Immune-Mediated Haemolytic Anaemia (IMHA) in Dogs
Written by Shula Berg BVSc CertAVP(GSAS) GPAdvCert(SASTS) MRCVS
Clinically reviewed by Elizabeth McLennan-Green BVM&S CertAVP(SAM) MRCVS
Table of Contents
Immune-mediated haemolytic anaemia, commonly referred to as IMHA, is a potentially serious condition seen mostly in dogs, but which can also occur in cats. Anaemia refers to a lack of red blood cells in the circulation, while haemolysis means the cells are being destroyed. In IMHA, a dysfunction of the immune system leads to the body attacking and destroying its own red blood cells.
IMHA can be primary or secondary. In primary IMHA there is no underlying cause. It is suspected, but not proven, that there may be a genetic predisposition to developing primary IMHA. It is seen more commonly in Cocker, Springer and Clumber Spaniels, Old English Sheepdogs and Maltese Terriers, though can affect any breed including crossbreeds. In secondary IMHA, the haemolysis occurs due to an underlying condition, such as a cancer, infection, or reaction to a drug.
Sometimes, IMHA is seen in combination with immune-mediated thrombocytopenia (IMTP). This is when platelets (small cells which help the blood to clot) are also destroyed.
The symptoms of IMHA are caused by the anaemia. Red blood cells are responsible for carrying oxygen around the body, and when there are not enough red blood cells pets become lethargic and may struggle to exercise. Inappetence is commonly seen. On clinical examination the mucous membranes (inside the lips and eyelids) are often very pale pink, white or yellow (due to jaundice). Patients with secondary IMHA may have other symptoms caused by an underlying condition.
IMHA can develop slowly, over several weeks, or very rapidly. In slow cases, the symptoms may not be as severe as the animal has time to adjust to the anaemia. In rapidly progressing cases, the sudden reduction in red blood cells can cause collapse and jaundice.
The first step in diagnosis of IMHA is a general blood test to check haematology and biochemistry. This will identify anaemia, which is often moderate to severe. Other abnormalities may also be present, such as low platelets.
Specific tests are needed to diagnose IMHA and distinguish it from other causes of anaemia. A saline agglutination test can be performed easily under the microscope using a blood sample. If this is positive it is very suspicious for IMHA, however, a negative result does not rule IMHA out. The definitive test is known as a Coomb’s test and must be performed at an external laboratory. Often a blood smear is also sent for specialist examination. It can take up to a week to receive results from a Coomb’s test, so it is often appropriate to start treatment before the diagnosis is confirmed, especially in severe cases.
Primary IMHA is a diagnosis of exclusion, so other causes of anaemia and underlying causes of secondary IMHA must be ruled out. This is most achieved with diagnostic imaging, such as x-ray and/or ultrasound examination, and further blood tests.
Treatment of primary IMHA predominantly involves suppression of the immune system to halt red blood cell destruction. This is typically achieved using glucocorticoids (steroids) but depending on the severity of disease, other drugs might be used in combination. Often these are human immunosuppressant drugs and are not specifically licensed for use in animals. Additional medications may be given to help manage symptoms, such as anti-nausea drugs, gastro-protectants, and antibiotics. Patients with IMHA are more likely to form blood clots, so low doses of an anti-coagulant, such as heparin or aspirin, are also commonly given.
Depending on the severity of disease, patients with IMHA may be managed at home, or may need to be hospitalised. Patients who are bright, eating and drinking can take medication at home but will need to return for regular blood tests, often daily initially. Patients who are lethargic, not eating or severely anaemic may require hospitalisation for intravenous medication and close monitoring.
Dogs who experience fast-onset IMHA often become severely anaemic very quickly. The body struggles to cope with this, and a blood transfusion may be required. This can be obtained from a donor dog or ordered from the pet blood bank. Blood transfusions carry a small risk of allergic reaction, but this is rare. More concerning is the possibility that the immune system will destroy the new cells. This risk is lowered by administering immunosuppressive treatment as above, but for some patients, blood transfusions only lead to short-term improvement.
Response to treatment is measured by looking at the packed cell volume (PCV), which tells us what percentage of the blood is red blood cells. A normal PCV for a dog is 35-50%. New red blood cells are constantly being released into circulation, so PCV can change rapidly. A positive response to treatment, with reduced destruction of red cells, can be seen as an increase in PCV within a few days.
IMHA is a serious disease and not all pets will respond to treatment. It is estimated that 50-75% of dogs respond well to treatment, however, the remaining cases may not survive or require euthanasia, due to lack of response to treatment or complications of the disease such as blood clots. Often those that are going to respond to treatment do so quickly, and a study found that most dogs who survived the first 14 days after diagnosis were still doing well at 6 months.
Patients who respond to treatment will need to take immuno-suppressants long term. Once they are stable, treatment can be very gradually withdrawn. Often one medication is stopped at a time, and typically the dose is reduced by 25% every few weeks – although this is case dependant and will be tailored to each pet. PCV is monitored closely throughout. Some patients will tolerate this well and eventually cope without medication, while others may need some degree of treatment for life.
Once a pet has had IMHA, they are at risk for relapse months or years later. There is no need to continue treatment if PCV is stable, however, monitoring every 6-12 months is wise and any clinical signs should be promptly investigated.
Piek, C.J. et al (2008). Idiopathic Immune-Mediated Hemolytic Anemia: Treatment Outcome and Prognostic Factors in 149 Dogs - Idiopathic Immune‐Mediated Hemolytic Anemia: Treatment Outcome and Prognostic Factors in 149 Dogs - Piek - 2008 - Journal of Veterinary Internal Medicine - Wiley Online Library
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Page last reviewed: 16th January 2024
Next review due: 16th January 2026