2012 Annual Report 1a.Objectives (from AD-416): Characterize and compare the pathology of infections with different Newcastle disease viruses (NDV) and recombinants to define NDV virulence mechanisms. 1b.Approach (from AD-416): Pathogenesis studies will be conducted in specific pathogen free (SPF) leghorn chickens to characterize the clinical form of any resultant disease and tissue tropism of the infection. Domestic and exotic field isolates as well as newly generated recombinant vaccine viruses will be tested. In brief the protocol is to inoculate 4-wk-old chickens by eye drop, a natural route of infection in contrast to pathogenicity testing done by systemic inoculation. Clinical observations will be recorded over a 14 day period and 2 birds will be euthanized for sampling at 2, 5, 10, and 14 days post inoculation. Birds dying from infection will be included in sampling. Tissues will be collected into 10% formalin for processing, sectioning and staining for histopathology, in situ hybridization (ISH) with a Newcastle disease virus (NDV) matrix gene riboprobe to detect NDV mRNA, and immunohistochemistry (IHC) with an NDV anti-peptide serum to detect expressed NDV nucleoprotein (N). Other riboprobes may also be used. Seroconversion detected in blood samples collected on days 10 and 14 and/or virus isolated from oral and cloacal swabs collected at each sampling will confirm infection in sampled birds that display no clinical signs of disease. Selected tissues may also be processed separately for virus isolation or for RNA isolation. Infections of other birds species may also be evaluated similarly if there appears to be a host predilection for some isolates. New plasmids for creation of recombinant viruses will be developed. 3.Progress Report: This project is related to objective 2.A. of this in-house project: Define host response pathways modulated by Newcastle disease viral infections. This objective has been partially achieved. Pathogenic effects on chickens after infection with viruses from West and South Africa, Central and South America have been completely characterized.