Immune-Mediated Haemolytic Anaemia (IMHA)

What is Immune-Mediated Haemolytic Anaemia (IMHA)?

Immune-Mediated Haemolytic Anaemia (IMHA) is a serious autoimmune condition that occurs when a dog's immune system attacks and destroys its own red blood cells because it has mistakenly identified them as 'foreign'.  Red blood cells are responsible for carrying oxygen around the body, so their rapid destruction can cause severe anaemia and can become life-threatening.  

IMHA can develop suddenly and requires prompt veterinary investigation and treatment.

Normally, the immune system protects the body from infections and other foreign substances.  With IMHA, however, antibodies and other parts of the immune system target the dog's red blood cells.  These cells may then be destroyed either within the bloodstream or by organs such as the spleen and liver.  The destruction of red blood cells is called haemolysis.  If red blood cells are being destroyed faster than the bone marrow can replace them, the dog develops anaemia.

IMHA can be:

• Primary (non-associative or idiopathic) – no underlying cause can be identified.

• Secondary (associative) – an underlying cause (or trigger) is suspected or identified.  These can include:

-   Certain infections

-   Cancer

-   Inflammatory diseases

-   Exposure to certain drugs

-   Other medical conditions

The evidence linking individual potential triggers to IMHA varies, and in many dogs no definite underlying cause can be established.

IMHA is an important cause of serious illness and death in dogs.  Certain breeds, including English Springer Spaniels, are thought to be more predisposed to it than average. 

What are the symptoms?

The signs of IMHA can develop rapidly and may vary depending on how quickly the red blood cells are being destroyed and how severely the dog is affected.

Common signs include:

• Lethargy and weakness

• Pale or white gums (rather than the normal healthy pink colour)

• Reduced exercise tolerance

• Rapid breathing or an increased breathing rate

• Increased heart rate

• Loss of appetite

• Fainting or collapse

• Yellowing of the gums, skin or whites of the eyes (jaundice)

• Dark or reddish-brown urine

• Vomiting

• Fever

Some dogs can deteriorate very quickly.  IMHA can also cause abnormal blood clotting, and affected dogs are at significant risk of developing thromboembolism, where a blood clot blocks a blood vessel.  This can cause additional, potentially life-threatening complications.

A dog showing sudden weakness, collapse, very pale gums, difficulty breathing or other signs of severe illness should receive urgent veterinary attention.

How is IMHA diagnosed?

There is no single test that can definitively diagnose every case of IMHA.  Vets use a combination of clinical examination, blood tests and tests for immune-mediated destruction of red blood cells.

Initial investigations will usually include:

• Complete blood count (CBC) – to establish whether the dog is anaemic and assess the red and white blood cells and platelets.

• Blood smear examination – red blood cells may show changes associated with immune-mediated destruction, including spherocytes (abnormal, sphere-shaped red blood cells that are smaller and darker than normal cells and lack a central pale area).

• Reticulocyte count – measures young red blood cells and helps determine whether the bone marrow is responding to the anaemia.

• Biochemistry tests – may identify increased bilirubin (a waste product formed during the normal breakdown of old red blood cells) and abnormalities affecting other organs.

• Urinalysis – can provide additional evidence of haemolysis (early destruction of red blood cells before the end of their normal lifespan) and assess kidney function.

Tests specifically looking for evidence that the immune system is attacking the red blood cells may include:

• Saline agglutination test (SAT) – looks for red blood cells that are clumping together because of antibodies.

• Direct antiglobulin test (DAT) or Coombs' test – detects if antibodies are stuck to red blood cells.

• Flow cytometry – can detect antibodies associated with red blood cells in some cases.

Guidelines recommend interpreting these tests together rather than relying on a single result.  A negative result from one test does not necessarily rule out IMHA.

