CWD Part Two

Looking Hard—Hardly Looking: Detecting Chronic Wasting Disease 

An Excerpt from Spring 2017 Fair Chase 

Another lesson learned from our first five decades of experience with chronic wasting disease (CWD) is that detecting CWD in captive and wild settings remains difficult despite the considerable effort expended. 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 as widely practiced to provide a basis for improving the efficiency and effectiveness of future efforts.

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 episodic (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. 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 CWD-endemic areas, it has been demonstrated that animals falling into certain categories are more likely to test positive.  These animals may have clinical signs of CWD (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. In addition to clinical targeting, spatial targeting via risk-based assessments, such as proximity to affected wild populations or captive cervids, also may enhance the effectiveness of CWD surveillance.

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CWD – Part 2

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 (e.g., harvested males aged 2 years or older) 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 for surveillance and have not tested adequate numbers of animals at a particular location.

Chronic wasting disease tends to be unevenly distributed in the wild. The notion that a survey sample of 300 assures 95 percent probability of detecting at least one case where prevalence is greater than or equal to 1 percent assumes infection is evenly distributed at that rate throughout the entire target population. However, CWD distribution typically is highly uneven within an affected population, and the target population itself often is distributed unevenly across the area being assessed. 

Toward a Sustained and Sustainable Effort to Control Chronic Wasting Disease

Eradicating CWD from North America appears infeasible given its extensive distribution and other epidemiological attributes as well as the limited number of available tools. With few exceptions—the detection of two positive deer in New York in 2005 and one positive deer in southeastern Minnesota in 2011 (although CWD has been found in several wild deer in 2016-17 in an adjacent county)—CWD in free-ranging cervids has persisted in affected areas in the face of widely varied control attempts. Faced with dim prospects for eradication, some affected jurisdictions now seem to have abandoned any further consideration of disease management and some have effectively dismantled surveillance and monitoring. In light of numerous wildlife conservation needs and ever-dwindling resources, we appreciate the allure but believe this approach should be reconsidered, and we strongly encourage wildlife managers to redouble efforts to collectively develop sustained approaches for CWD surveillance, monitoring, and control.

In contrast to the apparent success in eliminating New York’s small free-ranging focus (two wild deer with CWD were detected in 2005 in the vicinity of an affected captive herd), well-publicized early attempts to control CWD in Colorado and Wisconsin yielded little evidence of progress and thus gave initial appearances of failure. In recent years, however, evidence from some control attempts suggests that combinations of intensive deer removal around case clusters, as well as more sustained reduction of the affected population, may offer some measure of disease suppression. A sustained, localized culling program underway since 2003 has stabilized prevalence in northern Illinois whitetails as compared to the increasing trends in southern Wisconsin where disease control largely was suspended in 2007. Similar divergence in prevalence between deer harvested in Alberta and Saskatchewan may reflect the relative effectiveness of disease suppression efforts in Alberta, but also could be an artifact of more recent CWD emergence there. In northcentral Colorado, a combination of focal culling and broader, hunter-harvest population reduction (approximately 25 percent) in the early 2000s appears likely to have contributed to reduced prevalence, whereas estimated prevalence in other Colorado mule deer herds has increased since 2002.

One of the most common flaws in CWD control efforts to date has been initial underestimation of the affected area (often based on inadequate surveillance and erroneous assumptions about how long CWD has been present).  The outcome then gave the appearance that the control attempt had failed when in fact the approach was biologically sound but the application was either too small (spatially) or too short-lived. It follows that acquiring reliable distribution and prevalence data in the planning and early implementation stages may improve the efficacy of future CWD control efforts. Consequently we encourage wildlife managers to set realistic disease-control objectives and to use an adaptive management approach that incorporates future field data to refine objectives and strategies.

In addition to adopting and adaptively assessing approaches for stabilizing or suppressing CWD outbreaks, we encourage wildlife managers to consider how recent trends in cervid management may be contributing to disease establishment. Modeling suggests harvest-based control of CWD may be most effective when focused on male deer, perhaps because infection rates among adult male deer tend to be higher than among adult females. Conversely, then, harvest strategies intended to increase male to female ratios or adult male age structure could inadvertently facilitate CWD persistence. This may explain why the dramatic increases in prevalence observed since 2002 in Colorado in several affected mule deer herds coincide with changes in harvest strategies intended to reduce buck harvest and increase buck to doe ratios over the same period. Given the potential for unintended consequences, we encourage critical assessment of how this and other harvest strategies (e.g., season timing, baiting and/or feeding, “quality deer management”) may be affecting CWD dynamics.

Control efforts undoubtedly will be more difficult to champion and garner support for in sociopolitical climates ranging from apathetic to combative, particularly when control prescriptions impinge upon or conflict with commercial cervid enclosures and/or hunting by the general public. The human dimensions of managing wildlife diseases in general—and CWD in particular—present a substantial challenge for those determining the management objectives and actions. For example, surveys of hunters and landowners in Wisconsin identified several factors that contributed to hunter opposition to the state’s CWD management plan including: opposition to deer population goals (initially zero); conflicts with traditions; uncertainty about the likelihood of success; questions about agency credibility; and no sense of urgency.

We believe there are two important motivations for responsible wildlife managers to make progress toward sustainable containment and control strategies for CWD in the coming decades. First, data from several sources suggest that an affected whitetail population will not thrive in the long-term.  For example, researchers studying of an affected whitetailed-deer population in Wyoming recently found that CWD-positive deer were 4½ times more likely to die annually than CWD-negative deer, while bucks were 1.7 times more likely to die than does.  The researchers concluded that “the strong population-level effects of CWD suggest affected populations are not sustainable at high disease prevalence under current harvest levels.”  Second, we believe that existing data on CWD prions and experience with other animal prion diseases suggest minimizing human exposure to these agents would be prudent.

The final overarching lessons learned over the past five decades relate to how wildlife and animal health professionals should (and probably should not) approach the control of CWD. In contrast to advances in our understanding of CWD biology and ecology, the science informing effective management and control strategies remains relatively incomplete. However, recent insights and modest strides seem to offer a path forward, and adaptive approaches for containing CWD within limited geographic areas and for reducing infection and transmission rates deserve further attention.