Chris Geremia, John Treanor, and P. J. White
Chronic wasting disease (CWD) is a contagious, fatal disease of cervid populations, including deer, elk, and moose for which there is no vaccine or known treatment (Williams et al. 2002, Baeten et al. 2007). CWD is from a category of diseases known as transmissible spongiform encephalopathies, which are also known as prion diseases. Other diseases in this category include mad cow disease in cattle, scrapie in goats and sheep, and Creutzfeldt-Jakob disease in humans. Prion diseases are believed to be caused by the accumulation of a misfolded variant of native prion (a protein naturally found within all mammals) which leads to neurodegeneration and, ultimately, death. Infected animals may appear normal initially, but typically lose weight, drink and urinate more frequently, drool excessively, have a wide-based stance with lowered head and ears, have a blank facial expression, lose their fear of humans and, eventually, become isolated as infection progresses. However, these signs are not specific to CWD and a definitive diagnosis is made by laboratory testing of the brain or lymph nodes collected from dead animals.
The disease course is prolonged with deer capable of transmitting infectious prions within months of exposure, and infected animals remaining alive for up to two years (Fox et al. 2006, Miller et al. 2008, Geremia et al. 2015). CWD is transmitted by direct animal-animal contact (e.g., saliva, blood) or, indirectly, through infectious particles that can persist in the environment (e.g., carcasses, antlers, soils, plants; Mathiason et al. 2006, Tamgüney et al. 2009, Pritzkow et al. 2015). The debilitating effects of CWD are progressive with minimal pathophysiological effects during the first year after exposure, followed by rapid deterioration of motor function, such as gut control, vision, and hearing. CWD reduces adult deer survival with effects increasing over time (Miller et al. 2008, Geremia et al. 2015); though some research suggests the disease does not influence survival in populations subject to intensive harvest (Magle et al. 2012). CWD is not believed to influence reproduction (Dulberger et al. 2010, Magle et al. 2012).
There is general uncertainty regarding how CWD epidemics will effect cervid populations because epidemics are prolonged, playing out over decades. Simulation models have predicted a variety of outcomes from limited population decline and sustained low disease prevalence to local extinction within decades of disease introduction (Gross et al. 2001, Wasserberg et al. 2009, Wild et al. 2011, Almberg et al. 2011, Jennelle et al. 2014,Monello et al. 2014, Potapov et al. 2015). Empirical studies have confirmed that under high prevalence (>20% in females) deer populations have sustained short-term declines, and CWD has been implicated as a proximal cause acting through decreases in adult female survival (Miller et al. 2008, Edmunds 2013, Devivo 2015). However, similar effects have been muted across larger spatial areas because high prevalence has remained a local phenomenon (Geremia et al. 2015). If epidemics lead to widespread population reductions in Yellowstone, CWD could indirectly alter the structure and function of this ecosystem during future decades; adversely affect species of predators and scavengers; and have serious economic effects on the recreation-based economies of the area.
The National Park Service is tasked with maintaining native plants and animals by preserving and restoring the natural abundances, diversities, dynamics, distributions, habitats, and behaviors of native plant and animal populations and the communities and ecosystems in which they occur (National Park Service 2006). The origin of CWD is unknown, but it is clear that its distribution and prevalence were influenced by human activities such as the translocation of deer and elk between game farms, research facilities, and zoological parks and, also, by concentration of wildlife through artificial feeding, loss of habitat, and changes in movement patterns due to fragmented landscapes. By causing deer and elk to increase in numbers and become more sedentary in many areas, anthropogenic actions have increased the chances of animal-to-animal transmission and exposure from a contaminated environment, which may result in a disease process that is incongruent with a naturally functioning ecosystem. Thus, it is appropriate for the National Park Service to consider CWD management actions (National Park Service 2006).
CWD has continued to spread north and west across the state of Wyoming during the past 15 years. In 2015, CWD was detected in a mule deer harvested in a hunt area approximately 9.3 miles from the southeastern boundary of Yellowstone National Park. Elk that summer in the park could commingle with deer in this area. This plan for Yellowstone National Park outlines a step-wise surveillance plan for CWD and describes what actions the park will take to suppress disease prevalence and spread if it is detected.
A primary purpose of Yellowstone National Park is to preserve abundant and diverse wildlife in one of the largest remaining intact ecosystems on earth. Disease management actions such as depopulation or substantial population reductions by random culling may be inappropriate for the park because they would remove many more healthy animals than infected animals, substantially reduce the prey base for predators and scavengers, and result in fewer benefits (e.g., scientific knowledge) and reduced visitor enjoyment (e.g., recreational viewing).
Yellowstone National Park provides summer range for more than 10,000 deer and elk from multiple herds. Surveillance for the majority of these populations is difficult because they are widely distributed in high mountain habitats during summer and winter at lower elevations outside the park. Also, deer and elk from different populations intermix during summer, making it impossible to differentiate animals from different target populations and difficult to define sampling units. The current abundance of moose in and near the park is unknown due to the difficulty of conducting surveys and obtaining rigorous population estimates for this species at low densities.
