Evaluation of antemortem sampling to estimate chronic wasting disease prevalence in free-ranging mule deer

We conducted a field study to evaluate tonsillar biopsy immunohistochemistry (IHC) as a tool for diagnosing chronic wasting disease (CWD) in live, free-ranging mule deer (Odocoileus hemionus) and estimating CWD prevalence. Initially, we evaluated and refined techniques for collecting tonsillar biopsies from mule deer. Using a simple mouth gag and a 6-mm biopsy forceps, and taking the biopsy starting at the rostral rim of the tonsillar sinus, we obtained 155/161 (96%) samples that yielded ≥1 lymphoid follicle. To compare antemortem and postmortem survey techniques and assure biopsy-based estimates would not substantially underestimate "true" prevalence, we examined tonsillar biopsies from 161 free-ranging mule deer from 2 populations where CWD is endemic. We then calculated prevalence (p_b) and compared this to prevalence (p_h) estimated from tonsil samples from 161 deer harvested or culled in spatial and temporal proximity to our study areas; we considered the latter a close approximation of "true" prevalence. Biopsy-based prevalence estimates exceeded prevalence estimated by tonsillar IHC of samples from harvested or culled deer. Although 95% CIs for p_h - p_b included 0 for area-specific estimates, biopsy-based estimates were ≥3 times higher than harvest-based estimates in both study areas. Moreover, when data from both study areas were combined, p_h (= 0.025) was lower than p_b (= 0.081) and the 95% Cl for p_h - p_b (-0.104 to -0.007) did not include 0. Observed differences in prevalence most likely reflected spatial or temporal variation in populations (or subpopulations) of deer sampled. Tonsillar biopsy IHC appears to be reliable for detecting CWD infections in live mule deer and estimating prevalence in affected populations, thereby representing a new tool with potential utility in CWD management, particularly in areas where harvest-based sampling is infeasible.