Immune mediated hemolytic anemia (IMHA)

Immune mediated hemolytic anemia (IMHA), also known as auto-immune mediated hemolytic anemia (AIHA), is a disease in which the body's immune system, which is designed to attack and kill germs, attacks and kills the body's own red blood cells. The attack begins when antibodies, which are molecules made by the immune system to target germs, instead attach to and target the animal's own red blood cells for destruction. The red blood cells carry oxygen to the tissues, and the animal cannot survive without adequate oxygenation of the tissues.

The causes of IMHA remain largely unknown. While some cases of IMHA may be associated with a triggering event (cancer, infection, and perhaps even vaccinations), these events do not explain why the immune system misdirects its arsenal of weapons against the animal it is meant to protect. IMHA is a rapidly life-threatening disease. Even with appropriate treatment, this disease can be fatal – survival rates are approximately 50%.

Clinical signs include lethargy, decreased appetite, weakness, pale or yellow tinged gums, increased respiratory rate and red or dark yellow/orange/brown urine. When red blood cells rupture they release their hemoglobin, which is then converted to bilirubin by the liver. Increased bilirubin levels are responsible for icterus (jaundice) – the yellow staining that occurs on the skin, whites of the eyes and mucous membranes.

The diagnosis is usually made by finding a severe anemia and evidence of agglutination (red cells clumping together), spherocytes (a change in the appearance of red cells due to loss of a portion of their membrane), and a Coomb’s test (evidence of antibody covered red cells).

In the acute phase, many dogs require a blood transfusion because of the severity of their anemia. Corticosteroids (prednisone) are the cornerstone of treatment, but other immunosuppressants such as azathiaprine, cyclosporine or leflunomide are also added to allow the corticosteroids to be weaned because of their side effects (increased drinking, urination, appetite, panting, muscle loss/weakness). In most cases the immunosuppressant drugs can eventually be discontinued several months after remission is achieved.

Published on July 23, 2010.