IMHA in dogs: What do you need to know?

IMHA stands for Immune Mediated Haemolytic Anaemia. It’s a bit of a mouthful so is often known as its less tongue-twisting abbreviation. It’s not as complicated as it sounds and here we will attempt to demystify it.

What is IMHA?

IMHA belongs to a group of diseases known as autoimmune diseases. These occur because the body’s own cells are incorrectly identified as foreign by the immune system. These cells are then attacked and destroyed. With each autoimmune disease, different cells are targeted. With IMHA, red blood cells are targeted and destroyed leading to reduced numbers (anaemia). Two thirds of dogs with IMHA also experience a similar destruction of platelet cells. Platelets are involved in clot formation, and a lack of them (known as thrombocytopenia) leads to abnormal bleeding and is known as Immune Mediated Thrombocytopenia (ITP). When the two conditions occur together we call it Evans syndrome.

What causes this?

Body cells have a protein on the surface called an antigen. The immune system uses this protein to identify its own cells. In IMHA, red blood cell antigens are falsely recognised as foreign which stimulates the production of an antibody by the immune system. The antibody attaches to the antigen and causes the cell to swell and burst. Cells also become more likely to clump (agglutinate) giving higher risks of abnormal clotting.

When the cause of a disease is unknown we call it idiopathic. This is the case in around 70% of IMHA, so we call it idiopathic IMHA, or primary IMHA.

In the other 30% there is an underlying cause called secondary IMHA. Certain infections, mainly spread by parasites like ticks, can lead to secondary IMHA. Most are uncommon in the UK, but should be considered, especially if your pet has travelled abroad. Certain medications have been known to trigger IMHA, such as some antibiotics and painkillers. Some forms of cancer can also trigger IMHA. Recent vaccination has been suggested to trigger the disease but lacks evidence at present. Vaccination prevents serious life-threatening diseases and in the majority of dogs the benefits vastly outweigh the risks.

There seems to be a genetic predisposition in cocker spaniels and Old English sheepdogs, but any breed can be affected.

What are the symptoms?

Symptoms are largely due to the resulting anaemia. Signs may be vague such as weakness, lethargy and a poor appetite. Red blood cells are needed to carry oxygen around the body, so there is often a fast heart rate and breathing rate as the dog tries to compensate for the lack of oxygen. Usually we see pale gums, but there may be a yellow tinge to the gums as when red blood cells rupture they are broken down into a yellow tinged product called bilirubin.

Damage through lack of oxygen within any organ can cause varied signs and, as there is a tendency for red blood cells to clump, clots may form anywhere.

How is IMHA diagnosed?

After an examination and taking a history we may be suspicious.

Firstly we confirm the presence of anaemia. Anaemia has many causes so we need to further define the type of anaemia present. Most dogs with IMHA have a regenerative anaemia, where the body is stimulated to produce more red blood cells to make up for lost ones. In rare cases where the bone marrow is affected then a non-regenerative anaemia is possible.

Next we rule out other causes of regenerative anemia, and other rarer causes of haemolysis like onion poisoning or heavy metal toxicity, and identify whether your pet has had any recent medications or vaccinations. We may carry out further tests to identify infectious causes or cancers.

We often examine the blood under the microscope, looking for certain types of cells known as spherocytes, which are small, round red blood cells often associated with IMHA. We may see more immature red blood cells (reticulocytes) telling us the anaemia is regenerative. We often send samples to a lab for confirmation alongside other specific tests such as the Coombs test, which looks for antibodies on the red blood cells. A saline agglutination test looks for clumping of red blood cells when mixed with saline. It is quick and relatively inexpensive, but can be hard to interpret.

Can this condition be treated?

The first-line treatment for many autoimmune conditions is a corticosteroid called prednisolone, which suppresses the immune system. High doses may be needed initially, aiming to lower and possibly stop medication in the long run. A second immune suppressing drug can be tried in severe cases, or if prednisolone causes severe side effects or doesn’t work. Azathioprine is most commonly used as it is inexpensive, but takes time to work and requires handling precautions. Cyclosporine is licenced for use in dogs but expensive, has an unpredictable dose range and side effects.

A last resort is to help stop red cell destruction in the spleen by removing it (splenectomy). Pets by this stage are very sick and the procedure risky. Many owners would sadly have to consider euthanasia as an alternative.

If we think there’s a risk of excessive clotting we may prescribe medications such as aspirin to mitigate this.

A blood transfusion may seem obvious, but new cells may be destroyed quickly, worsening the signs. However, damage caused by the lack of oxygen in severe anaemia may be a bigger risk, so a transfusion may be recommended.

We would monitor the response to treatment with blood tests regularly. Hopefully once a response is seen, we can taper any medication gradually, whilst continuing to sample for any worsening in parameters. We may also want to monitor for any side effects of the medications.