IMMUNE MEDIATED HEMOLYTIC ANEMIA (IMHA)
IMMUNE MEDIATED HEMOLYTIC ANEMIA or “IMHA”
Immune-mediated hemolytic anemia is a condition where the patient’s immune system begins attacking his or her own red blood cells. What occurs on a microscopic level is this: the branch of the immune system that produces antibodies begins to direct them against the patient’s own red blood cells. The red blood cells become quickly coated with tiny antibody proteins, essentially marking these red blood cells for destruction. When too many red blood cells are destroyed the patient is said to be “anemic,” and will feel cold and weak. Because the red blood cells are being destroyed internally (and not lost through bleeding), the patient will become yellow-tinged (“jaundiced”) rather than pale (see picture below).
Red blood cells are coated with Y-shaped antibodies which mark them for removal / destruction
REMOVING OLD RED CELLS/RECYCLING IRON: THE NORMAL SYSTEM
Red blood cells have a natural life span from the time they are released from the bone marrow to the end of their oxygen-carrying days when they become too stiff to move through the body’s narrow capillaries. A red blood cell must be supple and flexible to participate in oxygen delivery and carbon dioxide removal so when the cell is no longer functional, the body has a system to destroy it and recycle its components.
When old red blood cells circulate through the spleen, liver, and bone marrow, they are plucked from circulation and destroyed, a process called “extravascular hemolysis.” Their iron is sent to the liver in the form of a yellow pigment called “bilirubin” for recycling. The proteins inside the cell are broken down into amino acids and used for any number of things (burning as fuel, building new protein etc.) The spleen uses immunological cues on the surface of red blood cells to determine which cells are plucked out of circulation. In this way, red cells parasitized by infectious agents are also removed from circulation along with the geriatric red cells. When the immune system marks too many cells for removal, problems begin.
THE TESTS SHOW ANEMIA
Part of a general evaluation includes blood testing. If your pet seemed obviously pale or jaundiced (yellow-tinged), your vet may run a test called a “packed cell volume” right off the bat. This test can be run in most veterinary offices and involves spinning a small amount of blood in a small glass tube at high speed to separate the red blood cells from the serum (the blood’s liquid phase). By comparing the blood tube to a chart, it will become obvious if your pet has a low red blood cell count. If your pet has hemolytic anemia, the serum will be bright orange instead of its normal off-white color.
The anemia may come up on a full blood panel submitted to a reference laboratory. These laboratories perform a test called a “hematocrit” which is slightly different from a packed cell volume but essentially measures the same thing: the percentage of blood volume made up by red blood cells. This number should be 43 - 59 for dogs and 29 - 50 for cats.
“Anemia” is the condition where one’s red blood cell count is low. Anemia can be mild or severe and can represent bleeding, red blood cell destruction (as in IMHA) or simply lack of red blood cell production. Once a patient is found to be anemic, it is important to determine why.
THE TESTS SHOW RESPONSIVE ANEMIA
Anemia due to poor red blood cell production by the bone marrow is called a “non-responsive anemia.” Such anemias are caused by chronic inflammatory diseases (like inflamed skin, infected teeth, or other long standing irritations), kidney failure, cancers of various types, or certain drugs (especially agents of chemotherapy).
Normally when red blood cells are lost, the drop in blood oxygen that results causes hormonal changes leading to increased production of red blood cells by the bone marrow. These are called “responsive anemias” because the bone marrow is responding. Bleeding and immune mediated red blood cell destruction are both “responsive anemias.”
There are several ways to determine if the anemia is responsive or not from the blood panel results. Most blood panels run by reference laboratories include a portion called a “complete blood count” or “CBC” which reviews red blood cell count, size, shape, maturity as well as white blood cell types and ratios. A patient with a responsive anemia will have a very active bone marrow. Red blood cells will be released early leading to a variety of sizes and redness of red blood cells circulating in the blood (less mature red blood cells are larger and paler than mature cells). Further, red blood cell precursors called “reticulocytes” are released. (These may be thought of as red blood cells so immature they can’t truly be called “red blood cells.) If the bone marrow stimulus is especially strong, red blood cells may be released still containing cell nuclei.
