Polycythemia vera with unusual complications in a cat

Abstract Background Polycythemia vera (PV) is an uncommon hematologic disorder in cats that typically manifests as erythrocytosis. Cases with concurrent increases in other cell lines such as leukocytes are rarely reported, and vascular complications other than neurologic abnormalities have not been well described. To the best of the authors’ knowledge, this is the first report to document ocular and auricular vascular involvement in a cat with PV. Case presentation A 7-year-old neutered male domestic shorthair cat presented with hematemesis, diarrhea, and episcleral congestion with elevated intraocular pressure (IOP). A complete blood count revealed marked erythrocytosis with leukocytosis. After excluding relative and secondary causes through thorough diagnostic evaluation, PV and glaucoma were diagnosed. Two sequential phlebotomies were performed, and hydroxyurea (15 mg/kg PO BID; Hydrine cap., Korea United Pharm, Sejong, Korea) therapy was initiated. The gastrointestinal signs subsequently resolved, and the IOP stabilized with concurrent topical antiglaucoma treatment. A right-sided aural hematoma developed approximately seven months after treatment initiation. As there was no evidence of underlying dermatologic disease or trauma, the lesion was considered a vascular complication related to blood hyperviscosity and endothelial dysfunction; this condition resolved after several drainage procedures and adjustment of the hydroxyurea dosage. Dosage adjustments based on serial hematologic monitoring led to stabilization of the patient’s red and white blood cell counts, and the cat remains clinically stable on hydroxyurea with no adverse drug reactions. Conclusions This case report describes feline PV presenting with atypical manifestations including hematemesis and ocular and auricular vascular involvement. The findings highlight that feline PV may be associated with diverse complications other than neurologic abnormalities and underscore the importance of careful hematologic monitoring during cytoreductive therapy.