Immune-mediated haemolytic anaemia (IMHA) is a condition where the animal’s immune system attacks its’ own red blood cells. Antibodies are produced, which coat the red blood cells, marking them for destruction. The spleen will enlarge as it processes more damaged red blood cells than normal. The liver recycles iron from the large numbers of destroyed red blood cells and is overwhelmed, leading to spillage of bilirubin (a yellow pigment) everywhere. This can lead to jaundice (where the skin and eyes become a yellow or orange colour). If there are not enough red cells, then there is also not enough oxygen circulating to the tissues and this can become a life-threatening crisis. This is a serious disease with high mortality rates. Mortality can range from 20% up to 80%!
Why does this happen?
Unfortunately, 60 – 70% of IMHA cases do not have apparent causes. Antibiotics and certain drugs (e.g. penicillins, methimazole etc) as well as cancers have been implicated in stimulating this immune response. Other possibilities suggested included red blood cell parasites, vaccination and insect bites. Certain dog breeds such as cocker spaniels, poodles, Old English Sheepdogs and Irish setters are more predisposed to developing IMHA. In cats, IMHA can specifically originate from infection with Feline Leukemia Virus or a red blood cell parasite called Mycoplasma Haemofelis.
Weakness, lethargy, and a loss of interest in food are most common. Urine can be dark orange or even brown. Gums are pale or white, or sometimes yellow. The sclera of the eyes can also sometimes be yellow. Fever is also a possibility.
If your vet suspects IMHA (often because your pet might have very pale gums), then a test called a Packed Cell Volume (PCV) can be quickly run in clinic. This immediately lets the vet know if the pet has a low number of red blood cells. Your vet might also perform an autoagglutination test. In severe cases of IMHA, the red blood cells can clump together (because their antibody coatings stick together) on a microscope slide. This can be visible to the naked eye.
Often, a full laboratory blood test will also be required to confirm a diagnosis of IMHA; this will review red blood cell count, size, shape, maturity as well as white blood cell types and ratios. Anaemia can be classified as responsive or non-responsive. Responsive means that the bone marrow is actively producing new immature red blood cells, called reticulocytes, in response to the loss of red blood cells. The laboratory blood test will let us know if these reticulocytes are present. This is important information to know as it helps the vet classify the specific type of anaemia your pet might have, and therefore narrows down the list of possible causes. Often, responsive anaemias are due to destruction (as in IMHA) or bleeding, possibly internal. White blood cells are commonly stimulated greatly in this disease, and therefore will be very high in number on the blood test as well.
The Coomb’s Test is considered to be the current ‘standard’ method of diagnosis and is another laboratory test where the antibodies coating the red blood cell surfaces can be specifically identified. It is often used in conjunction with the above mentioned general blood test.
1. Immune suppression
Corticosteroid hormones in high doses are the cornerstone of immune suppression. Prednisone and dexamethasone are the most popular medications selected. These hormones are directly toxic to lymphocytes, the cells that produce antibodies. If the patient’s red blood cells are not coated with antibodies, they will not have been targeted for removal so stopping antibody production is an important part of therapy. These hormones also suppress the activity of the reticuloendothelial cells that are responsible for removing antibody-coated red cells. Your pet is likely to be on high doses of corticosteroids for weeks or months before the dose is tapered down, and there will be regular monitoring blood tests (usually via repeat Packed Cell Volume measurements). Many patients will also always need to be a low dose life-long to prevent recurrence.
Unfortunately, there can be some side effects of corticosteroids when they are used long-term. These include increased thirst, increased appetite, panting, predisposition for urinary tract infection, thin skin and re-distribution of body fat.
If no or limited response is seen with corticosteroids, supplementation with stronger immune suppressive medication is required. In IMHA, we use azathioprine, cyclophosphamide and cyclosporine. Sometimes, depending on the individual, a combination of corticosteroids and these other drugs will be recommended. This should be discussed further with your vet.
2. Blood transfusion
If the red blood cells have dropped to a dangerously low level and your pet is critically ill, then a blood transfusion may be required. It is not uncommon for a severely affected patient to require many transfusions. General supportive care in hospital for days or even a week is also needed to maintain your pet’s fluid balance and nutritional needs.