A pair of reticulocytes are shown among some red blood cells.
These findings indicate the anemia is responsive. This means either red blood cells are being lost via bleeding, possibly internal bleeding, or they are being destroyed by the immune system.
Which is it?
THE TESTS SUGGEST IMMUNE MEDIATED DESTRUCTION? RATHER THAN BLEEDING
There are several clues in blood testing that tell us if our patient is bleeding or destroying red blood cells. One might think it would be obvious if the patient were bleeding but if the bleeding is internal it may not be so obvious.
ICTERUS (ALSO CALLED “JAUNDICE”)
We have mentioned this but let’s recap anyway. Icterus is the yellow color that is taken up by a patient’s tissues when the liver is overwhelmed with bilirubin, the iron-containing by-product of red blood cell destruction. Normally red blood cells are removed from the circulation when they become old and inflexible. Their iron is recycled in the liver. With so many red cells being destroyed, the liver is overwhelmed and bilirubin (a yellow pigment) spills out everywhere, coloring urine, gums, skin, and the eyes orange.
Is immune mediated red cell destruction the only cause of icterus? Absolutely not. Liver failure also leads to icterus when the diseased liver cannot process normal amounts of bilrubin. In cats especially, bacterial endotoxin (the toxic cell walls of certain types of infecting bacteria) can lead to icterus. Usually, however, an responsive anemia together with icterus, suggests immune-mediated red cell destruction.
In severe cases of immune mediated hemolytic anemia, the immune destruction of red cells is so blatant that the red cells clump together (because their antibody coatings stick together) when a drop of blood is placed on a microscope slide. Imagine a drop of blood forming not a red spot but a yellow spot with a small red clump inside it. This finding is especially forboding.
Classically, in IMHA the stimulation of the bone marrow is so strong that even the white blood cells lines (which have very little to do with this disease but which also are born and incubate in the bone marrow along side the red blood cells) are stimulated. This leads to white blood cell counts that are spectacularly high.
MORE TESTS NEEDED
COOMB’S TEST (ALSO CALLED A “DIRECT ANTIBODY TEST”)
This is a test designed to identify antibodies coating red blood cell surfaces. This test is the current state of the art for the diagnosis of IMHA but, unfortunately, it is not as helpful as it might seem. It can be erroneously positive in the presence of inflammation or infectious disease (which might lead to harmless attachment of antibody to red cell surfaces) or in the event of prior blood transfusion (ultimately transfused red cells are removed from the immune system). The Coomb’s test can be erroneously negative for a number of reasons as well. If the clinical picture fits with IMHA, often the Coomb’s test is skipped.
Remember, not all causes of hemolysis (red blood cell destruction) are immune-mediated. Onions in large amounts (and possibly garlic as well) will cause a toxic hemolysis. Zinc toxicity, usually from swallowing a penny minted after 1983, or from licking off a zinc oxide ointment applied to the skin, will cause hemolysis as well. In a young animal, a genetic red blood cell malformation might be suspected.
Once the diagnosis of Immune Mediated Hemolytic Anemia has been made.
TREATMENT AND MONITORING DURING THE CRISIS
The patient with IMHA is often unstable. If the hematocrit has dropped to a dangerously low level then blood transfusion is needed. It is not unusual for a severely affected patient to require many transfusions. General supportive care is needed to maintain the patient’s fluid balance and nutritional needs. Most importantly, the hemolysis must be stopped by suppressing the immune system’s rampant red blood cell destruction. We will review these aspects of therapy.
Corticosteroid hormones in very 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 a very important part of therapy. These hormones also suppress the activity of the Reticuloendothelial cells that are responsible for the removal of antibody coated red cells.
Corticosteroids may very well be the only immune suppressive medications the patient needs. The problem is that if they are withdrawn too soon the hemolysis will begin all over again. The patient 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. Expect your pet to require steroid therapy for some 4 months; many must always be on a low dose to prevent recurrence.
Corticosteroids in high doses produce excessive thirst, re-distribution of body fat, thin skin, panting, predisposition for urinary tract infection and other signs that constitute Cushing’s Syndrome. This is an unfortunate consequence of long term steroid use but in the case of IMHA, there is no way around it. It is important to remember that the undesirable steroid effects will diminish as the dosage diminishes.
