The effect of genetic susceptibility and targeting of sampling on the sensitivity of the surveillance system and certainty-of-freedom for classical scrapie in Finland in 2008-2014

We applied scenario tree modeling to study how the genetic distribution of the sheep population in Finland and the focusing on fallen stock would influence the surveillance sensitivity of scrapie. To incorporate the unevenly distributed susceptibility into the estimation we used data from GB where the genetic distribution and scrapie occurrence have been documented in both normally slaughtered and deceased animals. Finland's sheep population is more susceptible to scrapie than the sheep population in GB and surveillance is concentrated on fallen stock. As a result, there is high systemic sensitivity in Finland even with the moderate number of studied animals. The certainty of the freedom-of-disease status is clearly elevated by the low probability of previous disease occurrence and low probability of introduction. The results highlight the need to change the concept from surveillance system sensitivity to freedom-of-disease status and to also consider the risk of introduction and the cumulative nature of the disease prevalence information due repeated surveillance efforts.