What Is Immune-Mediated Haemolytic Anaemia?
Immune-mediated haemolytic anaemia, commonly abbreviated to IMHA, is a life-threatening condition in which a dog's immune system mounts an attack against its own red blood cells, leading to their rapid destruction. Red blood cells are responsible for carrying oxygen from the lungs to every tissue in the body. When they are destroyed faster than the bone marrow can replace them, the result is anaemia — a dangerously low red blood cell count that deprives organs of the oxygen they need to function.
IMHA can occur as a primary (idiopathic) condition, in which no underlying trigger can be identified, or as a secondary condition triggered by an identifiable cause such as a drug reaction, infection, toxin exposure, cancer, or vaccination. Primary IMHA is by far the more common presentation in dogs.
Breeds Most Commonly Affected
While any dog can develop IMHA, certain breeds appear to be significantly overrepresented. Cocker Spaniels, Poodles, and Irish Setters are among the breeds most frequently diagnosed. Other breeds with a recognised predisposition include Old English Sheepdogs and Bichon Frises. The condition tends to occur in middle-aged dogs and is seen more often in females than males. Understanding breed predisposition is useful, as it can help vets maintain a higher index of suspicion when these breeds present with signs of anaemia.
Recognising the Symptoms
The clinical signs of IMHA reflect the body's response to a falling oxygen supply. In mild to moderate cases, owners may notice lethargy, reduced exercise tolerance, pale or whitish gums, rapid breathing, and reduced appetite. As the condition worsens, dogs may become profoundly weak, collapse, or lose consciousness. Some dogs develop jaundice — a yellowing of the skin, whites of the eyes, and gums — caused by the excessive breakdown products of red blood cells accumulating in the bloodstream. Dark, reddish-brown or orange-tinged urine is another sign, resulting from haemoglobin spilling into the urine when red blood cell destruction is particularly severe.
Because IMHA can progress rapidly, sometimes over hours rather than days, it should be treated as an emergency. Any dog showing pale gums, laboured breathing, or sudden collapse requires immediate veterinary attention.
How IMHA Is Diagnosed
Blood Smear and Spherocytes
One of the hallmark findings in IMHA is the presence of spherocytes on a blood smear. Spherocytes are small, densely staining red blood cells that have lost portions of their outer membrane due to immune-mediated attack. They are round rather than biconcave and do not have the central pale area seen in normal red blood cells. Their presence is strongly suggestive of IMHA and distinguishes it from other causes of anaemia.
The Coombs Test
The direct Coombs test (also called the direct antiglobulin test) is used to confirm immune-mediated destruction. It detects antibodies or complement proteins that are bound to the surface of the red blood cells, effectively proving that the immune system is targeting them. A positive Coombs test, combined with the clinical picture and blood smear findings, is highly supportive of an IMHA diagnosis. However, a negative result does not rule out the condition, and clinical judgement remains essential.
Additional diagnostics typically include a full blood count (which will show a low packed cell volume and evidence of regeneration, such as reticulocytes), biochemistry panel, urinalysis, chest X-rays, and abdominal ultrasound to help identify any underlying secondary cause.
Treatment
Immunosuppressive Therapy
The cornerstone of IMHA treatment is immunosuppression — dampening down the immune system's attack on red blood cells. Prednisolone (a corticosteroid) is the standard first-line agent, given at high immunosuppressive doses. In severe or refractory cases, a second immunosuppressive drug is added. Azathioprine and mycophenolate mofetil are the most commonly used second agents. Mycophenolate is increasingly favoured due to its relatively rapid onset and better tolerability in some dogs compared to azathioprine. Treatment typically continues for several months, with doses gradually tapered as the packed cell volume stabilises and improves.
Blood Transfusions
Dogs with a critically low packed cell volume — typically below 12-15% — may require one or more blood transfusions to stabilise them while the immunosuppressive drugs take effect. Transfusions are not a cure; the transfused cells are just as susceptible to immune-mediated destruction as the dog's own red blood cells. However, they provide vital time for treatment to work and can be life-saving in the acute phase.
Managing Thromboembolism Risk
One of the most dangerous complications of IMHA is thromboembolism — the formation of blood clots that can lodge in the lungs, kidneys, or other vital organs. Dogs with IMHA are in a hypercoagulable (excessively clot-prone) state, and pulmonary thromboembolism is a leading cause of sudden death in these patients. To reduce this risk, low-dose aspirin is commonly co-administered, though some clinicians prefer other anticoagulants such as heparin or clopidogrel depending on the individual case and the severity of risk.
Strict cage rest is also important to reduce oxygen demand and the risk of clot dislodgement during the acute phase of illness.
Prognosis and Long-Term Outlook
IMHA carries a serious prognosis. Despite appropriate and aggressive treatment, mortality rates of 25 to 40 percent have been reported in published studies, with most deaths occurring within the first two weeks. Dogs that survive the acute crisis have a reasonable chance of long-term recovery, though relapses can occur and some dogs require ongoing low-dose immunosuppressive therapy indefinitely.
Close monitoring is essential during the recovery period. Packed cell volume should be checked frequently — initially daily in hospitalised patients and then every one to two weeks as an outpatient once the dog is stable. Any sudden drop in the packed cell volume should prompt an urgent reassessment of treatment.
What Owners Should Know
IMHA is one of the most emotionally and financially demanding conditions a dog owner can face. The need for hospitalisation, blood transfusions, and intensive monitoring means costs can be substantial. Equally important is understanding that the first few days of treatment are often the most uncertain, and a dog that appears to be improving can still deteriorate rapidly.
If your dog is in a predisposed breed and develops sudden lethargy, pale gums, or unusual breathing, contact your vet immediately. Speed of diagnosis and treatment initiation is one of the most important factors in determining outcome.