IMHA Toolkit

2. How does IMHA affect dogs?

Red blood cells carry oxygen. Normally, your dog's immune system acts like a highly trained security force, manufacturing tiny proteins called antibodies to tag and target harmful invaders like viruses, bacteria, or parasites.

In many dogs with IMHA, antibodies bind to red-cell surface antigens and mark those cells for immune-mediated destruction. Complement may also contribute in some cases [Garden et al., 2019].

Antibody-coated red cells may then be removed predominantly by macrophages in the spleen and liver, or destroyed within the circulation when complement-mediated mechanisms are involved. If red-cell destruction outpaces marrow replacement, clinically important anaemia develops.

3. Why is IMHA so dangerous?

The reduction in circulating red cells decreases the blood's oxygen-carrying capacity. This can contribute to marked lethargy, weakness, tachypnoea or panting, exercise intolerance and pale mucous membranes.

Canine IMHA is also associated with a complex prothrombotic state involving haemolysis, inflammation, endothelial and platelet activation, coagulation abnormalities and impaired fibrinolysis. Thrombi may develop in the pulmonary, portal or other vascular beds and are an important cause of morbidity and mortality [Swann et al., 2019].

4. What causes IMHA?

When diagnosing IMHA, your veterinary surgeon's first priority is to determine whether the disease is "Primary" or "Secondary".

Primary (Autoimmune) IMHA

This occurs when the immune system makes a mistake entirely on its own, with no external trigger. It is often referred to as "idiopathic" IMHA and is considered a true autoimmune disease.

Secondary IMHA

This occurs when an underlying trigger alters the red blood cells or hyper-sensitises the immune system, causing the body to attack its own cells. Common triggers include underlying infections (such as tick-borne diseases), recent administration of certain medications, or underlying cancers.

5. How is IMHA diagnosed?

There is no single diagnostic gold standard for IMHA. ACVIM recommends combining evidence of anaemia, immune-mediated red-cell destruction and haemolysis, while excluding important alternative explanations [Garden et al., 2019]:

• Packed Cell Volume (PCV) / Haematocrit: To quantify anaemia. A spun PCV can be particularly useful when agglutination may make automated haematocrit measurements unreliable.
• Blood Smear Review: To assess for spherocytosis and other red-cell abnormalities. Prominent spherocytosis supports immune-mediated destruction in dogs but is not, by itself, a complete diagnosis.
• Saline Agglutination Test (SAT): To assess persistent red-cell agglutination after saline dilution or washing. Persistent agglutination provides strong evidence of immune-mediated red-cell destruction.
• Evidence of haemolysis: Biochemistry, plasma/serum appearance, urine findings and bilirubin assessment can help demonstrate active red-cell destruction. Depending on the case, direct antiglobulin testing or flow cytometry may also contribute to evidence of immune-mediated destruction.

6. How is IMHA treated?

Treatment is individualised according to disease severity, complications and comorbidities, but commonly includes stabilisation, immunosuppression and thromboprophylaxis where appropriate:

• Immunosuppression: Prednisone or prednisolone is commonly used as first-line treatment. A second immunosuppressive drug may be considered in selected circumstances rather than being routinely required [Swann et al., 2019].
• Clot Prevention (Thromboprophylaxis): ACVIM strongly recommends thromboprophylaxis for dogs with IMHA unless severe thrombocytopenia or another important contraindication makes treatment inappropriate [Swann et al., 2019]. Options can include anticoagulants such as rivaroxaban, antiplatelet drugs such as clopidogrel, or selected combinations.
• Supportive Care: This may include blood transfusion, oxygen supplementation, fluid therapy when indicated and treatment of concurrent disease or complications. Gastroprotective medication is not automatically required for every dog and should be selected according to individual risk.

Veterinary professionals can review standard weight-based schedules on our Canine Dose Calculator.