DETECTING CWD - Another lesson learned with CWD is that detecting it in captive and wild settings remains difficult despite the considerable effort spent. Most states and provinces have, at least for a time since the early 2000s, engaged in extensive, if not intensive, surveillance to identify affected wild herds. Although these efforts were well-intentioned, many were too flawed or too short-lived to reliably indicate the absence of disease. We briefly review common shortcomings of CWD surveillance to provide a basis for improving the efficiency and effectiveness of future efforts.
There is no known, practical, “live animal test.” The only known and conclusive diagnosis involves a test or examination of the brain, lymph nodes, or tonsils after death.
Preferred approaches for detecting CWD in new locations (termed “surveillance” here) differ from approaches for following epidemic trends over time in affected populations (“monitoring”). We recommend that CWD surveillance of wild cervids be an ongoing activity in areas where it has not been detected previously. Monitoring may be more intermittent (e.g., at multi-year intervals) when resources are limited because infection rates in wild herds tend to change slowly.
Regardless of the purpose, CWD surveillance and monitoring should be undertaken at a meaningful scale, and any conclusions should reflect the highly patchy distribution of CWD in wild cervids. CWD distribution in the wild typically is highly uneven within an affected population, and the target population itself often is distributed unevenly across the area being assessed. In our experience, statements indicating that examination of a few hundred (or even a few thousand) harvested animals has proven a state’s freedom from CWD rarely are supported by the data in hand.
In areas where CWD is regularly found, it has been demonstrated that animals falling into certain categories are more likely to test positive. These animals may have clinical signs of CWD, such as emaciation and abnormal behavior, may have been killed by a vehicle or predator, or may be older-age, male deer. Consequently, it may be more cost-effective to concentrate testing on animals with a higher probability of infection when surveillance is conducted to detect CWD in new locations than testing large numbers of apparently healthy, hunter-harvested animals. The effectiveness of this type of surveillance assumes relatively even sampling effort over a geographic area, but it does have limitations. For example, clinical disease may not be observed in remote areas, vehicle-killed animals do not occur in roadless areas, and animals killed by predators may be consumed before sampling can occur.
For monitoring, random sampling (e.g., from harvested animals) provides relatively unbiased estimates of infection rates. Comparisons over time or between locations should be based on a common denominator to assure that conclusions are reliable. Even though affected areas emerge and grow slowly, infection rates may be remarkably high on first detection when jurisdictions rely on random sampling and have not tested adequate numbers of animals at a particular location.
James L. Cummins is executive director of Wildlife Mississippi, a non-profit, conservation organization founded to conserve, restore, and enhance fish, wildlife, and plant resources throughout Mississippi. Their web site is www.wildlifemiss.org.
