Complex Lymphatic Anomaly Presenting with Chylothorax, Chylous Ascites, and Generalized Subcutaneous Edema in a Young Cat: Comparative Insights Based on the Human ISSVA Classification.

In human medicine, the 2025 International Society for the Study of Vascular Anomalies (ISSVA) classification distinguishes localized lymphatic malformations from systemic disorders referred to as complex lymphatic anomalies (CLAs), including generalized lymphatic anomaly (GLA) and central conducting lymphatic anomaly (CCLA). GLA is characterized by multifocal proliferation of dilated lymphatic channels infiltrating soft tissues and visceral organs, whereas CCLA involves structural or functional abnormalities of the central lymphatic pathways such as the thoracic duct commonly resulting in chylous effusion. Reports of systemic lymphatic disease in veterinary species are exceedingly rare. A five-month-old intact male domestic shorthair cat presented with progressive dyspnea and was diagnosed with chylothorax and chylous ascites. Computed tomography (CT) revealed contrast leakage from both the thoracic duct and aortic hiatus, along with diffuse subcutaneous edema, indicating a systemic lymphatic disorder. Histopathology demonstrated proliferative lymphatic channels within the subcutaneous tissues, consistent with GLA, while imaging findings suggested concurrent CCLA, supporting classification as an overlapping CLA phenotype. Despite medical and surgical interventions, the cat developed progressive edema and recurrent pleural effusion and ultimately died. This case demonstrates that cats can develop systemic lymphatic disorders analogous to human CLAs and highlights the diagnostic value of CT lymphangiography. Furthermore, it underscores the utility of applying the ISSVA classification to enhance comparative understanding of lymphatic diseases in veterinary medicine.