Wildlife Disease and Toxicology II

Contributed Paper
ROOM: Room 130 – Cimarron

10:30AM Pathogen-Mediated Selection in Free-Ranging Elk Populations Infected by Chronic Wasting Disease
Ryan J. Monello; Nathan L. Galloway; Jenny G. Powers; Sally Madsen-Bouterse; Mary E. Wood; William H. Edwards; Katherine I. O’Rourke; Margaret A. Wild
Pathogens can exert a large influence on the evolution of hosts via selection for alleles or genotypes that moderate pathogen virulence. Genetic linkages, discordant interactions between parasites and the host genome, and environmental stochasticity have largely prevented observation of this process in wildlife species. We examined the prion protein gene (PRNP) in elk (Cervus elaphus nelsoni) populations that have been infected with chronic wasting disease (CWD), a contagious, fatal prion disease, and compared allele frequency to populations with no history of exposure to CWD. The PRNP in elk is highly conserved and a single polymorphism at codon 132 can markedly extend CWD latency when the minor leucine (L) allele is present. We determined population exposure to CWD, genotyped 1018 elk from five populations, and developed a hierarchical Bayesian model to examine the relationship between CWD prevalence and PRNP 132L allele frequency. The predicted probability that the correlation between disease prevalence and minor allele frequency was > 0 was 0.99. Populations infected with CWD for at least 30-50 years exhibited L allele frequencies that were on average twice as great (range = 0.23 to 0.29) than those from uninfected populations (range = 0.04 to 0.17). Despite numerous differences between the elk populations in this study, the consistency of increase in L allele frequency suggests pathogen-mediated selection has occurred due to CWD. Although prior modeling work predicted that selection will continue, allele frequencies in uninfected populations and the potential for new prion protein strains to arise suggest that it is prudent to assume balancing selection may prevent fixation of the L allele in populations with CWD.
10:50AM Drought as a Risk Factor for Hemorrhagic Disease in the Eastern United States
Sonja Christensen; Mark Ruder; David Williams; William Porter; David Stallknecht
Hemorrhagic disease (HD), caused by bluetongue viruses (BTV) and epizootic hemorrhagic disease viruses (EHDV), is the most important viral disease of white-tailed deer (Odocoileus virginianus) in the United States. The forces driving apparent increases in HD are poorly understood, particularly where the disease has recently been most severe in northern latitudes. Drought is suspected of being one of the risk factors for HD. We seek to evaluate the role of drought severity in both space and time on changes in HD reports across the Eastern US for the last 15 years. The objectives of this study were to: 1) develop a spatiotemporal model to evaluate if drought severity explains changing patterns of HD presence; and 2) to determine if this potential risk factor varies in importance over the present range of HD in the eastern United States. Historic data (2000-2014) from an annual HD presence-absence survey conducted by the Southeastern Cooperative Wildlife Disease Study and from the United States Drought Monitor were used for this analysis. For each of 23 states, data at the county level were based on reported drought status in the month of August. We used a generalized linear model to explain HD presence and evaluated spatial autocorrelation across the region of study. A relative risk estimate was calculated for each drought category by state and for three multistate regions. The probability of reported HD increased with each drought category in most states and in all regions. This increase was most pronounced in northern states; the effect was much reduced or non-existent in most southern states. These relationships suggest that drought severity does increase the probability that HD will be detected and reported at a county level, but in endemic areas in the southern US, the importance of drought as a predictor of HD risk is low.
11:10AM The Poor Get Richer? Spatial and Temporal Analysis of Genotypes Associated with Delayed Progression of Chronic Wasting Disease
Darren M. Wood; Amy B. Welsh; James M. Crum; Christopher W. Ryan
Chronic wasting disease (CWD) is a naturally occurring transmissible spongiform encephalopathy (TSE) found in free ranging cervids including white-tailed deer (Odocoileus virginianus). While understanding mechanisms for CWD transmission in free ranging populations has become important for initiating measures to control further spread, genetic variability within the highly conserved prion protein genes (Prnp) has shown to alter disease progression (i.e. prolonged incubation periods). Beginning in 2006, in conjunction with West Virginia Division of Natural Resources’ surveillance of CWD in white-tailed deer, retropharyngeal lymph nodes were collected and tested for the presence of the prion associated with CWD using enzyme-linked immunosorbent assay (ELISA). Samples testing positive through ELISA testing were confirmed through immunohistochemistry (IHC). Additionally, tissue samples consisting of ear notches or abdominal skin were collected, individually labeled and stored in 95% ethanol. Genomic DNA was extracted, standardized, and the coding region of the mature