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CWD: A 10-Year Retrospective

Chronic Wasting Disease (CWD) was first discovered in Wisconsin in February 2002. Much has changed in the last 10 years. The articles below provide a retrospective on those changes.

Disease Progression and Spread

Research Accomplishments

Management Approach of Other States

Wisconsin's Management Approach and Improvements

Disease Progression and Spread

The Wisconsin DNR began active surveillance for CWD in 1999 following increased awareness of interstate transport of elk from CWD-infected western game farms.  In February 2002, the DNR was notified that three deer harvested the previous fall from Deer Management Unit 70A in western Dane County had tested positive for CWD. This discovery launched an intensive surveillance effort in Wisconsin that continues today.  As of February 2012, nearly 172,000 wild white-tailed deer have been sampled, over 1,800 of which have tested CWD-positive.

There appear to be two main areas of CWD infection in Wisconsin. One is centered in western Dane and eastern Iowa counties. The second is located in northern Illinois and extends into southeastern Wisconsin. Illinois first detected the presence of CWD in this area in the fall of 2002 and as of April 2011, 336 CWD-positive deer have been found.

Surveillance has been continuously conducted since 2002 in the southern portions of the state and routinely on a rotating basis throughout the rest of the state. Sampling intensity across the majority of the state has been sufficient enough to have a high degree of confidence that CWD would be detected if the disease existed at 1% prevalence during the time of surveillance.

CWD has been found in 12 southern Wisconsin counties and the current CWD Management Zone encompasses all known locations of free-ranging deer that tested positive for CWD. 

Sex and Age Composition of Infected Deer

Analysis of the sex and age composition of positive deer over the past ten years in Wisconsin has shown that:

  •  disease prevalence is higher in older deer than in younger deer,
  •  and prevalence in males is approximately twice as high as in females. 

Overall, there has been an increasing trend in prevalence in all sex and age classes in the western and eastern Wisconsin core monitoring areas. Since 2002, prevalence in the western core has increased among:

  • adult males from about 8% to nearly 18%, 
  • adult females from about 3% to approximately 7%, 
  • yearling males from about 2% to about 6%, 
  • and yearling females from 2% to about 5%.

Prevalence increases are also evident in the eastern core where prevalence in adult males has increased from 2% to 6% between 2003-2011.

Very few fawns have tested positive for CWD (27 out of more than 15,000 tested since 2002).

Distribution Analysis

Analyses of the geographic distribution of disease show that the disease is not evenly distributed throughout the CWD Management Zone. 

Disease prevalence is much higher near the centers of each infection and declines with increasing distance from the center.

In a few square mile sections of land near the centers of the two infections, overall prevalence (includes deer of all ages and sexes) has been 8-20%. Analyses by wildlife disease scientists at the University of Wisconsin find that these spatial patterns are consistent with two separate disease introductions. These introductions likely occurred more than 20 years ago with growth in prevalence near the points of introduction and expansion to current distribution.

Get more information about the location of CWD in Wisconsin and the progression of the disease.

Research Accomplishments

During the past 10 years there has been an international effort to learn as much about CWD as possible. During that time, the WDNR has served an important role in generating new information by conducting in-house research, directly funding university research and by collaborating with others to share data and tissue samples from harvested deer. The DNR continues to make scientific research a priority in management.

This research has expanded our understanding of many facets of the disease including:

  1. genetic susceptibility of white-tailed deer to CWD,
  2. deer social organization and movement patterns,
  3. effects of artificial feeding and baiting on deer interactions,
  4. the effect of social organization and genetic relatedness on CWD transmission,
  5. the potential for scavengers feeding on deer carcasses to contribute to disease spread,
  6. how the landscape may influence the spread of the disease, and
  7. attitudes and behaviors of hunters and landowners in the CWD affected area.

In addition, several research paths have resulted in direct, significant cost and time savings for CWD management in that:

  1. improved diagnostic tools for detecting CWD has shortened the time required to notify most hunters of their test results,
  2. research on the binding of prions to soils has helped to identify safe and cost-effective methods of carcass disposal,
  3. an evaluation of the characteristics of deer that tested positive for CWD has led to more cost-effective strategies for detecting CWD in areas where it has not been found. 

Completed and ongoing research continues to fill important knowledge gaps regarding how CWD is transmitted among deer and how it spreads across southern Wisconsin.  In addition, hundreds of surveys and discussions with hunters and landowners have led to a better understanding of opinions, attitudes and behaviors relative to CWD and its management.  The results of the variety of research studies conducted during the past 10 years in Wisconsin and around the country were used in developing the current CWD Response Plan.

