Landscape connectivity predicts chronic wasting disease risk in Canada

Biogeography, climate, and land use create a mosaic of parasite risk in native bumble bees

Host-parasite interactions are crucial to the regulation of host population growth, as they often impact both long-term population stability and ecological functioning. Animal hosts navigate a number of environmental conditions, including local climate, anthropogenic land use, and varying degrees of spatial isolation, all of which can mediate parasitism exposure. Despite this, we know little about the potential for these environmental conditions to impact pathogen prevalence at biogeographic scales, especially for key ecosystem service-providing animals. Bees are essential pollination providers that may be particularly sensitive to biogeography, climate, and land-use as these factors are known to limit bee dispersal and contribute to underlying population genetic variation, which may also impact host-parasite interactions. Importantly, many native bumble bee species have recently shown geographic range contractions, reduced genetic diversity, and increased parasitism rates, highlighting the potential importance of interacting and synergistic stressors. In this study, we incorporate spatially explicit environmental, biogeographic, and land-use data in combination with genetically derived host population data to conduct a large-scale epidemiological assessment of the drivers of pathogen prevalence across >1000 km for a keystone western US pollinator, the bumble bee Bombus vosnesenskii. We found high rates of infection from Crithidia bombi and C. expoekii, which show strong spatial autocorrelation and which were more prevalent in northern latitudes. We also show that land use barriers best explained differences in parasite prevalence and parasite community composition, while precipitation, elevation, and B. vosnesenskii nesting density were important drivers of parasite prevalence. Overall, our results demonstrate that human land use can impact critical host-parasite interactions for native bees at massive spatial scales. Further, our work indicates that disease-related survey and conservation measures should take into account the independent and interacting influences of climate, biogeography, land use, and local population dynamics.

Effect of legal regulation of supplemental feeding on space use of red deer in an area with chronic wasting disease

Supplemental feeding of cervids during winter is a widespread management practice, but feeding may increase the risk of disease transmission. Therefore, legal regulations to limit supplemental feeding are often implemented when dealing with severe infectious diseases, such as chronic wasting disease (CWD) in cervids. However, it is currently unclear whether these regulations result in decreased spatial clustering and aggregation as intended. Supplemental feeding is expected to restrict the movement of cervids. Therefore, a ban on feeding may also result in wider space use and a risk of geographic spread of disease. The space use of 63 GPS-marked red deer (Cervus elaphus) was investigated before (n = 34) and after (n = 29) the implementation of a legal regulation aimed at limiting the supplemental feeding of cervids during winter in a CWD-affected region of Nordfjella, Norway. Snow depth was the main determinant of the space use for red deer. A moderate reduction in the number of GPS positions in spatial clusters was evident during periods of deep snow once the ban was in place. Sizes of core areas (Kernel 50%), home ranges (Kernel 95%), and dispersion (MCP 100%, number of 1 km2 pixels visited per deer) declined from January to March and with increasing snow depth. Dispersion (number of 1 km2 pixels visited per deer) did not depend on snow depth after the ban, and red deer used larger areas when snow depth was high after the ban compared to before. The ban on supplementary feeding had no effect on size of core areas or home ranges. Several potential factors can explain the overall weak effect of the ban on space use, including the use of agricultural fields by red deer, other anthropogenic feeding, and landscape topography. This study highlights that snow depth is the main factor determining space use during winter, and it remains to be determined whether the moderate reduction in spatial clustering during deep snow after the ban was sufficient to lower the risk of disease transmission.

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.

Spatial and temporal dynamics of space use by free-ranging domestic dogs Canis familiaris in rural Africa

Variation in the spatial ecology of animals influences the transmission of infections and so understanding host behavior can improve the control of diseases. Despite the global distribution of free-ranging domestic dogs Canis familiaris and their role as reservoirs for zoonotic diseases, little is known about the dynamics of their space use. We deployed GPS loggers on owned but free-ranging dogs from six villages in rural Chad, and tracked the movements of 174 individuals in the dry season and 151 in the wet season. We calculated 95% and core home ranges using auto-correlated kernel density estimates (AKDE₉₅ and AKDE_core ), determined the degree to which their movements were predictable, and identified correlates of movement patterns. The median AKDE₉₅ range in the dry season was 0.54 km² and in the wet season was 0.31 km² , while the median AKDE_core range in the dry season was 0.08 km² and in the wet season was 0.04 km² . Seasonal variation was, in part, related to owner activities; dogs from hunting households had ranges that were five times larger in the dry season. At least 70% of individuals were more predictably "at home" (<50 m from the household) throughout the day in the dry season, 80% of dogs demonstrated periodicity in activity levels (speed), and just over half the dogs exhibited periodicity in location (repeated space use). In the wet season, dogs mostly exhibited 24-h cycles in activity and location, with peaks at midday. In the dry season, dogs exhibited both 12- and 24-h cycles, with either a single peak at midday, or one peak between 06:00 and 12:00 and a second between 18:00 and 22:00. Strategies to control canine-mediated zoonoses can be improved by tailoring operations to the local spatial ecology of free-ranging dogs. Interventions using a door-to-door strategy in rural Chad would best conduct operations during the dry season, when access to dogs around their household more reliably exceeds 70% throughout the day. Given the importance of use in hunting for explaining variation in dog space-use, targeting approaches to disease control at the household level on the basis of owner activities offers potential to improve access to dogs.

