1

Introduction

Chronic wasting disease (CWD; Williams and Young, 1980) is a fatal prion disease affecting the central nervous systems of multiple cervid species (i.e., species in the “deer family”), including free-ranging and captive mule deer, white-tailed deer, elk (also known as wapiti), moose, caribou (sometimes called reindeer), red deer, and sika deer (cases in sika deer have thus far only been reported in South Korea). Box 1.1 describes what prions and prion diseases are. A syndrome now understood to be CWD was first observed affecting captive mule deer in Colorado in 1967 (Williams and Young, 1980; Williams and Young, 1992). As of August 1, 2024, CWD cases had been reported in 35 U.S. states and five Canadian provinces (see Figure 1.1). CWD has independently emerged among cervids (moose, red deer, and reindeer) in the European countries of Sweden, Norway, and Finland (EFSA Panel on Biological Hazards, 2019; Sola et al., 2023). It was introduced into South Korea via trade of live cervids from Canada (Sohn et al., 2002; Kim et al., 2005).

CWD prions are thought to be transmitted both directly and indirectly between cervids, and have been shown to persist in the environment for long periods (in a review by Osterholm et al., 2019; Miller and Wolfe, 2023). Cervids infected with CWD have been observed to have measurably shorter lifespans compared to uninfected compatriots (e.g., Miller et al., 2008; Monello et al., 2014; Edmunds et al., 2016; DeVivo et al., 2017; Haley et al., 2020a; J. Ballard, personal communication to NASEM, 2023). Rates of infection tend to increase over time (see Box 5.1 in Chapter 5), eventually exceeding 20% among adult females and twice that among adult males in some wild deer populations (e.g., Miller et al., 2008; Edmunds et al., 2016; DeVivo et al., 2017; J. Ballard, personal communication to NASEM, 2023; Fisher et al., 2022; Saskatchewan Ministry of Environment¹; Wisconsin Department of Natural Resources²). Prevalence as high as 80% has been reported in captive herds (Keane et al., 2008a). In addition to increasing numbers of infected animals, outbreaks in some states have shown marked geographic expansion over time such that, in the most extreme examples, cases have been recorded across virtually entire states (see Figure 1.1). The emergence of CWD can become a significant mortality factor in affected cervid populations when a sufficiently high proportion of adult females becomes infected (DeVivo et al., 2017; Edmunds et al., 2016; J. Ballard, personal communication to NASEM, 2023; Miller et al., 2008; Haley et al., 2020a; Fisher et al., 2022; Gilbertson et al., 2022; Monello et al., 2014).

A considerable foundation of knowledge about CWD was assembled in the first two decades after its formal description by Williams and Young (1980, 1992). This nascent understanding of host range, diagnostic and surveillance approaches, epidemiology, risk factors, and control strategies (see reviews by Williams and Young, 1992; Williams and Miller, 2002, 2003; Williams et al., 2002) informed the first national response plans undertaken in the United States during the early 2000s (Chronic Wasting Disease Task Force, 2002; National CWD Plan Implementation Committee, 2002; Gillin and Mawdsley, 2018). Research, surveillance, and management of CWD intensified after the disease was declared a national emergency in the United States (Geist et al., 2017) and have become a major focus of research activity within the U.S. Geological Survey (USGS). The U.S. Department of Agriculture (USDA) also provides research through the Agricultural Research Service and management guidance to the captive cervid industry through the Animal and Plant Health Inspection Service, Veterinary Services. The Association of Fish and Wildlife Agencies’ technical report (Gillin and Mawdsley, 2018) provides a detailed literature review that describes best management practices and points to the many CWD-related questions that remain unanswered. Those questions regard topics such as the precise mechanisms of transmission, the evolution of different prion variants (strains), the effects of animal density on transmission, the transmissibility of CWD to other species, the effects of human factors on the disease spread, and many others.

In 2020, Congress passed America’s Conservation Enhancement (ACE) Act (P.L. 116-188), which directs the U.S. Fish and Wildlife Service (USFWS) to lead a CWD Task Force to “leverage the collective resources of Federal, State, and local agencies, [tribal communities], and foreign governments, and resources from private, nongovernmental entities” and develop an action plan for addressing CWD in the nation. The ACE Act encouraged the Secretaries of the Interior and Agriculture to commission from the National Academies of Sciences, Engineering, and Medicine (the National Academies) “a special resource study to identify the predominant pathways and mechanisms of the transmission of chronic wasting disease in wild, captive, and farmed populations of cervids in the United States.”

CHARGE TO THE COMMITTEE

In collaboration with the USDA and USFWS, the USGS requested that the National Academies convene an ad hoc multidisciplinary committee of experts to draw conclusions about the state of knowledge on CWD. Box 1.2 is the statement of task provided to the committee. The committee was to examine published and ongoing research regarding infectious doses, modes of transmission, the means of geographic spread, the effectiveness of interventions to reduce transmission or spread, and implications on human society of CWD.