General State Response to the Threat of CWD in Captive and Wild Cervids

CWD has been found in captive and wild cervid herds in at least 18 states and two Canadian provinces.  Each has a response plan in place with varying protocols for surveillance and monitoring.  The widespread view held by most agencies is that eradication of CWD is at present not possible. In general, the programs and structures in place are designed with the following CWD management goals and guidelines:

  1. Conduct surveillance sufficient enough to establish the distribution and intensity of CWD in affected areas.
  2. Contain the disease within the known limits of its distribution, while minimizing geographic spread and local disease intensity.
  3. Prohibit or limit deer baiting activities to prevent high-density aggregation of animals which can lead to increased transmission probability.
  4. Quarantine or depopulation of captive cervid farms with infected animals.
  5. Place restrictions on inter-state transportation of farm-reared cervids.
  6. Dispose of infected carcasses by way of sealed landfills or incineration.
  7. Use monetary incentives, increased hunting season length, landowner permits and sharp-shooting to reduce the density of infected and susceptible deer on the landscape.

In addition, in several (assumed) CWD-free states adjoining regions with known infection, surveillance programs are being strategically conducted to maintain an early detection system in the case of CWD discovery/spillover.

Examination of wild deer data from Wisconsin, Colorado and Wyoming (states with known widespread distribution of CWD) show that without control efforts, CWD prevalence can reach high levels and become geographically widespread. Additionally, results from predictive models and monitoring data from Colorado and Wyoming suggest CWD can reduce deer populations—in some cases, drastically. Prevalence in adult male mule deer on some local winter ranges in Colorado more than doubled during a six-year period (1997–2002), reaching levels of 25–40%. A study in Boulder, Colorado showed that prevalence among 46 adult male mule deer sampled was 41% and prevalence among 69 adult female mule deer was 20%. The study concluded that high prevalence and shortened lifespan of infected deer is sufficient to have produced the observed 45% population decline. Preliminary findings from research in Wyoming have estimated a prevalence of 28% among white-tailed deer and have documented shorter lifespans among CWD positive animals.

In contrast to other states, Illinois has pursued a strategy of expanded public hunting regimes supplemented by localized, intensive sharpshooting in an effort to increase population turnover. The goal is to prevent the spread of CWD and eventually eliminate CWD from the affected populations. Sharpshooting is used to augment the hunting season kill by significantly reducing post-hunt local deer populations in known CWD areas. This effort has contributed more than 20% of the deer removed from the four Illinois counties that have been the primary focus of management efforts during the past five years.  Illinois is currently evaluating the effectiveness of their first five years of CWD management.Preliminary analyses indicate that they have achieved both local herd reductions and a corresponding decline in local prevalence levels in certain age and sex classes.

The Wisconsin and Illinois efforts to manage and respond to CWD are inextricably linked to the success or failure of one another. To formally acknowledge this fact, the two states signed a memorandum of understanding in mid 2010. It is imperative that the two states continue to work together on a mutual goal for CWD management to have a chance at success.

CWD Management Perspectives and Improvements

When Wisconsin initiated CWD management efforts in 2002, the goal was to eradicate the disease from the state. Surveillance efforts since then show the disease is found in a much larger area of southern WI than initial data indicated.Given the lack of tools available to counter CWD over a broad spatial extent, eradication was not possible and the current goal for CWD management is to minimize the area of Wisconsin where CWD occurs and the number of infected deer in the state.

To that end, the WDNR has made (and will continue to make) a number of improvements to reduce costs of CWD management, facilitate reliable broad-scale testing, streamline data collection and management, and foster cooperative relationships with various community groups. Some of these measures include:

  1. Improved operational efficiencies such as field lymph node collection for testing, rather than head removal, which reduces handling and processing costs.
  2. Cooperative agreements with landfills to safely dispose of deer carcasses in accordance with guidelines developed through research with UW Madison. This safe and cost saving measure is an improvement over previous disposal methods of chemical digestion and incineration (car-killed deer, butcher waste).
  3. Implementation of an extended landowner hunting season within the CWD Management Zone from January–March. This season is open to landowners and those they allow to hunt on their property. It provides additional hunting opportunity and gives landowners tools to lower local deer herds in areas of overabundance and high CWD prevalence.
  4. Streamlining of detection efforts using risk factor analysis which allows sampling of fewer deer (focus on higher risk groups) to achieve a high level of confidence to detect CWD if it is present.
  5. Establishing cooperative relationships with taxidermists and private businesses throughout the state (cooperative deer registration/sampling stations) to improve CWD surveillance, and with dozens of butchers to provide venison to food pantries within the CWD management zone.

Given the uncertainties regarding mechanisms of transmission, limited tools for management and declining public support and funding, eliminating CWD from Wisconsin is unlikely.  However, a healthy deer population is essential to maintain Wisconsin’s deer hunting culture, thus there remains a need and responsibility for the DNR to continue monitoring CWD prevalence and distribution to make informed decisions about its management.


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