include bovine spongiform encephalopathy (BSE), scrapie in sheep and goats, and Creutzfeldt-Jakob disease (CJD) in humans (3). CWD was identified in the late 1960s and recognized as a spongiform encephalopathy by Williams in 1980 (1).
Clinical signs of CWD include weight loss and behavioral changes such as altered stance, pacing, excessive salivation, and hyperexcitability that progress over weeks or months (1). The infectious agent of CWD is the abnormally folded prion protein (the prion) designated PrP^Sc, which is distinguished from the normal cellular prion protein (PrP^c) by its resistance to proteolysis, propensity for aggregation, and insolubility in detergents (4). Misfolded prion (PrP^Sc) can initiate conversion of PrP^c to PrP^Sc and replicate through a yet unknown mechanism. The exact role that PrP^Sc plays in prion disease remains unclear, but PrP^Sc is known to accumulate in the central nervous system (CNS) (1).
CWD continues to emerge and spread in free-ranging and captive cervids throughout the United States and Canada. Effective therapeutics or and management practices for animal populations in areas to which CWD is endemic do not currently exist. Long-term effects of CWD on cervid ecosystems remain unclear, but the potential for economic consequences is serious because of the role cervids play in the hunting, tourism, and agricultural industries. Moreover, the zoonotic potential of CWD is uncertain, and exposure to CWD-contaminated meat and material will only increase as the disease continues to spread and the incidence increases in areas to which CWD is endemic. We discuss current CWD prevalence and distribution and broadly review surveillance efforts to date. We also present a detailed conceptual model for transmission of the CWD agent and provide an update on CWD interspecies transmission, strains, and zoonotic potential. In addition, we suggest key research needs that may offer hope of slowing or halting the continued emergence of CWD.