prion gene amplified through previously published primers, CWD-LA and CWD-13. Single nucleotide polymorphisms (SNPs) resulting in an amino acid change at codon numbers 95, 96, 116, and 226, previously found to be associated with delayed disease progression were examined. While < 3% of individuals testing positive for chronic wasting disease (n = 143) were found to have heterozygous for the less susceptible genotype at codon 96 (n = 4), 41% of negative individuals (n=43) were heterozygous (n=16) and 5% (n=3) were homozygous for the less susceptible genotype at codon 96. However, unlike previous studies, no polymorphisms have been detected at codons 95, 116, 226. Chi-square analysis shows a significant relationship (p < 0.000) between CWD test result and genotypes associated with a delayed incubation rate at the 96th codon. These results indicate a likely relationship between genetic diversity within the Prnp gene and CWD detection.
11:30AM Genetic Susceptibility of Eastern Cervid Populations to Chronic Wasting Disease
William Miller; W. David Walter
Genetic variation within the prion gene of many cervids has been linked to reductions in perceived susceptibility to chronic wasting disease (CWD). Previous research suggests that certain nonsynonymous polymorphisms can influence genotype-specific infection and prevalence rates. These factors may affect the sustainability of cervid populations considering they influence future demography, genetics, and disease susceptibility. Understanding the frequency and distribution of prion genotypes may provide insights into future epidemiology and infection intensity, particularly in areas of recent infection. The objective of this study was to assess perceived genetic susceptibility for two sympatric cervid species (white-tailed deer, Odocoileus virginianus; and elk, Cervus canadensis) in areas of recent infection in the eastern U.S. Prion sequences were collected from 159 deer and 256 elk and analyzed in program R. Genotype frequencies were compared to previous studies to determine whether infection risk was greater in this region versus western populations. Spatial interpolation was used to compare genotype frequencies among subpopulations. Both species exhibited greater frequencies of the most susceptible genotypes than other regions for loci most associated with CWD status (deer: codon 96; elk: codon 132), with differences of as much as 27.9%. Eastern deer populations did exhibit reduced frequencies of the most susceptible genotype at codon 95 (up to 8.2%), but it was still in high frequency (91.1%). Both species exhibited spatial heterogeneity in genotype frequencies, with deer subpopulations ranging from 43.8% to 100% for all disease-associated loci and elk subpopulations ranging from 69.2% to 100%. These findings indicate that eastern cervid populations may be more susceptible to CWD than western populations, although infection has not reached elk populations in this region. The genetic variability observed among subpopulations indicates that susceptibility is not uniform and that fine-scale patterns of prion gene heterogeneity likely exist. This may influence future epidemiological patterns in the eastern U.S.
11:50AM Advent and Increasing Prevalence of Chronic Wasting Disease in Elk at Wind Cave National Park, South Dakota
Glen A. Sargeant; Margaret A. Wild; Daniel E. Roddy; Gregory M. Schroeder
Until recently, long-term persistence at low prevalence (< 1%) was thought to be characteristic of chronic wasting disease (CWD) in elk (Cervus elaphus). Low prevalence alleviated concern about effects on population and deflected interest in epizootiology to deer (Odocoileus spp.). To promote greater appreciation for potential effects on elk populations, we review the relatively short history of CWD at Wind Cave National Park (WICA). The first documented cases of CWD in South Dakota were discovered in 7 captive elk herds, including one held adjacent to WICA, during 1997-98. Testing of hunter-killed elk and deer during 1998-2001 revealed just 1 additional case, in a white-tailed deer, suggesting CWD was uncommon or not present in wild elk. A case was nevertheless observed at WICA in 2002. During 2005-2017, we used GPS collars to monitor the survival of 339 elk, evaluated 417 female elk for pregnancy, estimated population ratios annually, used vital rates to project population growth, and estimated CWD prevalence from 244 adult elk culled in 2017. Within ~5 years of the first documented case, CWD was a leading cause (3.4% annually) of adult mortality (13.2%). Within ~10 years, adult mortality from CWD reached 9.4 – 12.5% and total mortality reached 18.7%. In context with other vital rates (subadult pregnancy = 9.1%; adult pregnancy = 76.8%; juvenile survival < 0.5), mortality from CWD was unsustainable. Faced with increasing prevalence of CWD and elk numbers that exceeded park objectives for protection of vegetation and other wildlife, the NPS reduced elk numbers at WICA by ~50% during 2017 and documented CWD prevalence of 13.7% (90% HDI = [0.102, 0.174]). Future sustainability of the WICA elk population will depend on a density-dependent decrease in CWD transmission or compensatory increase in recruitment, which are subjects of ongoing study.


Contributed Paper
Location: Albuquerque Convention Center Date: September 26, 2017 Time: 10:30 am - 12:10 pm