CHRONIC WASTING DISEASE MODELING: AN OVERVIEW

Chronic wasting disease (CWD) is an infectious and fatal prion disease occurring in the family Cervidae. To update the research community regarding the status quo of CWD epidemic models, we conducted a meta-analysis on CWD research. We collected data from peer-reviewed articles published since 1980, when CWD was first diagnosed, until December 2018. We explored the analytical methods used historically to understand CWD. We used 14 standardized variables to assess overall analytical approaches of CWD research communities, data used, and the modeling methods used. We found that CWD modeling initiated in the early 2000s and has increased since then. Connectivity of the research community was heavily reliant on a cluster of CWD researchers. Studies focused primarily on regression and compartment-based models, population-level approaches, and host species of game management concern. Similarly, CWD research focused on single populations, species, and locations, neglecting modeling using community ecology and biogeographic approaches. Chronic wasting disease detection relied on classic diagnostic methods with limited sensitivity for most stages of infection. Overall, we found that past modeling efforts generated a solid baseline for understanding CWD in wildlife and increased our knowledge on infectious prion ecology. Future analytical efforts should consider more sensitive diagnostic methods to quantify uncertainty and broader scale studies to elucidate CWD transmission beyond population-level approaches. Considering that infectious prions may not follow biological rules of well-known wildlife pathogens (i.e., viruses, bacteria, fungi), assumptions used when modeling other infectious disease may not apply for CWD. Chronic wasting disease is a new challenge in wildlife epidemiology.

Assessment of spatial genetic structure to identify populations at risk for infection of an emerging epizootic disease

Understanding the geographic extent and connectivity of wildlife populations can provide important insights into the management of disease outbreaks but defining patterns of population structure is difficult for widely distributed species. Landscape genetic analyses are powerful methods for identifying cryptic structure and movement patterns that may be associated with spatial epizootic patterns in such cases.We characterized patterns of population substructure and connectivity using microsatellite genotypes from 2,222 white-tailed deer (Odocoileus virginianus) in the Mid-Atlantic region of the United States, a region where chronic wasting disease was first detected in 2009. The goal of this study was to evaluate the juxtaposition between population structure, landscape features that influence gene flow, and current disease management units.Clustering analyses identified four to five subpopulations in this region, the edges of which corresponded to ecophysiographic provinces. Subpopulations were further partitioned into 11 clusters with subtle (F ST ≤ 0.041), but significant genetic differentiation. Genetic differentiation was lower and migration rates were higher among neighboring genetic clusters, indicating an underlying genetic cline. Genetic discontinuities were associated with topographic barriers, however.Resistance surface modeling indicated that gene flow was diffuse in homogenous landscapes, but the direction and extent of gene flow were influenced by forest cover, traffic volume, and elevational relief in subregions heterogeneous for these landscape features. Chronic wasting disease primarily occurred among genetic clusters within a single subpopulation and along corridors of high landscape connectivity.These results may suggest a possible correlation between population substructure, landscape connectivity, and the occurrence of diseases for widespread species. Considering these factors may be useful in delineating effective management units, although only the largest features produced appreciable differences in subpopulation structure. Disease mitigation strategies implemented at the scale of ecophysiographic provinces are likely to be more effective than those implemented at finer scales.

The ecology of chronic wasting disease in wildlife

Prions are misfolded infectious proteins responsible for a group of fatal neurodegenerative diseases termed transmissible spongiform encephalopathy or prion diseases. Chronic Wasting Disease (CWD) is the prion disease with the highest spillover potential, affecting at least seven Cervidae (deer) species. The zoonotic potential of CWD is inconclusive and cannot be ruled out. A risk of infection for other domestic and wildlife species is also plausible. Here, we review the current status of the knowledge with respect to CWD ecology in wildlife. Our current understanding of the geographic distribution of CWD lacks spatial and temporal detail, does not consider the biogeography of infectious diseases, and is largely biased by sampling based on hunters' cooperation and funding available for each region. Limitations of the methods used for data collection suggest that the extent and prevalence of CWD in wildlife is underestimated. If the zoonotic potential of CWD is confirmed in the short term, as suggested by recent results obtained in experimental animal models, there will be limited accurate epidemiological data to inform public health. Research gaps in CWD prion ecology include the need to identify specific biological characteristics of potential CWD reservoir species that better explain susceptibility to spillover, landscape and climate configurations that are suitable for CWD transmission, and the magnitude of sampling bias in our current understanding of CWD distribution and risk. Addressing these research gaps will help anticipate novel areas and species where CWD spillover is expected, which will inform control strategies. From an ecological perspective, control strategies could include assessing restoration of natural predators of CWD reservoirs, ultrasensitive CWD detection in biotic and abiotic reservoirs, and deer density and landscape modification to reduce CWD spread and prevalence.

