Veterinary Medicine - August 2007 - (Page 527)

OPEER-REVIEWED CE Overcoming the diagnostic and therapeutic challenges of canine immune-mediated thrombocytopenia Determining that the immune system is the cause of a dog’s decreased platelet count can be difficult. These clinicians walk you through the diagnostic and treatment process so you can help patients with this life-threatening bleeding disorder. Johanna Heseltine, DVM, MS, DACVIM Anthony Carr, Dr. med. vet., DACVIM (internal medicine) P latelets play an essential role in normal hemostasis. In primary hemostasis, platelets bind to the damaged vessel wall, forming an initial platelet plug. The platelet plug is then stabilized by fibrin, which is generated through the activation of clotting factors in secondary hemostasis. Immune-mediated thrombocytopenia is the most common acquired cause of abnormal primary hemostasis in dogs.1 It involves antibodies binding to Johanna Heseltine, DVM, MS, DACVIM* Department of Small Animal Clinical Sciences College of Veterinary Medicine Oklahoma State University Stillwater, OK 74078 Anthony Carr, Dr. med. vet., DACVIM (internal medicine) the surface of platelets, causing premature platelet destruction by macrophages.2 Besides increased platelet destruction, other non-immune-mediated mechanisms of thrombocytopenia include decreased platelet production by the bone marrow, platelet sequestration (primarily in the spleen), and increased platelet consumption or loss.3 Immune-mediated thrombocytopenia is often diagnostically and therapeutically challenging. Understanding the underlying pathophysiology and principal differential diagnoses for thrombocytopenia in dogs is vital to develop an appropriate diagnostic and therapeutic approach. PATHOPHYSIOLOGY Platelets are produced from the fragmentation of bone marrow megakaryocytes and are then released into the circulation where their average life span is eight to 12 days. Aged platelets are removed from the circulation by tissue macrophages, especially in the spleen.1,4 Immune-mediated thrombocytopenia is mainly a disorder of accelerated platelet destruction.2 In this disease, antibodies bind to platelet membranes, enhancing platelet clearance by the mononuclear phagocytic system. This process usually occurs in the spleen although the liver also plays a role.1,2 Thrombocytopenia develops as Department of Small Animal Clinical Sciences Western College of Veterinary Medicine University of Saskatchewan Saskatoon, SK S7N 5B4 Canada *Dr. Heseltine’s current address is Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, SK S7N 5B4, Canada. platelet destruction exceeds compensatory production by the bone marrow.1 Antiplatelet antibodies are directed against normal platelet surface antigens.2 Platelet integrin alphaIIBbeta3 (formerly glycoprotein IIb/IIIa) on the platelet surface is highly immunogenic and is thought to be the most frequent target antigen.5 Platelet-surface-bound antibodies in dogs with immune-mediated thrombocytopenia are frequently immunoglobulin G (IgG).2 Antiplatelet antibodies may also alter platelet function and further compromise primary hemostatic function.1,5 However, the clinical relevance of this altered platelet function is unclear since immature platelets have increased hemostatic function that may compensate for the platelet dysfunction.1 Immune-mediated thrombocytopenia can be classified as primary or secondary.1 Primary immune-mediated thrombocytopenia Primary immune-mediated thrombocytopenia has also been called idiopathic thrombocytopenic purpura. In this spontaneous autoimmune disorder, autoantibodies are directed against specific portions of the platelet membrane. It occurs in the absence of an underlying disease1,2 and can only be diagnosed once causes of secondary VETERINARY MEDICINE August 2007 527

Table of Contents Feed for the Digital Edition of Veterinary Medicine - August 2007

Contents A Veterinary Medicine Interview Veterinary Voices Idea Exchange Diagnosis and Treatment of Solar Dermatitis in Dogs Diagnostic Imaging for Linear Foreign Bodies in Cats Overcoming the Diagnostic and Therapeutic Challenges of Canine Immune-Mediated Thrombocytopenia Ophthalmic drugs: What’s new? CE Form Product Preview Advertiser Index Marketplace/Classifieds Mind Over Miller

Veterinary Medicine - August 2007

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