Platelets (thrombocytes) are produced in the bone marrow. Platelets circulate in the blood. When your pet gets cut, platelets stick to the edges of the cut and to one another, forming a plug. They then release chemicals that react with fibrinogen and other plasma proteins, leading to the formation of a blood clot.
If your pet has a condition called thrombocytopenia, his platelet count is low. When the platelet count is low, it is harder for clotting to occur.
Signs of thrombocytopenia vary, but can include petechial (small, pinpoint) hemorrhages in the skin or mucous membranes (lining of the mouth, for example), nosebleeds, hematuria (blood in the urine), melena (blood in the feces), and bleeding into the joints or body cavities. The lower the platelet count is, the more likely it is that your pet will show signs. (Some animals may show no warning signs at all, and the thrombocytopenia may be discovered only during routine health exams.)
Major causes of thrombocytopenia are immune-mediated thrombocytopenia (an autoimmune disease), bone marrow cancers, infectious diseases, hereditary diseases, drug/vaccination reactions, vasculitis (leading to deep vein thrombosis), and disseminated intravascular coagulation.
Thrombocytopenia is not common in dogs and cats, although there are several breeds of dogs that are predisposed to it. Animals with cancer are at higher risk than those without cancer, but it can occur in a dog/cat of any age, breed, or sex.
Diagnostic tests include: blood tests (platelet count, complete blood count, serum biochemistry), urinalysis, chest or abdominal radiographs (to look for diseases that could be causing the thrombocytopenia), tests for specific infectious diseases (Rocky Mountain spotted fever, ehrlichiosis, etc.), bone marrow aspiration, and tests for immune system function.
Treatment is based on the cause of the thrombocytopenia, and may include antibiotics, corticosteroids, blood transfusions, etc.
Prognosis depends on the cause of the thrombocytopenia. A mild, self-limiting vaccine-induced thrombocytopenia has an excellent prognosis, while a thrombocytopenia due to cancer might have a much worse prognosis.