EVALUATION OF A TEST AND CULL STRATEGY FOR REDUCING PREVALENCE OF CHRONIC WASTING DISEASE IN MULE DEER ( ODOCOILEUS HEMIONUS)

Demographic Risk Factors Vary in the Invasion Front of Chronic Wasting Disease in West Virginia, USA

After detecting chronic wasting disease (CWD) in white-tailed deer (Odocoileus virginianus) in Hampshire County, West Virginia, USA, in 2005, we investigated the change of CWD apparent prevalence and potential factors influencing infection risk during the invasion front. Over eight sampling years (2006-2012 and 2017) during a 12-yr period within a 101-km2-area monitoring zone, we sampled and tested a total of 853 deer for CWD by ELISA and immunohistochemistry. Bayesian logistic regression of risk factors included collection year, age class, sex, and adjusted body weight (weight after accounting for sex, age, kidney fat index, and number of fetuses). In the whole-herd model (n=634), collection year, age, and adjusted body weight were associated with increased odds of CWD, whereas an age-weight interaction had a negative relationship. We found that males drove the positive associations with age and adjusted body weight, whereas females were responsible for the negative interaction effect. These findings suggest potential behavioral and physiological mechanisms related to sex that may influence CWD exposure. Older males exhibited higher CWD prevalence, aligning with previous studies. Notably, the novel finding of adjusted body weight as a risk factor in males warrants further investigation, and this study highlights the need for future research on social behavior and its role in CWD transmission within white-tailed deer populations.

Where we stand on chronic wasting disease: A systematic literature review of its prevalence patterns, impacts, and management interventions

With high fatality and no cure, chronic wasting disease (CWD) has infected cervids in multiple regions, including the United States, Canada, Europe, and South Korea. Despite the rapid growth of literature on CWD, the full scope of its ecological, social, and economic impacts and the most effective and socially acceptable management strategies to mitigate the disease is unclear. Of 3008 initially identified published peer-reviewed papers, 134 were included in a final systematic literature review to synthesize the current knowledge on CWD transmission patterns, impacts, and the effectiveness of management interventions. The number of publications on CWD has increased steadily since 2000 with an average of six papers per year. Most papers were related to CWD prevalence (39 %), human behavior (33 %), CWD impacts (31 %), and management interventions (16 %). Environmental factors such as soil, water, and plants were identified as the most common transmission medium, with a higher prevalence rate among adult male cervids than females. Hunters showed a higher risk perception and were more likely to change hunting behavior due to CWD detection than non-hunters. Ecological impacts included the decreased survival rate accompanied by lower population growth, eventually leading to the decline of cervid populations. Culling was found to be an effective and widely implemented management strategy across countries, although it often was associated with public resistance. Despite potentially high negative economic impacts anticipated due to CWD, studies on this subject were limited. Sustained surveillance, ongoing research, and engagement of affected stakeholders will be essential for future disease control and management.

Transmission, Strain Diversity, and Zoonotic Potential of Chronic Wasting Disease

Chronic wasting disease (CWD) is a prion disease affecting several species of captive and free-ranging cervids. In the past few decades, CWD has been spreading uncontrollably, mostly in North America, resulting in a high increase of CWD incidence but also a substantially higher number of geographical regions affected. The massive increase in CWD poses risks at several levels, including contamination of the environment, transmission to animals cohabiting with cervids, and more importantly, a putative transmission to humans. In this review, I will describe the mechanisms and routes responsible for the efficient transmission of CWD, the strain diversity of natural CWD, its spillover and zoonotic potential and strategies to minimize the CWD threat.

Apparent stability masks underlying change in a mule deer herd with unmanaged chronic wasting disease

The contagious prion disease "chronic wasting disease" (CWD) infects mule deer (Odocoileus hemionus) and related species. Unchecked epidemics raise ecological, socioeconomic, and public health concerns. Prion infection shortens a deer's lifespan, and when prevalence (proportion of adults infected) becomes sufficiently high CWD can affect herd dynamics. Understanding population responses over time is key to forecasting long-term impacts. Here we describe unexpected stability in prevalence and abundance in a mule deer herd where CWD has been left unmanaged. High apparent prevalence (~30%) since at least 2005 likely drove observed changes in the proportion and age distribution of wild-type native prion protein (PRNP) gene homozygotes among deer sampled. Predation by mountain lions (Puma concolor) may be helping keep CWD in check. Despite stable appearances, prion disease nonetheless impairs adult survival and likely resilience in this deer herd, limiting its potential for growth despite refuge from hunter harvest and favorable habitat and winter conditions.

Spatio-temporal changes in chronic wasting disease risk in wild deer during 14 years of surveillance in Alberta, Canada

