Spatial analysis of chronic wasting disease in free-ranging white-tailed deer (Odocoileus virginianus) in Illinois, 2008-2019.

Understanding the geographic distribution and clustering of chronic wasting disease (CWD) among free-ranging white-tailed deer (Odocoileus virginianus) populations can inform disease management practices. We used a retrospective analysis of surveillance data to evaluate CWD's spatial and temporal dynamics within 16 CWD-infected northern Illinois counties. Of 42,541 deer samples collected and tested for CWD from recreational hunter harvest between 2008 and 2019, we recorded 359 (0.84%) CWD-positive samples. We observed variability in CWD cases over time and space. By county, the median CWD-positive proportion was 0.84%, varying from a minimum of 0.14% in McHenry County to a maximum of 6.28% in Boone County. Across years, there were differences among CWD-positive proportions with a median of 0.90%, ranging from a minimum of 0.27% in 2012 to a maximum of 1.60% in 2019. We used a retrospective discrete Poisson scan statistic model to evaluate the space-time clustering of CWD-positive deer. We identified a statistically significant (p < .001) primary cluster C1 (area = 23.59 km ; RR = 10.48), occurring from 2010 to 2015 in the north-central part of the study area, and a secondary cluster C2, occurring from 2014 to 2019 (area = 9.27 km ; RR = 3.88) in the north-west of the study area. Detected CWD-positive space-time clusters suggest that the risk of CWD is not random. Space-time clusters of CWD can be used to evaluate the effectiveness of the Illinois CWD management programme. The area surrounding the older C1 cluster has undergone longer and more intense CWD management compared with C2. Currently, the older C1 cluster is no longer as high risk compared with the newer cluster C2, suggesting that management efforts in C2 should be increased. However, all CWD clusters should be targeted with surveillance, prevention and management programmes, including reducing deer densities to limit further spread of CWD.