Space–time distribution of classical scrapie in Italian sheep: assessing the effectiveness of the National Genetic Selection Plan

Scrapie is a contagious neurodegenerative disease of sheep and goats sustained by prions. The aim of this study was to assess the effectiveness of selective interventions in Italy by describing the temporal trends and geographical distribution (a) of scrapie prevalence and (b) of the application of the National Genetic Selection Plan (NGSP). Poisson models were used to study temporal trends of disease prevalence (in terms of prevalence ratios, PR), whereas NGSP genotyping data from rams tested between 2005 and 2022 were used to study the temporal trend and geographical distribution of the percentage of susceptible animals. In addition, change-point regression analysis was used to identify changes in the trend. The trend in prevalence at the national level shows an increase in the period 2005-2015 (PR = 1.09, 95% CI 1.04-1.15), followed by a sharp decline between 2016 and 2022 (PR = 0.88, 95% CI 0.83-0.93). The proportion of susceptible rams shows a decreasing temporal trend, both along 2005-2022 (PR 0.97, 95% CI 0.96-0.98) and in the two periods (respectively, 2005-2015: PR 0.92, 95% CI 0.89-0.96; 2016-2022: PR 0.94, 95% CI 0.89-0.98). Change-point regression analysis identified a nationwide trend shift between 2015 and 2020, with Sardinia showing an earlier change between 2012 and 2015. The analysis of temporal trends showed geographical heterogeneity associated with different control strategies, especially in Sardinia, where the early targeted interventions in 2009 anticipated most of the provisions of the second NGSP.