Modeling Risks and Mitigation Options for the Chronic Wasting Disease (CWD) in Scandinavia

Chronic wasting disease (CWD) is a contagious neural prion-disease affecting deer populations in North America with severe ecological and societal consequences. CWD is fatal and infectious prions spread and remain in the environment for many years even without animals present. The recent appearance of CWD in reindeers in Norway called for a drastic culling operation to prevent further spreading of the disease. This appears to have stopped the spreading of CWD among reindeers, but due to the persistence of CWD prions in the environment a reappearance of new cases among reindeer or other species in the future cannot be excluded. To evaluate the risks and the effectiveness of alternative management (monitoring and culling) options, we developed a model of CWD dynamics and management. The model includes stochastic population and spatial dynamics of the four relevant deer species in northern Sweden and Norway: reindeer (Rangifer tarandus), roe deer (Capreolus capreolus), red deer (Cervus elaphus) and moose (Alces alces). Transmission of CWD is modelled via direct contacts and via the environment. The model was parameterized and calibrated based on CWD studies from USA, data from the Norwegian CWD cases, and local deer population and vegetation data. The results indicate that without management, a CWD epidemic can be initiated by a single infected reindeer and would spread to other deer species. It would lead to dramatic population declines of reindeer and red deer and would also reduce the populations of roe deer and moose. The disease prevalence would stabilise at a about 50% after 50 years, as observed in some areas in the USA. A management strategy to cull only visibly sick animals, even with very efficient detection, cannot prevent a catastrophic development but merely slow the outbreak. To prevent an outbreak and the establishment of CWD it is necessary to cull all individuals, not only visibly sick ones, of an affected species in a relatively large area (30x30 km in our model) once a case is detected. Further, to prevent a slow buildup of CWD in the environment and eliminate the risk of outbreaks in the future it is necessary to expand this area of culling even further. Although the model has not yet been thoroughly validated due to scarcity of data, the results suggest that the drastic culling done in Norway was appropriate and necessary to prevent establishment of CWD and that further monitoring and potential culling is required to prevent outbreaks in the future.