Primary Erythrocytosis (Polycythaemia) in a Neutered Male Dog Presenting With Right Hindlimb Lameness

ABSTRACT

A 2‐year‐old male entire Cocker Spaniel was referred for investigation of progressive right hindlimb lameness and polyuria and polydipsia. Investigations conducted by the primary care veterinary surgeon revealed erythrocytosis (haematocrit of 78%, reference interval: 37%–55%) and urine concentration consistent with polyuria and polydipsia (specific gravity 1.014). Upon referral, physical examination revealed a dark red oral mucosa and right hindlimb lameness, characterised by intermittent non‐weight bearing of the limb when standing. Neurological examination only revealed a mild short‐strided gait, and orthopaedic examination was unremarkable. Secondary causes of erythrocytosis were excluded by normal arterial blood gas, low erythropoietin levels and normal thoracic and abdominal imaging, resulting in a diagnosis of primary erythrocytosis (PE). Focal ultrasonography identified a right external iliac venous thrombus, suspected to be the cause of the right hindlimb lameness and presumed to be associated with the PE. Initial management involved repeated phlebotomies, followed by long‐term treatment with hydroxyurea (31 mg/kg twice weekly) and rivaroxaban (2 mg/kg once daily). At the 5‐month recheck, the dog was asymptomatic, with resolution of the iliac thrombus and a normal haematocrit. This case highlights an unusual presentation of PE in a dog, not previously described in the veterinary literature, and re‐examines the discussion on the optimal management of this rare condition, with a focus on the necessity of antithrombotic therapy.