The effectiveness of harvest for limiting wildlife disease: Insights from 20 years of chronic wasting disease in Wyoming.

Effective, practical options for managing disease in wildlife populations are limited, especially after diseases become established. Removal strategies (e.g., hunting or culling) are used to control wildlife diseases across a wide range of systems, despite conflicting evidence of their effectiveness. This is especially true for chronic wasting disease (CWD), an untreatable, fatal prion disease threatening cervid populations across multiple countries, for which recreational harvest has been suggested as an important disease control strategy. Using observational data to evaluate whether harvest effectively limits CWD prevalence has been difficult because statistical relationships between harvest and disease prevalence can arise from a causal effect of harvest (i.e., harvest's impacts on prevalence via changes in transmission or demographic structure) or from a number of alternative mechanisms. For instance, correlations between harvest and disease prevalence can also be driven by disease's impacts on population size and harvest (i.e., reverse causality) or from confounding variables (e.g., habitat or geographic location) that impact both harvest rates and disease prevalence. We analyzed two decades of surveillance data (2000-2021) from 10 mule deer herds in Wyoming, using statistical approaches informed by causal inference theory, to test for the effects of harvest on CWD prevalence. Herds with consistently high harvest pressure across 20 years had significantly lower prevalence. Our models predicted that harvesting 40% of adult males per year across 20 years would maintain prevalence below 5% on average, whereas if only 20% of males were harvested in each year, prevalence would increase to >30% by year 20. Moreover, shifting the relative harvest pressure within a herd over a shorter period (3 years) reduced subsequent prevalence, albeit to a smaller degree. Although high harvest is unlikely to completely eradicate CWD, our analysis suggests that maintaining hunting pressure on adult males is an important tactic for slowing CWD epidemics within mule deer herds. Our study also provides guidance for future analyses of longitudinal surveillance data, including the importance of demographic data and appropriate time lags.