MORE IMMUNE SUPPRESSION
If no response at all is seen with corticosteroids, supplementation with stronger immune suppressive agents is necessary. The two most common medications used in this case are: azathioprine and cyclophosphamide. These are very serious drugs reserved for serious diseases. Please follow the links above to read more about specific side effects concerns etc.
Cyclosporine is an immune-modulator, made popular in organ transplantation technology. It has the advantage over the two above medications of not being suppressive to the bone marrow cells. It has been a promising adjunctive therapy in IMHA but has two major problems: first, it is extremely expensive and second, blood level monitoring is necessary to ensure that the dosage is appropriate. This adds dramatically to the expense of treatment but ultimately may provide the results not possible with other drugs.
Leflunomide is a new immuno-modulator that is meant for patient with immune mediated diseases when corticosteroids either do not work or cannot be used. It is expensive (approx $600 per month) but we may be hearing more about it in the future.
Human Gamma Globulin transfusion is a treatment that is reserved for patients for whom more traditional methods of immune suppression have been ineffective. The gamma globulin portion of blood proteins includes circulating antibodies. These antibodies bind the reticuolo-endothelial cell receptors that would normally bind antibody coated red blood cells. This prevents the antibody coated red blood cells from being removed from the circulation. Human Gamma Globulin therapy seems to improve short term survival in a crisis but, unfortunately, availability of the product is limited and it is very expensive.
WHY DID THIS HAPPEN TO YOUR PET?
When something as threatening as a major disease emerges, it is natural to ask why it occurred. Unfortunately, if the patient is a dog, there is a good chance that there will be no answer to this question. Depending on the study, 60-75% of IMHA cases do not have apparent causes.
In some cases, though, there is an underlying problem: something that triggered the reaction. A drug can induce a reaction that stimulates the immune system and ultimately mimics some sort of red blood cell membrane protein. Not only will the immune system seek the drug but it will seek proteins that closely resemble the drug and innocent red blood cells will be consequently destroyed. Drugs most commonly implicated include penicillins, trimethoprim-sulfa, and methimazole.
Drugs are not the only such stimuli; cancers can stimulate exactly the same reaction (especially hemangiosarcoma).
Red blood cell parasites create a similar situation except the immune system is aiming to destroy infected red blood cells. The problem is that it gets over-stimulated and begins attacking the normal cells as well.
Breeds predisposed to the development of IMHA include: cocker spaniels, poodles, Old English Sheepdogs, and Irish setters.
COMPLICATIONS OF IMHA
This particular complication is the leading cause of death for dogs with IMHA (between 30-80% of dogs that die of IMHA die due to thromboembolic disease). A “thrombus” is a large blood clot that occludes a blood vessel. The vessel is said to be “thrombosed.” “Embolism” refers to smaller blood clots, spitting off the surface of a larger thrombus. These mini-clots travel and occlude smaller vessels thus interfering with circulation. The inflammatory reaction that normally ensues to dissolve errant blood clots can be disastrous if the embolic events are occurring throughout the body.
Heparin, a natural anticoagulant, may be used in hospitalized patients (or in patients with predisposing factors for embolism) as a preventive.
* It should be noted that 4-7 days are required for the bone marrow to generate a response. If hemolysis occurs suddenly there may not have been adequate time for a response. When this occurs, if there is any question about the responsive nature of the anemia, continued monitoring of the Complete Blood Count will show a clear response in an appropriate time period. It should also be noted that in an especially unlucky patient, the red blood cell destruction may extend to the pre-red blood cells (reticulocytes, nucleated red cells and other precursors) within the bone marrow. If these cells are also destroyed, the condition is especially dangerous and it will take weeks rather than days to begin to see a response to treatment. The lack of circulating immature red cells will lead this anemia to test as “non-responsive.”
IMHA is a very serious disease associated with a high mortality rate. Sadly, many dogs have succumbed. Several pet owners have used these sad events to create outstanding informational web sites on IMHA in tribute and to help others. We have found these site especially noteworthy and recommend them highly:
Page last updated: 3/14/2013