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ITP IN DOGS

By Gail D. Mason, DVM, MA, DACVIM | Nov 30, 2018
Blood Cells

Immune-mediated thrombocytopenia (ITP) is a fairly common disease in dogs and in people. It refers to destruction of blood platelets by the dog’s own immune system. Platelets are small fragments of a precursor cell called a megakaryocyte that is formed in bone marrow. The result of this natural fragmentation is several small, cloud-shaped circulating elements that are not related to white or red blood cells. Platelets are an integral part of the body’s mechanism to repair small injuries that occur in blood vessels. They accomplish this by forming clumps and essentially “plug” the leaky blood vessel. If the platelet count becomes too low for any reason, the body cannot maintain blood vessel integrity and spontaneous bleeding can occur. This phenomenon can become life threatening.

Platelets are also involved in the first steps of normal blood clotting which is necessary during trauma, injury, or surgery. As small as they are, dogs cannot live without them. A normal “platelet count” for dogs is approximately 175,000-300,000 per microliter of blood. If the platelet count is less than 100,000 per microliter, then the patient has low platelets or “thrombocytopenia.” This abnormality can go undetected. However, if the platelets drop below 50,000 per microliter, then the dog becomes at high risk of spontaneous hemorrhage, even without an injury. A decrease in platelets can occur secondary to decreased production in the bone marrow or increased loss/destruction.

What does “immune-mediated” mean? A normal immune system has safeguards in place to recognize the body’s own cells as being “friendly.” When foreign cells or certain infections are present in the body, the immune system detects these invaders and creates “antibodies” (smart bombs) to attach to and destroy the enemy. In the case of dogs with ITP, the immune system overreacts and attacks its own platelets. Why does this happen? Most of the time, an exact cause is not found. Less commonly, it can be an aberrant response to certain infections, cancers, vaccinations, or medications.

The rate of destruction can be slow or rapid in onset. The faster it occurs, the more likely the dog will show symptoms. Platelets are destroyed at 10 times the normal rate which rapidly lowers the platelet count. Most of the time, a healthy bone marrow will respond to the deficiency by increasing platelet production. These newly released platelets are larger than normal but function well. Unfortunately, they often cannot keep up with the ongoing destruction.

What are the symptoms of thrombocytopenia? In patients that have ITP from an unknown cause, the disease is remarkably “silent.” Most dogs act normally. The first symptoms that are often noted include: 1) nosebleeds (epistaxis); 2) small, pin-point red spots (petechiae) observed on the gums, whites of the eyes or belly; and 3) bruising of the skin (ecchymoses) of the belly and neck. Bruising as we know it in humans is otherwise very rare in dogs. If you observe any of these symptoms in your dog, immediate attention is warranted.

Your veterinarian would generally start by asking you questions about your dog’s health history and any recent vaccinations, medications, or illnesses. After a physical examination, a blood test called a CBC (complete blood count) would likely be performed. This test will not only determines the platelet count but also gives information as to the health of the other cells in the bloodstream. Sometimes the platelet count is too low to be accurately read by blood analyzers, and a manual count must be performed. If your doctor is concerned that there may be underlying disease causing the thrombocytopenia, he/she may recommend further tests such as radiographs, tests for infection, and/or an abdominal ultrasound.

Treatment for ITP depends on the severity of the disease. In asymptomatic dogs, outpatient treatment with drugs that suppress the immune system’s ability to destroy the platelets is recommended. This virtually always includes prednisone (a steroid) which is rapid- acting, effective, and generally safe. Antibiotics may also be used if infection is suspected.

If your dog has known bleeding, then hospitalization for treatment and monitoring is indicated. In patients that have lost significant blood through spontaneous hemorrhage, multiple drugs as well as a blood transfusion may be required. It is important to note that in most dogs who develop this disease, treatment is highly successful and reasonably rapid. Fortunately, once an increase in the platelet count is achieved, it does not usually relapse unless medications are discontinued prematurely.