2.9. Gross Lung Pathology, Histopathology, and Immunohistochemistry

At necropsy, lung tissue from each pig was examined in a blind fashion by a single pathologist and given a subjective score for the severity of gross lung lesions using an established scoring system [15] that estimated the percentage of lung tissues affected by pneumonia and consolidation. Formalin-fixed tissues of the lung (two sections from the caudal lobe and one from each of middle, accessory, and cranial lung lobes), tonsils, tracheobronchial lymph nodes, thymus, heart, spleen, kidney, and brain were submitted to ISU VDL for histopathological examination. Lung and brain tissues were also tested by PRRSV immunohistochemistry (IHC). For microscopic and IHC lung scores, one pathologist blind to treatments scored each of the five lung sections for all pigs and then averaged those numbers to give a unique value per pig. The microscopic sections were examined and assigned a score for the severity of interstitial pneumonia (0 to 6) as previously described [15,45]. The IHC score was based on a 0 to 3 system, as previously described [46].