merck Radiographic findings in horses with RAO are peribronchial infiltration and overexpanded pulmonary fields (flattening of the diaphragm). Thoracic radiographs are of little benefit in confirming the diagnosis of RAO and may not be necessary in horses with characteristic clinical signs, unless there is no response to standard treatment after 14 days of therapy. However, they may be helpful in identifying the most important differential diagnoses, including interstitial pneumonia, pulmonary fibrosis, or bacterial pneumonia.