Immune Mediated Haemolytic Anaemia (IMHA)


What is IMHA?

Immune Mediated Haemolytic Anaemia (IMHA) is a condition where the body’s immune system, which normally fights infection, starts to damage and destroy red blood cells. This results in the affected animal becoming anaemic (having a low red blood cell count).

What causes IMHA?

In some dogs and cats IMHA occurs without any underlying cause. In this situation it is referred to as primary (or idiopathic) IMHA. However, in other cases there is an underlying reason or trigger factor that results in the body damaging and destroying red blood cells, and this is termed secondary IMHA. Possible trigger factors include certain infectious or inflammatory diseases, particular classes of drugs or, sadly in some patients, underlying cancer. Recent vaccination has also been suggested to be a trigger factor in some dogs.

What are the signs of IMHA?

Common clinical signs that are seen with IMHA include:

  • Being quiet and lethargic
  • Pale or sometimes jaundiced (yellow) mucous membranes
  • Increased breathing rate and effort
  • Decreased appetite
  • Running a fever (increased body temperature)

How is IMHA investigated?

A thorough history and physical examination may give important information regarding any potential underlying cause for IMHA – for example whether the patient has been receiving any drugs, was vaccinated recently or has a history of travelling abroad.

If IMHA is suspected from the history and clinical findings, blood tests will be performed to determine the degree of anaemia. The red blood cells will be looked at under the microscope, where there may be tell-tale signs suggesting that they are being destroyed by the immune system. Other blood tests will be carried out to check general body organ function, and urine tests will often be performed to check for any evidence of urinary tract infection.

Advanced imaging of the chest and abdomen is frequently recommended for patients with suspected IMHA. This may involve X-rays, CT scanning or an ultrasound scan. These investigations help to rule out any evidence of inflammatory disease or cancers that could be acting as triggering factors for the IMHA. The diagnosis of primary IMHA is usually made by ruling out other underlying problems.

How is IMHA treated?

Primary IMHA is usually treated with drugs to dampen down the immune system which has become overactive i.e. immunosuppressive therapy. Most often this means treatment with steroids (see Steroid Therapy Information Sheet) over a period of approximately six months, although sometimes additional immunosuppressive treatment is also required. In pets which are very anaemic a blood transfusion may be needed, and a blood-typing test is often performed before the transfusion. A transfusion can help to stabilise the patient while further investigations are performed, and it gives time for treatment of the IMHA to start working. Dogs with IMHA can be at risk of forming blood clots, so anti-clotting medications such as aspirin are often used as part of the treatment regime.


In patients where an underlying trigger factor for IMHA is identified, this condition also needs to be treated if possible. In some situations this can be simple, for example by stopping a medication. However, in other situations, for example where there is underlying cancer, treatment can be more challenging. Even those patients that are diagnosed with secondary IMHA are likely to need immunosuppressive therapy.

Why should I bring my pet to NDSR for diagnosis and treatment of IMHA?

Our medicine service is led by a team of recognised, accredited Specialists and we aim to provide the best possible treatment for your pet in our state-of-the-art hospital. Our medicine team is supported by imaging and anaesthesia Specialists, and dedicated out-of-hours vets and nurses. We have extensive experience of managing critically ill patients, often with complex medical complaints.

What is the long term outlook for pets with IMHA?

The long term outlook for patients with IMHA can be variable.

Those patients with primary IMHA that respond to immunosuppressive therapy can do very well, although it is possible for relapses to occur, either during the course of therapy or when treatment has been discontinued.

The outlook for patients with secondary IMHA will often depend on the severity of the underlying disease process.

Unfortunately, in a proportion of patients it can prove difficult to control either the immune mediated red blood cell destruction or the secondary complications associated with IMHA. In some cases either the anaemia itself or its complications, such as blood clots to the lungs (pulmonary thromboembolism), can be life threatening.


If you have any queries or concerns, please do not hesitate to contact us.

Arranging a referral for your pet

If you would like to refer your pet to see one of our Specialists please visit our Arranging a Referral page.