CWD can be confirmed through laboratory analysis of central (obex, spinal cord) or peripheral (retropharyngeal lymph node, tonsil, rectal mucosa associated lymphoid tissue) nervous system tissue. Tissue samples can be obtained by anesthetizing live animals and obtaining a biopsy of tonsil or rectal mucosa associated lymphoid tissue (Wolfe et al. 2002, 2007) or from necropsying carcasses. Current screening tests include enzyme-linked immunosorbent assay (ELISA) and immunohistochemistry (Spraker et al. 2002), though other prion amplification methods are being evaluated (Wyckoff et al. 2015). Immunohistochemistry is currently the most reliable test for detecting CWD in animal tissues. However, it is still an imperfect test and a negative test result does not guarantee a CWD-free animal, particularly early in the disease course (Fox et al. 2006, Wolfe et al. 2007). Furthermore, pathogenesis of CWD may differ among deer and elk and within species based on composition of the prion precursor gene, especially with regards to the timing of misfolded prion deposition in different tissues (Keane et al. 2009, Spraker et al. 2009, Wolfe et al. 2014). These differences reduce the ability to detect CWD early in the disease course.
In most areas and especially during the early phases of CWD epidemics, CWD is likely to occur at low prevalence that is difficult to detect (Samuel et al. 2003). As an example, for surveillance to reliably provide early detection of CWD, sample sizes must be sufficient to detect relatively low infection rates with considerable confidence. At a disease prevalence of 1%, we would need to sample a minimum of 450 animals from a population of approximately 10,000 animals to be 99% confident of detecting one animal with CWD.
Given these challenges, we propose a stepwise surveillance program. Initially, we will implement targeted surveillance aimed to opportunistically test deer and elk that are more likely to be infected with CWD. In these deer, the CWD-detection probability per animal sampled would be higher than other, more random methods of surveillance (e.g., harvests) and, as a result, fewer samples should be needed to detect a CWD-positive animal (Walsh 2012). In general, prevalence tends to be twice as high in males compared to females (Miller and Conner 2005, Jennelle et al. 2014). Also, prevalence is higher in mule deer compared to elk or white-tailed deer living in the same area. In addition to targeting species and demographics, we will:
If CWD is detected in the park, we will:
We will measure CWD prevalence over 5- to 10- year horizons given the slow progression of CWD epidemics, difficulty of obtaining enough samples to identify the presence of CWD and, if it is found, accurately estimate prevalence. Tissue samples will be tested using ELISA as an initial screening and any samples testing positive will be reevaluated using immunohistochemistry.
Laboratory studies have shown that prions bind to clay minerals and clay-laden whole soils with dramatic increases on infectivity (Johnson et al. 2006, 2007). Cervids ingest soil both deliberatively and inadvertently during foraging and grooming. Prions propagated in lymphoid tissue of the oropharynx and gut may bind to ingested soil particles enhancing infectivity both within and outside the host (Johnson et al. 2006, 2007; Walter et al. 2011).
The structure of the prion precursor (PRNP) gene within hosts may afford some general protection against infection. A nucleotide substitution in the 225th codon of mule deer PRNP leads to an amino acid phenylalanine (F) for serine (S) substitution (Jewell et al. 2005). This amino acid change may disrupt or slow prion propagation, such that the 225F/S substitution has been associated with slower disease progression in mule deer (Fox et al. 2006, Wolfe et al. 2014). Similar non-synonymous nucleotide substitutions in white-tailed deer (O’Rourke et al. 2004, Johnson et al. 2006) and elk (Green et al. 2008) have been associated with longer incubation or reduced prevalence in individuals possessing the rarer allele.
We will begin risk mapping for CWD based on soils, genetics, and movement patterns of deer and elk inside Yellowstone, and distances to known infected deer and elk populations outside the park. We will collaborate with other agencies and researchers currently studying elk, mule deer, and CWD in and near Yellowstone to unify existing information to (1) identify deer and elk movement patterns and identify spatial population structure, (2) characterize deer and elk PRNP genotypes and soil conditions at population and subpopulation scales, (3) integrate soil, PRNP, and population structure information with known distances to CWD to predict areas more likely for CWD introduction, and (4) develop stepped-up surveillance strategies if CWD is detected in the park.
We will establish 5% (males) and 2-3% (females) as thresholds in 5- to 10- year prevalence levels that would trigger an evaluation of whether active disease suppression was feasible and warranted. This decision was based on several important aspects of CWD epidemiology and management:
If prevalence reaches 5% (males) or 2-3% (females), we would evaluate the science and technology at the time and consider implementing disease management actions (e.g., selective culling) to maintain prevalence below these thresholds.
There is no evidence that CWD poses a risk to human health, but a related animal disease, bovine spongiform encephalopathy (mad cow disease), has been causally linked to the human form of that disease known as variant Creutzfeldt-Jakob disease. This has raised concerns about the possibility of CWD crossing the species barrier and infecting humans that consume meat from infected elk and deer. While current evidence indicates that the differences between mad cow disease, Creutzfeldt-Jakob disease, and CWD are significant, there is still ongoing research to establish whether CWD can cross the human species barrier. Thus, health experts warn that no part or product of any animal with evidence of CWD should be fed to any species (human or any domestic or captive animal). At this time, Yellowstone does not plan to donate meat from predator or vehicle killed animals or to local food banks or those in need.
For public safety and resource protection, the Superintendent’s Compendium (Yellowstone National Park 2014; 36 CFR 1.6(a), 1.5(a)(2)) prohibits the transport of heads and spinal cords from deer, elk, or moose through the park if they were harvested in a state or province with CWD diagnosed in their wildlife populations, except for the following portions of the carcass:
Currently, those states and provinces include Arkansas, Colorado, Illinois, Iowa, Kansas, Maryland, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, New Mexico, New York, North Dakota, Ohio (hunting preserve), Pennsylvania, South Dakota, Texas, Utah, Virginia, West Virginia, Wisconsin, Wyoming, Alberta, and Saskatchewan.
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Last updated: November 20, 2018