Chronic wasting disease detection and mortality sources in semi‐protected deer population

Krysten L Schuler,Robert W Klaver,R Terry Bowyer,Christopher S Jennelle,Jonathan A Jenks

doi:10.2981/wlb.00437

Abstract

Surveillance for wildlife diseases is essential for assessing population dynamics of ungulates, especially in free‐ranging populations where infected animals are difficult to sample. Chronic wasting disease (CWD) is an emerging infectious disease of concern because of the potential for substantial negative effects on populations of cervids. Variability in the likelihood that CWD is detected could invalidate traditional estimators for prevalence. In some instances, deer located after death cannot be tested for infectious diseases, including CWD, because of lack of availability or condition of appropriate tissues. We used various methods to detect infectious diseases that could cause mortality for deer Odocoileus spp. residing in Wind Cave National Park, South Dakota, USA, and we report survival estimates for animals in this population. We included 34 monthly encounters of deer resightings and 67 mortalities. We tested live deer by tonsillar biopsy for CWD and estimated pooled prevalence (mean ± SE) at 5.6 ± 3.0% over the three‐year study. Live deer potentially had exposure to several infectious diseases, including bluetongue, epizootic hemorrhagic disease, bovine viral diarrhea, West Nile virus, and malignant catarrhal fever, but no apparent morbidity or mortality from those diseases. We tested survival and influence of covariates, including age and sex, using known‐fate analysis in Program MARK. Those data best supported a model with time‐invariant encounter probability and an annual survival of 72.8%. Even without direct pressure from hunting within the park, average life expectancy in this population was 3.2 years. Only 68% of mortalities contained sufficient material for CWD sampling (because of predation and scavenger activity) and >42% of these were CWD‐positive. These findings underscore the possible biases in postmortem surveillance estimates of disease prevalence because of potential for subclinical infected animals to be removed by predators and not tested.

Highlights

In 2003, we tested 33 live deer (32 mule deer, one whitetailed deer) and had one chronic wasting disease (CWD)-positive 5.5-year-old female mule deer for a sample prevalence of 3.0%
In 2004, of 26 live deer tested by tonsillar biopsy (25 mule deer, one white-tailed deer), there was only one CWDpositive 4.5-year-old female mule deer detected for a sample
Based on rapidly escalating prevalence rates in the antemortem testing and high observed prevalence in postmortem testing, CWD likely had been in this area for several years before its discovery in 2002

Summary

Introduction

Tested animals contribute to determining prevalence levels, which are used to assess influences of harvest (Conner et al 2000, Grear et al 2006), transmission (Miller et al 2006), and population trends (Miller and Conner 2005); such data could contain inherent biases if failures in disease detection are not evaluated (Diefenbach et al 2004, Heisey et al 2014). We used both antemortem and postmortem testing for CWD and other infectious diseases: blue tongue, epizootic hemorrhagic disease, bovine viral diarrhea, West Nile virus, and malignant catarrhal fever in mule deer O. hemionus and white-tailed deer O. virginianus for disease screening. We predicted that deer populations still would have high survival rates despite mortality from CWD, other infectious diseases, predators, and humans

Methods

Results

Conclusion