Extreme erythrocytosis in a cat reverts after short-term treatment with hydroxyurea and the cat remains healthy for years.

Objective: This case report describes a cat with severe erythrocytosis (Hct, 80%), which after initial treatment with hydroxyurea has gone into remission for over 3 years.

Animal: A 1-year-old neutered male American Maine Coon crossbred cat.

Clinical presentation, progression, and procedures: A 1-year-old neutered male American Maine Coon crossbred domestic cat was presented with acute neurologic signs, systolic heart murmur, and extreme erythrocytosis (Hct, 80%; normal interval, 30% to 48%). There were no clinical signs of dehydration, and several diagnostic tests for absolute erythrocytosis did not identify an underlying cause. A presumptive diagnosis of primary erythrocytosis (polycythemia vera [P vera], a myeloproliferative disease) was made.

Treatment and outcome: Repeated phlebotomies were declined by the owner, and thus the cat was treated with oral hydroxyurea. The neurologic signs, heart murmur, and erythrocytosis resolved within 2 months (Hct, 41%). Treatment with hydroxyurea was continued for 2 years and then discontinued. The Hct remained in the normal range (between 37% and 44%) during a 3-year observation period.

Clinical relevance: This case illustrates the challenges of determining a precise cause of erythrocytosis. The extreme erythrocytosis reverted after treatment with hydroxyurea and did not recur even after drug withdrawal, suggesting an undefined singular or multifactorial cause of the erythrocytosis rather than a primary absolute erythrocytosis, such as P vera. The reversibility of this cat's erythrocytosis suggested that in select cases the discontinuation of treatment is warranted.