Chronic wasting Dsease detection and mortality sources in semi-protected deer population

Abstract

Surveillance for wildlife Dseases is essential for assessing population dynamics of ungulates, especially in free-ranging populations where infected animals are Dfficult to sample. Chronic wasting Dsease (CWD) is an emerging infectious Dsease of concern because of the potential for substantial negative effects on populations of cervids. Variability in the likelihood that CWD is detected could invalidate traDtional estimators for prevalence. In some instances, deer located after death cannot be tested for infectious Dseases, incluDng CWD, because of lack of availability or conDtion of appropriate tissues. We used various methods to detect infectious Dseases that could cause mortality for deer Odocoileus spp. resiDng 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 Dseases, incluDng bluetongue, epizootic hemorrhagic Dsease, bovine viral Darrhea, West Nile virus, and malignant catarrhal fever, but no apparent morbiDty or mortality from those Dseases. We tested survival and influence of covariates, incluDng 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 Drect 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 finDngs underscore the possible biases in postmortem surveillance estimates of Dsease prevalence because of potential for subclinical infected animals to be removed by predators and not tested.