Occurrence, transmission, and zoonotic potential of chronic wasting disease

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.

Figure 1

US states and Canadian provinces reporting chronic wasting disease (CWD) cases. A) Year or season CWD was first identified/confirmed in captive (C) or free-ranging (F) cervids. Underlying map shows geographic distribution of CWD (from the US Geological Survey National Wildlife Health Center, updated October 2011, www.nwhc.usgs.gov/disease_information/chronic_wasting_disease/). Light gray shading, current CWD in free-ranging populations; dark gray shading, known distribution of CWD in free-ranging populations before 2000. All locations are approximations based on best available information. B) Cumulative totals of states and provinces that have reported CWD cases in captive or free-ranging cervids. Totals also include South Korea (2001, captive). Many states have reported captive cases for only 1 or 2 years.

Figure 2

Annual surveillance of free-ranging cervids for chronic wasting disease (CWD). A) Number of US states and Canadian provinces conducting limited or extensive CWD surveillance of free-ranging cervids. B) Number of cervids tested by species each year/season. Other/unspecified includes black-tailed deer, moose, caribou, and data that could not be separated by species. C) Number of CWD-positive cervid samples (CWD cases) by species each year/season. Less than 5 moose were positive. Data were obtained from state and provincial wildlife agencies. Asterisks indicate preliminary or approximated 2010 data.

Figure 3

Conceptual model of horizontal transmission of chronic wasting disease (CWD). Items in italics are poorly studied or unknown in cervid CWD.