Immune-Mediated Hemolytic Anemia (IMHA) in Dogs
Drs. Foster & Smith, Inc.
Race Foster, DVM

Immune mediated hemolytic anemia (IMHA) is a disease in which the body attacks its own red blood cells. In dogs with IMHA, red blood cells are still being manufactured in the bone marrow, but once released into the circulation, they have a shorter-than-normal life span, since they are constantly being attacked and destroyed by abnormal antibodies in the blood. Antibodies are normally formed by the dog's immune system when needed to destroy invading bacteria or viruses. However, with IMHA, the dog's immune system is not working normally and sees its own red blood cells as foreign, and therefore, produces antibodies to destroy them.

A second abnormality often seen in IMHA is a decreased number of platelets. A low numbers of platelets is called thrombocytopenia. Platelets are tiny structures, which circulate within the bloodstream and function in the formation of clots when vessels are cut or broken. Over the course of a day, many of the body's small vessels normally break and the platelets clot the blood so no bleeding occurs. A simple bruise is nothing more than a collection of blood that has leaked out of broken blood vessels. When low platelet numbers are diagnosed, clotting does not occur correctly, and there can be internal bleeding, or increased numbers of nosebleeds, bleeding from the rectum etc. Uncontrolled bleeding further decreases the number of red blood cells caused by the abnormal antibodies.

What are the symptoms of immune mediated hemolytic anemia?

A dog with IMHA will have a lower-than-normal number of red blood cells within the blood. This is termed anemia. The lips, gums, and eye margins will appear pale (or yellow in the later stages of the disease) and not the normal pink to red color. Commonly, the dog will be tired and lethargic as there are not enough red blood cells to carry oxygen to the tissues. Fainting may occur due to low oxygen levels in the brain.

As the red blood cells are being destroyed, hemoglobin (the oxygen carrier molecule of red blood cells) builds up to high levels within the body. The liver attempts to break it down and remove it. Elevated levels of the breakdown products cause a yellowish color to the skin and membranes that we refer to as jaundice. The urine may contain hemoglobin and appear dark or tea-colored. Additionally, the heart beats much more rapidly to pump the thinner blood faster through the tissue. This is an attempt to compensate for low oxygen levels.

If the platelets are also low in number, bleeding from the nose or blood in the stool may be seen.

How is immune mediated hemolytic anemia diagnosed?

It is quite easy to diagnose anemia using a simple blood test called a 'hematocrit'. More information on the health of the blood can be obtained through doing a complete blood count. Once the anemia is diagnosed, it is more difficult to determine that the cause of the anemia is immune mediated.

There is no specific test for immune mediated hemolytic anemia. Instead, the results of various tests help point to the diagnosis. First, the anemia associated with IMHA is usually regenerative. This means the body quickly tries to replace blood cells that are lost. One can determine if an anemia is regenerative by looking at the cells under the microscope, using special stains. The shape of the red blood cells is also a good indicator. Red blood cells are usually flat, with a concave dip in both the top and bottom. In IMHA, the red blood cells are usually round. Finally, testing can be done to determine if antibodies are present on the surface of the red blood cells. These tests include a Coomb's test and Direct Immunofluorescence Flow (DIF) cytometry. Since platelets and clotting factors may be altered in IMHA, a coagulation panel and evaluation of the platelets is often recommended.

Many times other testing is performed to evaluate the health of the other organs. These tests may include a urinalysis and a chemistry panel.

What are the risks of immune mediated hemolytic anemia?

IMHA is serious and left untreated, usually results in death. Even with treatment, some dogs with IMHA will die. Complications involving the blood clotting system are common and need to be continually monitored.

What is the management of immune mediated hemolytic anemia?

Many dogs who are first presented to the veterinarian with signs of IMHA require transfusions, and may need supplemental oxygen. The basic treatment of IMHA involves the use of corticosteroids such as prednisone or dexamethasone to stop the body from attacking its own red blood cells. Sometimes prednisone is not sufficient, and more potent immunosuppressive drugs such as azathioprine are needed. Sometimes low-dose aspirin is added to the treatment, but should only be used if prescribed by your veterinarian. Treatment must continue for weeks to months to assure the body does not continue to destroy the red blood cells.

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