Thoracic radiographic findings in cats with feline infectious peritonitis

Objectives The objective of this study was to describe thoracic radiographic findings and associated histopathological changes (where available) in cats with feline infectious peritonitis (FIP). Methods This was a retrospective descriptive study. Cats were included if they had a definitive diagnosis of FIP (based on histopathology and immunohistochemistry) or a presumptive diagnosis of FIP (based on case review by two veterinary internists), and contemporaneous orthogonal thoracic radiographs. Radiographs were reviewed retrospectively by a veterinary radiologist and veterinary radiology resident and assessed for the following: presence of pleural space disease; unstructured interstitial, bronchial, alveolar and/or nodular pulmonary patterns; lymphadenopathy; and cardiovascular abnormalities. Archived histopathologic specimens were reviewed by a veterinary pathologist. Results In total, 35 cats were included: 18 with definitive FIP and 17 with presumptive FIP. Radiographs were abnormal in 32/35 cats and normal in 3/35 cats. Pleural effusion was present in 13/35 cats and was either bilateral (11/13) or unilateral (2/13) in distribution. The lungs were radiographically abnormal in 25/35 cats, with the most common abnormality being an unstructured interstitial pattern (21/25), with bronchial (11/25) and alveolar (10/25) patterns less common. Pulmonary nodules were identified in 3/25 cats. Mixed pulmonary patterns were frequent (18/25). Sternal lymphadenopathy was present in 16/35 cats. An enlarged cardiac silhouette was noted in 6/35 cats, attributable to myocarditis (3/6), pericardial effusion (1/6), a high output state (1/6) or unrelated cardiomyopathy (1/6). Common histopathologic lesions included pulmonary edema (16/17), fibrinosuppurative pleuritis (13/17) and histiocytic vasculitis causing pneumonia (10/17); myocarditis (5/14); and lymphadenitis (2/2). Histologic lung changes were more common in patients with moderate to severe radiographic pulmonary changes. Conclusions and relevance Thoracic radiographic findings in cats with FIP may include variably distributed pleural effusion; interstitial, bronchial, and/or alveolar patterns; pulmonary nodules; lymphadenopathy; and cardiomegaly. FIP should be considered in cats with these radiographic changes and supportive clinical findings.