Immune-mediated haemolytic anaemia (IMHA) is a life-threatening condition in which the immune system destroys the dog's own red blood cells. It is one of the most serious acute welfare emergencies in veterinary medicine, with mortality rates of 20–80% reported in different studies. Rapid diagnosis and aggressive treatment are essential.
Pathophysiology
IMHA occurs when autoantibodies (IgG or IgM) bind to red blood cell (RBC) surface antigens, marking them for destruction by macrophages in the spleen and liver (extravascular haemolysis) or by complement activation in the bloodstream (intravascular haemolysis). Primary (idiopathic) IMHA has no identifiable underlying cause; secondary IMHA is triggered by drug reactions, infections, cancer, or vaccines (rarely). Cocker Spaniels, English Springer Spaniels, Irish Setters, and Poodles are predisposed.
Clinical Signs
- Profound lethargy and weakness
- Pale, white, or yellow (icteric) mucous membranes
- Rapid, laboured breathing (compensating for reduced oxygen delivery)
- Tachycardia
- Collapse in severe cases
- Dark red-brown urine (haemoglobinuria) in intravascular haemolysis
- Splenomegaly on abdominal palpation
Diagnosis
PCV (packed cell volume) is severely reduced — often <20% in acute IMHA. Blood smear shows spherocytes (small, densely staining RBCs without central pallor) and autoagglutination. The Coombs test (direct antiglobulin test) demonstrates antibody coating on RBCs. Slide agglutination test: a drop of blood mixed with saline shows macroscopic clumping. Workup for secondary causes includes infection screening, radiography/ultrasound for neoplasia.
Treatment
Immunosuppression: High-dose prednisolone is first-line; azathioprine, mycophenolate, or ciclosporin are added in refractory cases or to reduce steroid doses.
Blood transfusion: Packed RBCs for life-threatening anaemia (PCV <12–15%) — cross-matching essential; transfusion provides time for immunosuppression to work.
Thromboprophylaxis: IMHA causes a hypercoagulable state — pulmonary thromboembolism (PTE) is a leading cause of death. Aspirin, heparin, or clopidogrel are used prophylactically.
Supportive care: IV fluids, cage rest, oxygen supplementation in severe cases.
Welfare and Prognosis
IMHA causes profound acute distress — extreme weakness, dyspnoea, and disorientation from hypoxia are significant welfare concerns requiring urgent treatment. Survivors typically require 3–6 months of immunosuppression tapering; relapses occur in approximately 15–20% of cases. Long-term monitoring after treatment cessation is important for early relapse detection.