Disease risk modeling is a key first step to understand the spatio-temporal dynamics of wildlife disease and to direct cost-effective surveillance and management. In Alberta, active surveillance for chronic wasting disease (CWD) in wild cervids began in 1998 with the first case detected in free-ranging cervids in 2005. Following the detection, a herd reduction program was implemented during 2005-2008 and in 2006 the ongoing hunter-based CWD Surveillance Program became mandatory in high-risk Wildlife Management Units (WMU). We used data collected during the CWD surveillance program to 1) document growth in sex-specific CWD prevalence (proportion of deer in sample that is CWD-positive) in hunter-harvest deer in 6 WMUs consistently monitored from 2006 to 2018, 2) document landscape features associated with where CWD-positive compared to CWD-negative deer were removed during hunter harvest and herd reduction in an early (2005-2012) and in a late period (2013-2017), and 3) to map the spatial risk of harvesting a deer infected with CWD in the prairie parklands of Alberta. In the 6 continuously monitored WMUs, risk of a harvested deer being CWD positive increased from 2006 to 2018 with CWD prevalence remaining highest in male mule deer whereas overall growth rate in CWD prevalence was greater in female mule deer, but similar to male white-tailed deer. We found no evidence that the 3-year herd reduction program conducted immediately after CWD was first detected affected the rate at which CWD grew over the course of the invasion. Risk of deer being CWD-positive was the highest in animals taken near small stream drainages and on soils with low organic carbon content in the early period, whereas risk became highest in areas of agriculture especially when far from large river drainages where deer often concentrate in isolated woody patches. The change in the influence of proximity to known CWD-positive cases suggested the disease was initially patchy but became more spatially homogeneous over time. Our results indicate that a targeted-removal program will remove more CWD positive animals compared to hunter harvest. However, the discontinuation of targeted removals during our research program, restricted our ability to assess its long term impact on CWD prevalence.

THE RELATIONSHIP BETWEEN HARVEST MANAGEMENT AND CHRONIC WASTING DISEASE PREVALENCE TRENDS IN WESTERN MULE DEER (ODOCOILEUS HEMIONUS) HERDS

We analyzed retrospective data on harvest management practices and corresponding chronic wasting disease (CWD) prevalence trends in 36 western US and Canadian mule deer (Odocoileus hemionus) management units (units). Our analyses employed logistic regression and model selection, exploiting variation in practices within and among jurisdictions to examine relationships between harvest management and apparent prevalence (the proportion of positive animals among those sampled). Despite notable differences in hunting practices among jurisdictions, our meta-analysis of combined data revealed strong evidence that the amount of harvest was related to CWD prevalence trends among adult male mule deer in the 32 units where prevalence at the start of the analysis period was ≤5%. All competitive models included the number of male deer harvested or number of hunters 1-2 yr prior as an explanatory variable, with increasing harvest leading to lower prevalence among males harvested in the following year. Competitive models also included harvest timing. Although less definitive than the number harvested, median harvest dates falling closer to breeding seasons were associated with lower prevalence in the following year. Our findings suggest harvest-when sufficient and sustained-can be an effective tool for attenuating CWD prevalence in adult male mule deer across western ranges, especially early in the course of an epidemic. Evidence of a broad relationship between the amount of harvest and subsequent changes in CWD prevalence among adult male mule deer provides an empirical basis for undertaking adaptive disease management experimentation aimed at suppressing or curtailing CWD epidemics.

Inferring Chronic Wasting Disease Incidence from Prevalence Data

Incidence of chronic wasting disease infection showed strong, positive correlation (r≥0.944) with apparent prevalence among female and male mule deer (Odocoileus hemionus) in seven herds previously studied in Colorado and Wyoming, US. With attention to monitoring method consistency and context, inferring that observed prevalence trends reflect underlying epidemic dynamics in mule deer herds appears justifiable.

HUNTING PRESSURE MODULATES PRION INFECTION RISK IN MULE DEER HERDS

The emergence of chronic wasting disease, an infectious prion disease of multiple deer species, has motivated international calls for sustainable, socially accepted control measures. Here, we describe long-term, spatially replicated relationships in Colorado, US, mule deer (Odocoileus hemionus) herds that show hunting pressure can modulate apparent epidemic dynamics as reflected by prevalence trends. Across 12 areas in Colorado studied between 2002-18, those with the largest declines in annual hunting license numbers (pressure) showed the largest increases in the proportion of infected adult (≥2-yr-old) male deer killed by hunters (prevalence); prevalence trends were comparatively flat in most areas where license numbers had been maintained or increased. The mean number of licenses issued in the 2 yr prior best explained observed patterns: increasing licenses lowered subsequent risk of harvesting an infected deer, and decreasing licenses increased that risk. Our findings suggest that harvesting mule deer with sufficient hunting pressure might control chronic wasting disease-especially when prevalence is low-but that harvest prescriptions promoting an abundance of mature male deer contribute to the exponential growth of epidemics.

Harvest strategies for the elimination of low prevalence wildlife diseases

The intensive harvesting of hosts is often the only practicable strategy for controlling emerging wildlife diseases. Several harvesting approaches have been explored theoretically with the objective of lowering transmission rates, decreasing the transmission period or specifically targeting spatial disease clusters or high-risk demographic groups. Here, we present a novel model-based approach to evaluate alternative harvest regimes, in terms of demographic composition and rates, intended to increase the probability to remove all infected individuals in the population during the early phase of an outbreak. We tested the utility of the method for the elimination of chronic wasting disease based on empirical data for reindeer (Rangifer tarandus) in Norway, in populations with (Nordfjella) and without (Hardangervidda) knowledge about exact disease prevalence and population abundance. Low and medium harvest intensities were unsuccessful in eliminating the disease, even at low prevalence. High-intensity harvesting had a high likelihood of eliminating the disease, but probability was strongly influenced by the disease prevalence. We suggest that the uncertainty about disease prevalence can be mitigated by using an adaptive management approach: forecast from models after each harvest season with updated data, derive prevalence estimates and forecast further harvesting. We identified the problems arising from disease surveillance with large fluctuations in harvesting pressure and hence sample sizes. The elimination method may be suitable for pathogens that cause long-lasting infections and with slow epidemic growth, but the method should only be attempted if there is a low risk of reinfection, either by a new disease introduction event (e.g. dispersing hosts) or due to environmental reservoirs. Our simulations highlighted the short time window when such a strategy is likely to be successful before approaching near complete eradication of the population.