Combining public participatory surveillance and occupancy modelling to predict the distributional response of Ixodes scapularis to climate change

Ixodes scapularis, a known vector of Borrelia burgdorferi sensu stricto (Bbss), is undergoing range expansion in many parts of Canada. The province of New Brunswick, which borders jurisdictions with established populations of I. scapularis, constitutes a range expansion zone for this species. To better understand the current and potential future distribution of this tick under climate change projections, this study applied occupancy modelling to distributional records of adult ticks that successfully overwintered, obtained through passive surveillance. This study indicates that I. scapularis occurs throughout the southern-most portion of the province, in close proximity to coastlines and major waterways. Milder winter conditions, as indicated by the number of degree days <0 °C, was determined to be a strong predictor of tick occurrence, as was, to a lesser degree, rising levels of annual precipitation, leading to a final model with a predictive accuracy of 0.845 (range: 0.828-0.893). Both RCP 4.5 and RCP 8.5 climate projections predict that a significant proportion of the province (roughly a quarter to a third) will be highly suitable for I. scapularis by the 2080s. Comparison with cases of canine infection show good spatial agreement with baseline model predictions, but the presence of canine Borrelia infections beyond the climate envelope, defined by the highest probabilities of tick occurrence, suggest the presence of Bbss-carrying ticks distributed by long-range dispersal events. This research demonstrates that predictive statistical modelling of multi-year surveillance information is an efficient way to identify areas where I. scapularis is most likely to occur, and can be used to guide subsequent active sampling efforts in order to better understand fine scale species distributional patterns.

Clay content and pH: soil characteristic associations with the persistent presence of chronic wasting disease in northern Illinois

Environmental reservoirs are important to infectious disease transmission and persistence, but empirical analyses are relatively few. The natural environment is a reservoir for prions that cause chronic wasting disease (CWD) and influences the risk of transmission to susceptible cervids. Soil is one environmental component demonstrated to affect prion infectivity and persistence. Here we provide the first landscape predictive model for CWD based solely on soil characteristics. We built a boosted regression tree model to predict the probability of the persistent presence of CWD in a region of northern Illinois using CWD surveillance in deer and soils data. We evaluated the outcome for possible pathways by which soil characteristics may increase the probability of CWD transmission via environmental contamination. Soil clay content and pH were the most important predictive soil characteristics of the persistent presence of CWD. The results suggest that exposure to prions in the environment is greater where percent clay is less than 18% and soil pH is greater than 6.6. These characteristics could alter availability of prions immobilized in soil and contribute to the environmental risk factors involved in the epidemiological complexity of CWD infection in natural populations of white-tailed deer.

Chronic wasting disease (CWD) in cervids

In April and May of 2016, Norway confirmed two cases of chronic wasting disease (CWD) in a wild reindeer and a wild moose, respectively. In the light of this emerging issue, the European Commission requested EFSA to recommend surveillance activities and, if necessary, additional animal health risk-based measures to prevent the introduction of the disease and the spread into/within the EU, specifically Estonia, Finland, Iceland, Latvia, Lithuania, Norway, Poland and Sweden, and considering seven wild, semidomesticated and farmed cervid species (Eurasian tundra reindeer, Finnish (Eurasian) forest reindeer, moose, roe deer, white-tailed deer, red deer and fallow deer). It was also asked to assess any new evidence on possible public health risks related to CWD. A 3-year surveillance system is proposed, differing for farmed and wild or semidomesticated cervids, with a two-stage sampling programme at the farm/geographically based population unit level (random sampling) and individual level (convenience sampling targeting high-risk animals). The current derogations of Commission Implementing Decision (EU) 2016/1918 present a risk of introduction of CWD into the EU. Measures to prevent the spread of CWD within the EU are dependent upon the assumption that the disease is already present; this is currently unknown. The measures listed are intended to contain (limit the geographic extent of a focus) and/or to control (actively stabilise/reduce infection rates in an affected herd or population) the disease where it occurs. With regard to the zoonotic potential, the human species barrier for CWD prions does not appear to be absolute. These prions are present in the skeletal muscle and other edible tissues, so humans may consume infected material in enzootic areas. Epidemiological investigations carried out to date make no association between the occurrence of sporadic Creutzfeldt-Jakob disease in humans and exposure to CWD prions.