Histologic patterns of chronic interstitial lung diseases in dogs, cats, and humans: a review

Interstitial lung diseases (ILDs) are diverse conditions affecting the lung parenchyma, with variable patterns and amounts of inflammation and fibrosis within the interstitium, resulting in impaired gas exchange. Classification of this disparate group of diseases in humans is achieved by collaboration among specialists, including clinicians, radiologists, and pathologists. In contrast, such collaborative frameworks are implemented less often in veterinary medicine. This discrepancy has been addressed in a published classification scheme derived from human medicine to facilitate future characterization, with 3 primary categories: 1) idiopathic, 2) ILDs with known causes, and 3) miscellaneous and other conditions. Differentiating between clinically defined entities and histologic patterns is essential because they often share similar terminology. Interpretation of lung samples from dogs and cats with ILDs requires pathologists to recognize and describe histologic patterns, which can be correlated retrospectively with clinical and imaging data to broaden our knowledge base. Here, we review chronic ILDs in dogs and cats, with comparisons to human counterparts, highlighting their definitions, current classification systems, histologic patterns, and established or proposed etiologies. We discuss the potential role of molecular and genetic approaches in advancing our understanding and diagnosis of ILDs. Chronic disorders encompassing the idiopathic and known entities are addressed, excluding miscellaneous ILDs or alveolar-filling disorders. Our goal is to better define ILD entities in domestic small-animal patient populations, which may aid antemortem identification and elucidation of treatment options for improved patient care.