Type II reactions can lead to several types of diseases in horses, including anemia, blood clotting problems, and skin and muscle disorders. They may be associated with other immune system disturbances, such as cancers of the lymphoreticular system, or triggered by a drug, vaccine, or infection. Most often, the triggering cause cannot be pinpointed.

Immune-mediated Hemolytic Anemia

This type of anemia is a severe and life-threatening disease in which the immune system sees its own red blood cells as foreign invaders, and produces antibodies to destroy them. Red blood cells are manufactured as usual in the bone marrow, but once released into the bloodstream, they are attacked and destroyed by antibodies. Signs of anemia may include fatigue, paleness of the lips and gums, and depression, along with jaundice in some cases. Other signs your veterinarian may find include an enlarged liver or spleen.

Cold Agglutinin (Hemolytic) Disease

Cold agglutinin (hemolytic) disease is also called cold antibody disease. It is a type of immune-mediated hemolytic anemia in which the body develops antibodies that attack red blood cells at temperatures lower than normal body temperature. It is more common in colder climates and seasons. The cause is usually not known, but it may follow a longterm infection, another autoimmune disorder, or cancer. The red blood cells are destroyed prematurely and bone marrow production of new cells cannot compensate for their loss. The severity of the anemia is determined by the length of time that the red blood cells survive and by the capacity of the bone marrow to produce new red blood cells. The condition is more likely to attack cooler parts of the body such as the nose, tips of the ears, legs, scrotum, and the skin over the penis. Diagnosis is based on a blood test. Medications are available to control this disease. Your veterinarian can prescribe the most appropriate for your horse.

Immune-mediated Thrombocytopenia

Immune-mediated thrombocytopenia is caused by the destruction of platelets (thrombocytes) by the immune system in much the same manner as red blood cells are destroyed in immune-mediated hemolytic anemia (see above). When an animal has thrombocytopenia, clotting does not occur correctly. Even minor injuries can cause uncontrollable bleeding, further decreasing the number of red blood cells. The most frequent signs are bleeding of the skin and mucous membranes.

The diagnosis is usually made based on the signs and response to treatment, rather than on blood tests. However, certain blood tests such as platelet counts and clotting profiles are helpful. Medication will likely be prescribed to treat this disease. Signs usually disappear after 5 to 7 days of treatment when platelet counts begin to rise. If the platelet count has not increased significantly after 7 to 10 days, additional or different medications, such as drugs that suppress the immune system, may be prescribed. If the blood loss is life threatening, transfusions of whole blood or plasma may be necessary.

Treatment is often continued for 1 to 3 months after the platelet counts return to normal. Some animals have persistent decreases in platelets even with drug treatment. If this is the case with your horse, you will want to discuss longterm treatment and maintenance options with your veterinarian.

Last full review/revision July 2011 by Christine Andreoni; Kevin T. Schultz, DVM, PhD