Veterinary Medicine - November 2007 - (Page 724)

CE OPEER-REVIEWED Is it immune-mediated neutropenia? Since no test definitively identifies an immune-mediated neutropenia in dogs and cats, diagnosing this condition and treating it appropriately rely on your excluding the common causes of neutropenia, such as infections and drug reactions. Belle Marie D. Nibblett, DVM Anthony P. Carr, Dr. med. vet., DACVIM (small animal internal medicine) N eutrophils are a part of the granulocytic cell line and play an essential role in initiating an immune response to invading pathogens, such as bacteria, and releasing chemotactic substances that attract other inflammatory cells, such as monocytes.1 Neutropenia (< 3,000 neutrophils/µl in dogs, < 2,500 neutrophils/µl in cats) as a solitary laboratory finding is often not associated with clinical signs and may be detected on routine screening of otherwise apparently healthy patients. Alternatively, neutropenic dogs may present with a fever or lethargy or other signs associated with infection. Prompt recognition and treatment are vital because if the neutropenia is not resolved, infections may be persistent or recurrent and may result in death.2-6 Immune-mediated neutropenia, also called steroid-responsive neutropenia, is a primary disorder or may occur secondary to infection, neoplasia, or the effects of certain drugs. It results when antibodies directed against neutrophil surface antigens or potentially against growth regulators of Belle Marie D. Nibblett, DVM Anthony P. Carr, Dr. med. vet., DACVIM (small animal internal medicine) granulopoiesis develop.7,8 It can occur concurrently with other immunemediated diseases, such as rheumatoid arthritis or systemic lupus erythematosis in dogs and people.9-11 Immunemediated neutropenia is a well-recognized cause of neutropenia in people, primarily affecting infants.12 Much of the available information about immune-mediated neutropenia in animals is extrapolated from the disease in people.12 Immune-mediated neutropenia is a relatively uncommon cause of neutropenia in cats and dogs, and recognizing it is challenging. It can only be identified by understanding the pathophysiology of neutropenia in general and ruling out its differential diagnoses. liferative disorders, and lymphoproliferative disorders • Increased consumption or destruction of neutrophils as in immunemediated destruction or severe infection in which use or destruction is in excess of bone marrow production and release of neutrophils. These causes of neutropenia are not mutually exclusive, and multiple mechanisms are frequently concurrent. Immune-mediated neutropenia represents the least common cause of neutropenia and results primarily from increased destruction of neutrophils when they become coated with antineutrophil antibodies.14 The spleen also sequesters antibody-coated blood cells, so increased margination may concurrently be present (Felty syndrome: neutropenia, rheumatoid arthritis, and splenomegaly). PATHOPHYSIOLOGY OF NEUTROPENIA Several mechanisms of neutropenia must be considered before diagnosing immune-mediated neutropenia.13 Neutropenia can occur because of • Increased margination or egress of neutrophils from the circulating pool to the marginated pool as may occur with infection, immune-mediated disorders, or hypersplenism • Decreased production resulting from bone marrow injury, such as that caused by certain drugs, or infiltration as in myelophthisis (normal hemopoietic tissue is replaced with fibrous tissue or a nonhemopoietic tissue such as carcinoma), myelopro- DIFFERENTIAL DIAGNOSES FOR NEUTROPENIA Neutropenia can occur for a variety of reasons including infections, drug reactions, and bone marrow and genetic diseases. Infections Overall, infections with bacteria, viruses, and fungi are the most common causes of decreased neutrophils. In the case of endotoxemia, sepsis, or localized infections, increased margination may be noted. If the infectious process is overwhelming, increased destruction Department of Small Animal Clinical Sciences Western College of Veterinary Medicine University of Saskatchewan Saskatoon, SK S7N 5B4 724 November 2007 VETERINARY MEDICINE

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Veterinary Medicine - November 2007 Contents Leading Off The Veterinary Medicine Interview: Dr. Lila Miller Clinical Exposures Idea Exchange A Challenging Case: Uveitis and Secondary Glaucoma in a Cat Is it Immune-Mediated Neutropenia? Pimobendan Treatment in Dogs with Congestive Heart Failure CE Form Product Preview Advertiser Index Marketplace/Classifieds Mind Over Miller Special Supplement by Lilly: Flea Bite Facts Q&A

Veterinary Medicine - November 2007

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