The committee’s charge does not include developing recommendations regarding CWD, nor does it include a discussion of the state of knowledge regarding transmission of CWD to humans. During discussions with the committee, the study sponsors indicated that this report will be used to inform discussions of the eventual CWD task force formed under the ACE Act and its decisions regarding an action plan for consistent and coordinated management of CWD. The audience for this report includes state, provincial, tribal, and federal wildlife and captive cervid managers and decision makers; conservation and hunting groups; the captive cervid industry; individuals and groups interested in CWD and cervid management; members of Congress; and the public at large (J. Malmberg, presentation to the committee, October 9, 2023).

COMMITTEE COMPOSITION

Eleven experts were convened specifically to deliberate the task described in Box 1.2. Expertise on the committee included veterinary medicine and cervid health, prion biology and ecology, pathology, wildlife epidemiology, wildlife management, captive cervid management, husbandry, statistical and spatial modeling, transmission

modeling, natural resource economics, testing methodologies, genetic resistance, and transmission surveillance. A range of perspectives was sought, including views based on public sector experience in the state (or provincial) and federal governments, experience managing other wildlife diseases, academic research experience, and familiarity with different interested and affected groups and perspectives. Committee member biographies are included in Appendix A.

APPROACH TO TASK

The committee regards CWD as fitting the criteria to be considered among a category of diseases described as emerging infectious diseases (EIDs). EIDs are defined as newly recognized, introduced, or evolved diseases, or those that have recently and rapidly changed in incidence or expansion in geographic, host, or vector range (Petersen, Petrosillo, and Koopmans, 2018). The steady and impressive identification of CWD cases across the United States over the last few decades in captive and free-ranging cervids is consistent with other EIDs. EIDs have been characterized by the interaction of multiple factors and drivers that converge to create a complex and dynamic environment enabling the emergence or expansion of a disease (IOM, 1992, 2003). The dynamics of CWD are impacted by the convergence of factors at the animal, ecosystem, and human interface, adding to CWD’s complexity and disease management challenges. Thinking of CWD as an EID allowed the committee to view CWD through a comprehensive scientific framework that helps to better understand both the disease and its prevention and control.

This report uses the term “CWD” in reference to the infectious North American phenotype of CWD unless noted otherwise. In addition to exploring the scientific literature related to CWD, the committee heard from its sponsors—three agencies with different mission statements—and from a variety of researchers, cervid farmers,

TABLE 1.1 Examples of Sectors and Constituents with Interest in CWD and Their Perspectives

The ACE Act recognizes that management of CWD and its wide-reaching impacts are highly complex issues. Management will involve the interplay of, and input from, a variety of interest groups from an array of disciplines, including animal, human, and environmental health professionals; wildlife biologists; laboratory researchers; law enforcement personnel; sociologists; economists; local, state, federal, and tribal leaders; and often members of the public—everyone from nature enthusiasts to sanitation workers. The management of the original severe acute respiratory syndrome (SARS) caused by the first identifiable strain of the SARS-related coronavirus (SARS-CoV-1) in 2003 also required the interplay of an array of groups and experts. The success was at least partially the result of the application of “One Health” concepts, forming the basis of what would become the Manhattan Principles (2004)—the 12 recommendations for holistic prevention of epidemic or epizootic diseases, protecting the integrity of ecosystems and ecosystem diversity, and benefiting people and domesticated animals. The One Health approach allows the development of solutions based on consideration of disease effects on a range of human, animal, environmental, social, economic, and political interests (see Figure 1.2). With rare exception, CWD solutions have been challenging to develop and implement. Widely accepted consensus on solutions often has been impossible to reach due to numerous interdependent and often contradictory factors. CWD could be considered a “wicked” problem—one that is complex and often intractable with no single definitive solution. Like other wicked problems, improved understanding of CWD and its appropriate management will require genuine collaboration between

diverse groups to develop long-term strategies and uncover mutually agreeable solutions, to a degree far beyond what has been demonstrated to date.

REPORT ORGANIZATION

This report is organized into eight chapters, with Chapters 2-7 each including a header box of bulleted chapter highlights. Chapter 2 provides a brief history of CWD in North America followed by descriptions of CWD and the state of knowledge regarding CWD strains, infectious doses, incubation period, clinical presentation, disease pathogenesis, and infectivity. Chapter 3 provides discussion of transmission mechanisms and routes of exposure, potential host range, and the potential for spillover of the disease into other animal species and humans. Chapter 4 focuses on diagnostic and surveillance methodologies and strategies followed by a discussion of the epidemiology and ecology of CWD in Chapter 5, including a discussion of the impact of CWD on cervid population dynamics. The effectiveness of interventions to control or reduce the spread of CWD in captive and free-ranging cervid populations is addressed in Chapter 6, and Chapter 7 provides a description of the state of knowledge with respect to the human and socioeconomic dimensions of CWD. Chapter 8 synthesizes the committee’s overarching conclusions regarding the state of knowledge regarding CWD. The conclusions are intended to inform the design of a strong action-oriented and integrated strategy to reduce the transmission and further geographic spread of CWD. The report is appended with answers to questions commonly asked by the lay public. This is provided as Appendix C.