Spatiotemporal epizootiology of chronic wasting disease in Wisconsin deer

Chronic wasting disease (CWD) is a fatal neurological disease of cervids caused by a misfolded protein with no vaccines or other cures to prevent infection and death. In the past decade, CWD has been recognized as one of the 10 most important challenges facing wildlife management. This paper evaluates the temporal and spatial patterns of CWD prevalence in southern Wisconsin white-tailed deer during the past 20 years. In most CWD areas, prevalence progresses from introduction to enzootic equilibrium in 15-20 years. In some monitoring areas, the disease grows at a much slower rate and remains below the equilibrium levels. Outbreaks are characterized by four distinct epizootic stages. Disease establishment (Stage 1) depends on the distance from a CWD focus. High rates of incidence and prevalence growth (Stage 2) are followed by slower growth as the relative number of susceptible deer declines (Stage 3). The rate of prevalence increase over time is higher in more forested ecoregions and in males (0.48 vs. 0.43) but similar in females (0.43 vs. 0.37) and yearlings (0.44 vs. 0.39). Habitat features, acting as surrogates for deer behavior and contact, may affect the rate of prevalence growth at a landscape (ecoregion) scale. Additionally, prevalence may be affected by deer management practices that favor higher deer abundance and more mature males. Finally, enzootic equilibrium (Stage 4) is higher in males (0.5), followed by females (0.36) and then yearlings (0.26). These equilibrium prevalence levels are high enough to have significant population impacts, reduce the abundance of mature males, and facilitate CWD spread by infected yearlings. Epizootic patterns suggest that CWD transmission has been frequency-dependent and primarily driven by direct contact with infected deer. Evidence for a meaningful change in the epizootic pattern from a 10-year management program to reduce deer abundance is lacking. The trajectory of CWD dynamics in Wisconsin suggests rapid growth in regional prevalence following introduction and increased spread across the landscape.