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Immune Mediated Haemolytic Anaemia (IMHA)

What is IMHA?

Immune mediated haemolytic anaemia is a condition whereby an animals’ own immune system destroys it’s red blood cells, leading to anaemia (too few red blood cells). This destruction can occur in the blood vessels (intravascular) or in organs such as the liver and spleen (extravascular). This is a serious condition and it has been reported that a third to a half of animals which develop IMHA will, sadly, not survive.

Usually the immune system only attacks the ‘adult’ red blood cells, so the body can, initially, create more red blood cells to replace those that are destroyed. Though occasionally the immune system attacks ‘immature’ red blood cells, in which case the body is unable to replace old, worn out red blood cells and anaemia develops this way.

There two main types of IMHA: Primary (auto-immune) and Secondary IMHA. In Primary IMHA, the immune system recognises and directly attacks specific ‘chunks’ of the red blood cell membrane. In secondary IMHA the body responds to a foreign substance (eg a virus or drug), but parts of this foreign substance ‘look like’ parts of red blood cells, or sometimes stick to the red blood cells, and so the immune system mistakenly attacks the red blood cells.

IMHA is more common in dog than cats, though cats are more likely to make a full recovery from the condition.

What are the signs of IMHA?

The signs seen with both types of IMHA are associated with anaemia (too few red blood cells), animals may be lethargic, anorexic, weak, be breathing fast, and have pale gums, they may have a fever, your vet may hear a heart murmur and a fast heart rate too and they may be able to feel an enlarged spleen or liver if this is present.

Generally, if the anaemia has developed slowly an animal will be better able to cope and show less severe signs. If the anaemia has developed suddenly, the animal hasn’t had a chance to ‘get used to’ the condition and so may be severely depressed, very weak or collapsed. They may be jaundiced - the skin, gums and eyes may look yellow.

What causes IMHA?

Autoimmune haemolytic anaemia is the most common type of IMHA in dogs, unfortunately, to date, there is no recognised cause. It typically affects young adult to middle aged dogs and is most common in Cocker Spaniels, English Springer Spaniels, Poodles, Old English Sheep dogs, and Collies. It is thought to be more common in female dogs, although some studies have found no difference between male and female occurrences.

Secondary IMHA can be due to infection (eg FeLV, mycoplasma haemofelis), Drugs (eg some antibiotics) allergies, recent vaccination and cancers (eg lymphoma, leukaemia) as well as zinc or onion poisoning. Secondary IMHA can affect any breed, sex and age of animal; IMHA in cats is usually secondary.

It is important to recognise when IMHA is secondary as, without removing the cause of the illness, treatment will not be successful.

It is also worth being aware that IMHA is one part of the disease Systemic Lupus Erythematosus, a serious and complex condition which affects multiple organs and systems within the body.

How is this condition diagnosed?

A simple blood test carried out at the vets will show a moderate to severe anaemia. Your vet will also be able to tell if the body is making replacement red blood cells, this process won’t be seen in the blood until about 5 days after the start of the RBC destruction as that’s how long it takes for the body to make new RBCs. So if there’s no signs of the body replacing the red blood cells, it could be because the illness started less than 5 days ago, or it could be because the ‘baby’ red blood cells are being destroyed by the immune system.

Your vet may be able to detect increased white blood cells. They may see ‘spherocytes’ which are red blood cells which have been attacked by the immune system but not completely destroyed, though these cells are difficult to detect in cats.

Your vet may be able to test the blood using saline to see if the red blood cells are sticking together, this is abnormal and would indicate IMHA, although the test doesn’t pick up all cases of IMHA.

A coombs’ test, a type of blood test, may be used to detect antibodies which are primed to attack red blood cells.

What is the treatment?

Because IMHA is caused by the immune system, the most important treatment is to suppress this. Steroids are the most commonly used medicine for this. Because steroids can sometimes irritate the gut, your vet may prescribe drugs to protect the gut from this irritation.

If a response to the steroid treatment is not seen within an expected timeframe (around 7 days) your vet may choose to change to a much stronger immune suppressing drug.

If the IMHA is secondary the cause needs to be removed, your pet may require antibiotics.

If the anaemia is very severe a blood transfusion may be necessary to replenish red blood cells.

Good nursing care is paramount to an animals’ recovery; when animals are very anaemic, they are unable to carry adequate oxygen to the cells around the body. It’s important to reduce the amount of oxygen required by these cells so that the body can keep up with oxygen demand. We do this by restricting exercise until the anaemia begins to resolve.

It’s usually 2-3 weeks before the number of red blood cells starts to increase after starting steroid treatment, once this response is seen, the steroids can be gradually tapered until no further treatment is needed, it can be up to 6 months before all medication can be stopped.

A common complication of IMHA is pulmonary thromboembolism (please see handout), especially in animals with sudden onset, severe anaemia that are receiving high dose steroids, this shows as sudden, severe breathing difficulties. DIC is also a complication which can occur in animals with IMHA, DIC is also related to the clotting of the blood and actually results in an inability of the animal to form blood clots. Both of these complications are life threatening, if your vet thinks these complications are likely, they may decide to give treatments to try and prevent them from occurring.

How can IMHA be prevented?

Unfortunately, as the cause of IMHA is unknown, there is no advice on prevention of this disease.

There are many causes of secondary IMHA and, there’s no way of telling whether your pet will develop IMHA as a result of exposure to these things.

Lucy King BVetMed
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