Multi-state Qualitative Chronic Wasting Disease Risk Assessment

Random active surveillance for Chronic Wasting Disease (CWD) in free-ranging populations of cervids is challenging due to low prevalence and heterogeneous distribution of the disease. Furthermore, geographic areas are at different levels of risk for exposure to CWD depending on the presence of various risk factors. Based on the assumption that areas of Virginia in proximity to concentrations of farmed or captive deer or elk are at the highest risk for the introduction of CWD into the free-ranging white-tailed deer population (Odocoileus virginianus), the Virginia Department of Game and Inland Fisheries (VDGIF) conducted a qualitative risk assessment that resulted in the stratification of the state into three risk categories (high, medium, and low risk) and the application of different surveillance strategies in each region. All captive cervid facilities within Virginia and the neighboring states (Kentucky, Maryland, North Carolina, Tennessee, and West Virginia) were designated a CWD risk category based on species present, movement histories and the state's CWD surveillance program for captive cervid facilities. The highest risk facilities were defined as facilities with known or suspected importations of white-tailed deer, elk (Cervus elaphus), mule deer (Odocoileus hemionus), black-tailed deer (Odocoileus hemionus columbianus), moose (Alces alces), as well as any hybrids or subspecies from out of state within the last five years and facilities that moved these species within the state from one of the above facilities within the last five years. In addition, any facility of unknown status or without adequate CWD surveillance was considered at high risk. These facilities with 10-mile buffers were mapped using GIS (ArcView, ESRI, Redlands, California), and the distribution of the high risk facilities was used to stratify the state into the risk categories. Based on a concentration of high risk facilities, we identified a region of northwest Virginia including sections of Shenandoah, Frederick, Clarke, and Loudoun counties at high risk for exposure to CWD. Subsequent to this risk assessment, West Virginia reported its first CWD-positive case adjacent to the previously defined high risk surveillance area. This risk assessment provided valuable information in delineating an area of highest concern where active surveillance was focused, allowing VDGIF to maximize the use of resources. This study also illustrates the importance of adopting a regional approach to assessing CWD exposure risk factors and designing appropriate surveillance strategies.

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