Congestive Heart Failure Secondary to Hypereosinophilic Syndrome with Cardiac Involvement in a Cat.

Hypereosinophilic syndrome is a rare systemic disorder in cats characterized by sustained eosinophilia and eosinophil-mediated tissue injury. A three-year-old castrated male domestic shorthair cat was presented for acute onset dyspnea and was diagnosed with left-sided congestive heart failure. Echocardiography revealed concentric left ventricular hypertrophy with hyperechoic and irregular endomyocardium, large irregular oscillating lesions involving the mitral valve apparatus, and severe left atrial dilation. Complete blood count revealed a marked leukocytosis (53.33 x103/μL; reference range: 5.5-19.5 x103/μL) and marked eosinophilia (11.20 x103/μL; reference range: 0.0-0.75 × 103/μL). Despite initial stabilization and medical management for congestive heart failure, the cat experienced cardiopulmonary arrest. Prior medical records documented a chronic history of eosinophilia. Necropsy and histopathology revealed eosinophilic and granulomatous mitral valvular and mural endomyocarditis with thrombosis, myocardial necrosis, and eosinophilic infiltration of multiple organ systems. This case report highlights hypereosinophilic syndrome as a rare cause of cardiac disease in cats but a relevant differential diagnosis for an echocardiographic phenotype mimicking infective endomyocarditis and provides echocardiographic and pathologic images of this uncommon disorder.