Congestive heart failure secondary to hypereosinophilic syndrome with cardiac involvement in a cat.

Hypereosinophilic syndrome should be considered when feline echocardiography mimics infective endocarditis alongside marked eosinophilia.

This case report describes a rare presentation of hypereosinophilic syndrome (HES) causing left-sided congestive heart failure (CHF) in a three-year-old castrated male domestic shorthair cat. The cat presented with acute dyspnea, and echocardiography demonstrated concentric left ventricular hypertrophy, hyperechoic and irregular endomyocardium, large oscillating lesions on the mitral valve apparatus, and severe left atrial dilation — findings closely mimicking infective endocarditis. Hematologic evaluation revealed marked leukocytosis (53.33 x10³/μL) and severe eosinophilia (11.20 x10³/μL), well above feline reference ranges. Historical medical records confirmed chronic eosinophilia, suggesting longstanding HES prior to cardiac decompensation. Despite initial stabilization and standard CHF management, the cat suffered cardiopulmonary arrest. Necropsy and histopathology confirmed eosinophilic and granulomatous mitral valvular and mural endomyocarditis with thrombosis, myocardial necrosis, and multi-organ eosinophilic infiltration. This case underscores HES as a rare but important differential diagnosis for cats presenting with echocardiographic features resembling infective endocarditis, particularly when accompanied by peripheral eosinophilia. Clinicians encountering similar echocardiographic phenotypes should include HES in their differential list and pursue complete blood count evaluation. Early identification of eosinophilia may facilitate timely immunosuppressive therapy, potentially altering disease progression before irreversible cardiac damage occurs.

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