The left lung was filled with 4% formalin for fixation and embedded in paraffin for histopathological examination. Sections of 3 mm thickness were stained with hematoxilin and eosin (HE). Histopathology was evaluated in a blinded manner. The following scoring system based on four criteria was used for each histological section: neutrophils in the alveolar or interstitial space, alveolar septal thickening, alveolar congestion and formation of hyaline membranes. Each criterion scored one point, if present, thus scores ranged from 0 (none criterion found) to 4 (all four criteria observed). Nine to ten sections per lung were evaluated and the mean was calculated for every lung and every group.