June 12, 2000
                 Written by: Celeste A. Clements, DVM, Diplomate ACVIM

It is cause for alarm when a pet begins to bleed spontaneously.
Sometimes the hemorrhaging is subtle, but other times, a dramatic volume of blood is released, and in these cases, the condition may be life-threatening.
The tendency for exaggerated bleeding often is due to the condition thrombocytopenia, or a reduction of blood platelets.Platelets are small, circulating blood elements that help stop bleeding by clumping together at the site of blood vessel damage. Without an adequate number of these platelets, blood loss can be significant. Spontaneous bleeding usually occurs when platelets are present at only about 10 percent of their normal levels.
Typical signs of thrombocytopenia are pinpoint or paintbrush bleeding into the skin or mucous membranes. These lesions are described as petechiae and ecchymoses, respectively.Nosebleeds, gastrointestinal tract and urinary tract bleeding also occur commonly; hemorrhage into joints, body cavities, and organs is less common.
Severe thrombocytopenia is much more common in canine patients than in feline patients. The disease has diverse causes, ranging from active bleeding during surgery to cancers to Rocky Mountain spotted fever. However, in dogs, the most common cause is the disease immune-mediated thrombocytopenia, or IMT.
Unfortunately, mortality rates of patients with IMT approach 50 percent: death is caused by severe blood loss, especially bleeding into the gastrointestinal tract, or bleeding into the brain.
IMT occurs when the platelets or precursors of the platelets in the bone marrow are targeted by the immune system. Most often, the disease is primary, or occurring spontaneously, but it also can be secondary to another disease, such as canine hemolytic anemia. Secondary IMT also may be triggered by the presence of certain   cancers, especially lymphosarcoma; or by certain medications, such as trimethoprim-sulfa antibiotics; and possibly, by vaccination. A genetic predisposition is suspected, since cocker spaniels, poodles, German shepherds, and old English sheepdogs are affected most often with IMT.

                 Diagnosing IMT

Definitive confirmation of IMT requires special immunologic testing that is relatively inaccurate and of limited availability. The veterinarian, however, may assume IMT upon exclusion of other common causes of low platelet numbers, such as rickettsial infection and disseminated intravascular coagulopathy. This assumption will be further bolstered by the presence of known triggers and/or laboratory evidence of platelet destruction, such as circulating platelet fragments, concurrent with larger immature and hyperfunctional platelets that suggest a positive bone marrow response. In addition, a positive response to immunosuppressive treatment supports a diagnosis of IMT.
Bone marrow sampling frequently is delayed until a limited therapeutic trial is attempted, if the clinical index of suspicion is high for infectious or immune causes of thrombocytopenia. The marrow of most patients with IMT will show substantial increases in megakaryocytes, platelet precursors. Rarely, antibodies will attack the bone marrow elements, leading to diminished numbers of megakaryocytes and circulating platelets.
Bone marrow cancers and some chronic feline viral infections are detectable only through bone marrow analysis. This procedure is frequently prioritized in cat patients or dog patients where bone marrow suppression is suspected. Also, diagnosis of lymphoid cancer may be masked by treatment with corticosteroids or other immunosuppressive agents, prompting earlier use of diagnostics, where suspected.

                 Treating the disease

Treatment is directed toward achieving a complete or partial remission that is sustained, resulting in a patient that is free of  bleeding tendency. Most patients with IMT will be treated for a  minimum of six months; some will not maintain remission as medications are withdrawn, so lifelong treatment is required.
Relapses often are more difficult to treat, so patience with drug withdrawal is critical.

Successful treatment of IMT entails removing any underlying cause, such as cancer or medication exposure; providing excellent supportive care, which may include transfusion with whole blood, packed red blood cells, or blood substitutes to address significant anemia; and a focus on specific and tailored immunosuppressive       therapy.
Corticosteroids are the cornerstone of treatment for dogs and cats with IMT: they offer an immediate reduction in immune-mediated platelet destruction. When the patient has severe thrombocytopenia or is intolerant of the steroid effects, or if there is a poor response to treatment, other immunosupressive agents are used in combination with prednisone, prednisolone or dexamethasone. Side effects may be less common with the ancillary treatments, but these medications require several weeks of treatment for maximal efficacy, and some are quite expensive.

In emergency situations where the IMT is assessed to be life-threatening, standard medical treatments may not be sufficient to stabilize the patient. Those patients may benefit from the use of less common medications that aid in platelet release from the bone marrow or treatments that cleanse the blood of circulating immune complexes. Surgical splenectomy is rarely indicated for the treatment of IMT. Transfusion of blood products with concentrated platelets from donor dogs rarely makes a sustained impact on the clinical progress.

Veterinary specialists are eagerly awaiting improvements in immunosuppressive therapies that will enhance the outcomes for patients with severe immune-mediated thrombocytopenia. Future progress would bring hope to owners of patients affected by this severe disease.