Grading of the histological lesions from lung sections. A. Absence of histological lesion in the lung parenchyma of an EIAVNEG horse (#91-rom). B. Mild lesions of broncho-interstitial pneumonia in an EIAVPOS horse (#1-rom) revealed peribronchiolar and interstitial lymphocytic accumulation with mild thickening of the interlobular septa. C. Moderate lesions in EIAVPOS horse (#78-fr) with accumulation of lymphocytes within the septa and moderate thickening of the alveolar walls. D. Severe lesions of interstitial pneumonia in EIAVPOS horse (# 2-rom) with peribronchiolar lymphocyte infiltration, partial destruction of bronchiolar epithelium, peribronchiolar accumulation of smooth muscle cells and thickening and partial destruction of the alveolar septal leukocyte exudate. (►) Interstitial lymphocyte accumulation; (✶) thickening of the interlobular septa; (➔) destruction of alveolar walls; () peribronchiolar accumulation of smooth muscle cells; a: alveoli; b: bronchiole.