Retrospective evaluation of outcomes in dogs with non‐associative immune‐mediated haemolytic anaemia treated with rivaroxaban as a sole or adjunctive thromboprophylaxis agent: 34 cases (2018‐2024)

OBJECTIVES: To describe clinical outcomes, thromboembolic and haemorrhagic complications, and treatment characteristics in dogs with non-associative immune-mediated haemolytic anaemia treated with rivaroxaban at a UK referral hospital. MATERIALS AND METHODS: Retrospective descriptive study of dogs diagnosed with non-associative immune-mediated haemolytic anaemia and prescribed rivaroxaban as part of their management protocol between January 2018 and July 2024. RESULTS: Thirty-four dogs met inclusion criteria. Median admission haematocrit was 14.1% (range 7.0% to 25.5%). Primary management of immune-mediated haemolytic anaemia was with corticosteroids (34/34; 100%), which were the sole agent in 8/34 (24%). Adjunctive therapies included mycophenolate mofetil (26/34; 70.6%), azathioprine (1/34; 2.9%) and leflunomide (1/34; 2.9%). Rivaroxaban was administered at a median total daily dose of 1.2 mg/kg/day (range 1.03 to 1.97 mg/kg/day); 29/34 dogs (85.3%) received concurrent clopidogrel. Confirmed or suspected thromboembolic events occurred in 7/34 dogs (20.6%), all during initial hospitalisation. Gastrointestinal bleeding occurred in 5/34 dogs (14.7%); all five were receiving concurrent corticosteroids, mycophenolate mofetil and rivaroxaban. Two bleeding events were classified as major adverse events, including one fatal case. Overall, 26 dogs (76.5%) survived to discharge. Remission was confirmed in 19/26 discharged dogs (73.1%) at a median of 33 days (range 15 to 156). Relapse occurred in 2/19 dogs (10.5%) post-remission. Of 21 dogs with follow-up available, 20 (95.2%) were alive at last known contact. CLINICAL SIGNIFICANCE: This represents one of the largest UK-based cohorts of dogs with non-associative immune-mediated haemolytic anaemia treated with rivaroxaban, demonstrating acceptable survival and remission rates with this thromboprophylaxis protocol. The occurrence of clinically significant gastrointestinal bleeding exclusively in dogs receiving concurrent corticosteroids, mycophenolate mofetil and rivaroxaban represents an important safety signal that warrants consideration and further study.