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Evidence-based management of ITP (Proceedings)

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Apr 01, 2010

Dogs with immune-mediated thrombocytopenia (ITP) usually present with platelet counts low enough to be considered life-threatening, although overt signs of bleeding are uncommon. Owners usually only note anorexia or lethargy, although in some cases epistaxis, cutaneous bruising/petecchiae/echymoses, or oral bleeding may be noted. Very rarely patients will present with complications that require immediate intervention such as pleural effusion. As a specialist, the most common reason for referral to me of ITP patients is inadequate or no response to appropriate immunosuppression. As with IMHA, this often is due to unfamiliarity with some of the treatment options beyond glucocorticoids, or a difficult-to-diagnose underlying disease. This presentation will briefly review the more common causes of secondary ITP and the diagnostic tests which I routinely consider prior to instituting therapy, and then discuss in-depth treatment options for dogs with this disease.

ITP: Diagnostic Work-Up

            Primary versus Secondary causes of ITP
Development of anti-platelet antibodies may be idiopathic (primary ITP), or may occur secondary to a number of infectious or neoplastic diseases, or certain drugs. The diseases listed below can cause thrombocytopenia through other mechanisms, without the presence of anti-platelet antibodies. Differentiation can be difficult, as there are no widely available tests which allow detection of anti-platelet antibodies. An anti-megakaryocyte antibody test is available through some specialty laboratories, but this must be performed on bone marrow aspirate samples. As a very general rule primary ITP results in much lower peripheral platelet counts than ITP secondary to concurrent diseases. Primary ITP dogs usually present with platelet counts less than 20,000; dogs with secondary ITP often have platelet counts higher than this. However clinicians should remember that these are general rules only—I have seen many cases of secondary ITP (particularly with RMSF or neoplastic diseases) where the peripheral platelet count is very low, in the 'primary ITP' range. Intense history-taking and physical examination are required in every case, as well as screening tests to ensure that there are no concurrent diseases. Diseases which are definitely associated with thrombocytopenia include:

      Increased consumption
Acute severe hemorrhage (e.g. gastrointestinal bleeding, rodenticide toxicity)
DIC
Vasculitis (e.g. sepsis/endotoxemia; tick-borne diseases)

      Sequestration
Portal hypertension
Splenomegaly (barbiturate or acepromazine overdose; hypersplenism, splenic infarction)
Splenic torsion

      Decreased production
Myelophthisis (bone marrow infiltration by tumors)
Estrogen excess (Sertoli cell tumor; iatrogenic)
Myelofibrosis
Infections: Ehrlichia sp., Leishmania infantum; Histoplasma capsulatum; FeLV

      Increased destruction (Immune-mediated thrombocytopenia)
Drugs: sulfonamides, other antibiotics, anti-thyroidal drugs
Recent vaccination?
Neoplasia: lymphoma, malignant histiocytosis
Infectious diseases: Ehrlichia sp., Babesia sp.; RMSF; Bartonella sp.?, Other autoimmune diseases: SLE