Looking for an underlying cause:

Once IMHA is suspected or diagnosed, a vet will also consider whether the condition is primary or secondary.  This is important because treating an underlying disease, where one is identified, can form an important part of managing secondary IMHA.  Depending on the individual dog, investigations may include:

• Further blood tests

• Testing for infectious diseases

• Imaging such as X-rays or ultrasound

• Investigation for inflammatory disease

• Investigation for cancer

• Review of recent medications and other relevant history

What are the treatment options?

IMHA is a potentially life-threatening disease and treatment usually needs to begin promptly.  Treatment is tailored to the individual dog and may include a combination of the following:

• Immunosuppressive treatment: Aims to stop the immune system destroying the red blood cells.  Corticosteroids, such as prednisolone, are commonly used as the initial immunosuppressive treatment.  Some dogs may also require a second immunosuppressive drug, particularly when the disease is severe, the response to corticosteroids is inadequate or their side effects need to be reduced.  Drugs that may be considered include cyclosporine, mycophenolate mofetil and azathioprine.  The choice depends on the individual dog and the veterinary assessment, as adding a second immunosuppressive drug isn't beneficial in every case. 

Immunosuppressive treatment generally needs to be continued for some time and then gradually reduced (tapered) once the disease is under control.  Dogs require regular monitoring throughout treatment.

• Blood transfusions:  Dogs with severe anaemia may require a blood transfusion to improve oxygen delivery to the body's tissues while treatment takes effect.  

• Preventing blood clots:  Because dogs with IMHA are at increased risk of potentially fatal blood clots (thromboembolism), antithrombotic treatment is an important part of managing many affected dogs.  Drugs may include antiplatelet or anticoagulant medications, depending on the individual case.

• Supportive treatment:  Depending on the severity of the disease, supportive care may include:

-   Intravenous fluids where appropriate

-   Treatment of nausea or loss of appetite

-   Oxygen therapy in severely affected dogs

-   Nutritional support

-   Treatment of any identified underlying disease or infection

-   Intensive monitoring of blood counts and organ function

Some dogs require hospitalisation, and severely affected dogs may need care in a veterinary referral or intensive-care facility.

What is the outlook for affected dogs?

The prognosis for IMHA is uncertain.  It is a serious disease, particularly during the first few weeks following diagnosis, and some dogs can deteriorate very rapidly despite treatment.  However, the outlook varies considerably between individual dogs.  Factors that may influence outcome include:

• How severely anaemic the dog is

• Whether significant blood clotting complications develop

• Whether the dog has low platelet numbers (thrombocytopenia)

• Whether bilirubin levels are increased (hyperbilirubinaemia)

• Whether an underlying disease is present

• How well the dog responds to treatment

• The dog's overall health and condition

A 2024 study of 104 dogs with IMHA in Ireland found that 16% had died within one month and 31% within three months of diagnosis.  Of the dogs that survived the initial three months, the long-term outcome was considerably more favourable, with a median survival time of more than seven years in that study.

These figures should not be interpreted as a prediction for an individual dog.  Studies differ in their diagnostic criteria, treatment protocols and populations, and individual outcomes can be very different.

Dogs that respond successfully to treatment may eventually be able to have their immunosuppressive medication gradually reduced and stopped.  However, careful monitoring is important because relapse can occur.

IMHA therefore remains a serious condition requiring prompt diagnosis, intensive treatment in many cases and ongoing veterinary monitoring.  Early recognition and appropriate treatment give an affected dog the best possible chance of recovery.

Important: This information is intended as a general guide and is not a substitute for veterinary diagnosis or treatment.  If IMHA is suspected, veterinary advice should be sought urgently.

If you would like any help or further information relating to IMHA, we would be happy to hear from you.  We would also be extremely grateful if ESS owners would help us to monitor this disorder in ESS by reporting online any confirmed diagnosis or the death of your ESS from the condition by clicking on either of the links below.  All information provided will be treated in the strictest confidence.

For any further information or advice, please contact the

ESS Breed Clubs Health Co-ordinators