What is IMHA?

1. What is IMHA?

Immune-mediated haemolytic anaemia, commonly shortened to IMHA, is a serious condition in which a dog’s immune system mistakenly attacks and destroys its own red blood cells.

Red blood cells are vital because they carry oxygen around the body. When too many are destroyed, the body cannot deliver enough oxygen to the muscles, brain and organs. This is what causes anaemia.

To understand the name:

• Immune-mediated means guided, prompted or caused by the immune system.
• Haemolytic means involving the breakdown or destruction of red blood cells.
• Anaemia means the body does not have enough healthy red blood cells to carry oxygen effectively.

2. How does IMHA affect dogs?

Normally, your dog’s immune system acts like a defence force, identifying and responding to harmful invaders such as bacteria, viruses or parasites.

In IMHA, a recognition error occurs. The immune system mistakenly identifies the dog’s own red blood cells as a threat. It produces antibodies that attach to the surface of those red blood cells, marking them for removal.

As these marked cells circulate through the body, organs such as the spleen and liver help remove and destroy them. If red blood cells are destroyed faster than the body can replace them, the dog becomes anaemic, weak and short of oxygen.

3. Why is IMHA so dangerous?

IMHA is dangerous because red blood cells can fall to life-threatening levels. Dogs may become suddenly weak, lethargic, pale, breathless or unable to cope with normal activity.

However, IMHA is not only dangerous because of anaemia. The disease can also create a strongly prothrombotic, or clot-prone, state. Inflammation, red cell destruction and changes in blood flow can make dangerous blood clots more likely.

Pathological blood clots, known as thrombosis, may affect vital organs. Thromboembolic disease is recognised as a major complication and cause of death in canine IMHA.

4. What causes IMHA?

IMHA may be described as primary or secondary. This distinction matters because it affects how the disease is investigated and managed.

Primary IMHA

Primary IMHA means no clear underlying trigger has been found. It is sometimes called idiopathic IMHA. In these cases, the immune system appears to attack red blood cells without an obvious external cause.

Some breeds appear to be more predisposed to IMHA, including Cocker Spaniels, English Springer Spaniels and Irish Setters, although IMHA can affect any dog.

Secondary IMHA

Secondary IMHA means the immune attack may be associated with another trigger or underlying condition. Possible triggers include:

• Infections, including some tick-borne diseases and blood parasites.
• Certain medications, toxins or recent immune stimulation in susceptible animals.
• Underlying cancers or severe inflammatory disease.

When secondary IMHA is suspected, treatment may need to address both the immune attack and the underlying trigger.

5. How is IMHA diagnosed?

Diagnosing IMHA usually requires several tests. The aim is to confirm that red blood cells are being destroyed and to look for evidence that the immune system is involved.

• Packed cell volume (PCV) or haematocrit: Measures the proportion of red blood cells in the blood.
• Complete blood count: Gives more detailed information about red cells, white cells and platelets.
• Blood smear review: Allows the veterinary team to look for features such as spherocytes, agglutination, parasites or other abnormalities.
• Saline agglutination test: Helps assess whether red blood cells are clumping together because of antibody coating.
• Biochemistry and urinalysis: Help assess organ function, bilirubin levels, hydration and possible complications.
• Additional testing: May be used to look for infections, cancer, inflammation or other underlying triggers.

6. How is IMHA treated?

Treatment is tailored to the individual dog. The main goals are to stabilise the patient, stop or reduce the immune attack, support oxygen delivery, monitor for complications and reduce the risk of dangerous clot formation.

• Immunosuppression: Corticosteroids such as prednisolone are commonly used to reduce immune destruction of red blood cells. Some dogs may also need a second immunosuppressive medicine.
• Thromboprophylaxis: Because thromboembolic disease is a major complication in canine IMHA, vets commonly consider antithrombotic medication unless there is a clear contraindication. Commonly discussed options include clopidogrel and rivaroxaban.
• Supportive care: Some dogs may need intravenous fluids, oxygen support, stomach protection, pain relief or blood transfusion depending on severity.
• Monitoring: Regular blood tests and clinical reassessment help guide treatment changes and detect relapse or complications.

Veterinary professionals can use our Canine IMHA Dosage Calculator to support weight-based dose checking alongside clinical judgement, current formularies and patient-specific assessment.