Genetic and Biological Characterization of Atypical Porcine Pestivirus (APPV) Associated with Congenital Tremor in Newborn Piglets Pestiviruses are enveloped RNA viruses belonging to the family Flaviviridae which comprises significant animal and human pathogens. A novel genetically distinct pestivirus designated Atypical porcine pestivirus (APPV) was recently discovered and subsequently demonstrated to be a major cause of congenital tremor (CT) in newborn piglets. Within the present project investigations of apparently healthy pigs from different parts of Europe and Asia (n=1 460) revealed remarkably high APPV genome (8.9%) and antibody (60%) detection rates which together with reports from North and South America indicate the global distribution and high abundance of APPV. Characterization of APPV variants from different origins demonstrated the high genetic variability of APPV. High genome (19%) and antibody (52%) detection rates of APPV in wild boar were demonstrated which indicates that wild boar may serve as a wild animal reservoir for APPV. Furthermore humoral immune response induced after a field infection with APPV was characterized. Findings indicate that E2 glycoprotein represents the immune dominant antigen for induction of neutralizing antibodies against APPV infections. Maternally derived antibodies (MDA) could be detected at a low titer and rapidly declined. Presence of MDA in new born piglets apparently did not confer protection against APPV infections. A subclinical horizontal infection was observed when healthy and infected piglets were mingled on the flat deck. Results provide strong evidence that protective immunity against APPV infection can be achieved after horizontal infection. In contrast to the classical pestiviruses typical persistent infection was not observed for APPV infection. Findings of this project help to understand the epidemiology and pathogenesis of this novel pathogen and are useful for the implementation of tailored prevention strategies.