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Immune-Mediated Hemolytic Anemia and Immune-Mediated Thrombocytopenia
-Carol Norris
Ettinger & Feldman, 5th Edition of the
Textbook of Veterinary Internal Medicine,
Client Information Series

Immune-mediated hemolytic anemia (IMHA) and immune-mediated thrombocytopenia (ITP) are diseases in which the body's own immune system attacks its red blood cells (IMHA) or platelets (ITP). Symptoms that develop are caused by a massive, often sudden, depletion of red blood cells or platelets. One of the major functions of red blood cells is to carry oxygen from the lungs to all other tissues in the body. When there are inadequate numbers of red blood cells (anemia), the body becomes starved for oxygen. As the pet owner, you may notice depression, listlessness, panting, loss of appetite, weakness, or reluctance to exercise in your pet.

The major function of platelets is to help form blood clots to stop bleeding. Destruction of large numbers of platelets can result in pinpoint bleeding in the skin or gums or may appear as nosebleeds. Less commonly, blood can be seen in the stool (which takes on a black appearance if it is digested or a bright red appearance if it is not) or urine. Severe anemia can result from excessive bleeding. Occasionally, IMHA and ITP occur together.

IMHA and ITP are more commonly seen in dogs than cats. It is believed that cocker spaniels, toy and miniature poodles, and Old English sheepdogs are breeds predisposed to develop IMHA. The latter two breeds and standard poodles are also at increased risk for developing ITP. Most affected dogs are middle-aged females. No breed or sex predilection is appreciated in cats. You should remember, however, that these conditions can develop in any dog or cat of any age, either sex, neutered or not. There is no scientific evidence that these diseases are caused by anything you feed your pet or by where your pet lives.

Both IMHA and ITP can be classified as "primary" or "secondary." In primary disease, no underlying cause of the immune destruction can be found after an exhaustive clinical and laboratory evaluation. In comparison, secondary IMHA or ITP occurs when the immune system inadvertently destroys its own red blood cells or platelets secondary to an immune attack directed against an underlying condition such as cancer, infection, a drug, or toxin exposure. If an underlying condition is present, it is critical to attempt to correct that problem while simultaneously treating the immune disease.

Treatment of IMHA and ITP relies on suppressing the immune system's attack against the red blood cells and platelets, respectively. The medication most commonly prescribed to shut off the immune system is a steroid hormone called prednisone. Side effects of this drug in dogs include an increase in water intake and urination, an increase in appetite, and panting; cats tend not to have significant side effects. Therapy must be continued until there is laboratory evidence that anemia has resolved and there is no ongoing destruction of red blood cells or platelets. This requires frequent recheck examinations to monitor the success of therapy. If the immune system has been adequately suppressed, the dosage of prednisone can be slowly tapered (often over a period of several months) and ultimately discontinued. Generally, most veterinarians like to check a dog or cat immediately before each decrease in prednisone dose. These frequent rechecks are extremely important. If inadequate suppression of the immune system occurs, additional drugs such as cyclophosphamide, azathiprine, or cyclosporine may be tried. These drugs can have more severe side effects than prednisone, so it is important to talk to your veterinarian about the potential risks of each medication and what problems you need to look for.

In some dogs and cats, the destruction of red blood cells or platelets is so severe that a life-threatening anemia can occur. Blood transfusions may be necessary to stabilize these pets until the bone marrow can keep up with the demand for red blood cells and platelets and until the drugs suppressing the immune attack have had time to work.

Prognosis for both diseases is highly variable and depends on the underlying cause if one is present, complications related to the disease or drug therapy, and the response to treatment. Relapses can occur months to years after the initial episode. Overall, if there is no severe underlying illness or significant complications and if your pet responds to therapy, prognosis for both diseases is generally good.

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