Veterinary Medicine - December 2008 - (Page 663)

Immune-mediated hemolytic anemia PEER-REVIEWED cellular mechanisms by which an agent can cause autoimmunity can include infection of immune cells, activation of lymphocytes from exposure to cytokines, cross-reactivity between microorganisms and host tissues, and the production of drug-speci c antibodies with the formation of immune complexes.2 The loss of self-tolerance, resulting in autoimmunity, is also a mechanism by which hemolytic anemia can occur. A recent report linked the class III antiarrhythmic agent amiodarone with a positive Coombs test result in two dogs. One of these patients developed hemolytic anemia, the other developed thrombocytopenia.5 Another report discussed intravascular hemolysis associated with a patent ductus arteriosus coil embolization in a dog.6 Examples of possible underlying conditions causing secondary IMHA are listed in Table 2 and include infections, drugs, and neoplasia, as well as other immunerelated disorders. Secondary IMHA is the common type of immune-mediated hemolysis in cats.1 In certain infectious diseases, such as mycoplasmosis and babesiosis, hemolysis caused by the organism is exacerbated by the body’s own immune response. If you suspect an infectious disease, administer antibiotics pending the results of antibody titers or polymerase chain reaction (PCR) testing. Additionally, to reduce the body’s response to the infectious agent, immunosuppressive medications often need to be administered. Once an underlying infectious agent is identi ed, the immunosuppressive medications can be used at lower doses and for an abridged treatment course. Rules of thumb for diagnosing and treating IMHA in dogs and cats These rules of thumb should help you avoid some of the common mistakes that occur in managing IMHA. • Work up patients for underlying disease. It is essential to work up patients for an underlying disease. Immunosuppression will worsen the condition if an underlying disease is not addressed. • In unusual cases, search for inherited abnormalities. In patients that seem to have an unusual presentation (e.g. young cats and dogs of particular breeds), consider researching and testing for the possibility of inherited RBC membrane and enzyme abnormalities. • Don’t blame lead. Lead toxicosis is usually not associated with hemolysis but instead is associated with a large number of nucleated RBCs and gastrointestinal and neurologic signs. • Taper drugs slowly. Often, clinicians taper immunosuppressive drugs too quickly, predisposing patients to relapse. Tapering drugs at three- to four-week increments appears to be adequate in most ﬁrst-time cases of primary IMHA. • Be patient with bone marrow disease. In cases in which autoimmunity is targeted at the bone marrow, it can take two months or more for a patient to respond to immunosuppressive medications. • Evans syndrome treatment resembles IMHA treatment. Evans syndrome is an immune-mediated disease directed toward erythrocytes (IMHA) and thrombocytes (immune-mediated thrombocytpenia). Similar underlying conditions that can trigger IMHA also trigger immune-mediated thrombocytpenia. The diagnostic work-up and treatment options are generally the same as in IMHA, except that aspirin and heparin must be avoided.1 • Avoid immunosuppression in toxin- and drug-related cases. Toxinand drug-related causes of hemolytic anemia (e.g. onion and zinc toxicosis) are often confusing because spherocytes are seen on a blood smear, the hallmark of immune-mediated disease. However, in general, immunosuppressive drugs are not used to control these conditions.2 • Rule out microangiopathic causes. Microangiopathic anemia involves any condition in which erythrocytes are physically damaged. Conditions such as heat stroke, splenic disease, liver disease, hemolytic-uremic syndrome, disseminated intravascular coagulation, and hemangiosarcoma and other tumors can all lead to RBC fragmentation and hemolysis. Generally, these diseases cause a mild anemia and are diagnosed by identifying the underlying disease and schistocytes (small, misshapen erythrocyte fragments) on blood smears. • Don’t confuse severe illness with a poor prognosis. Patients may present with a host of clinical signs that suggest a poor prognosis, yet every animal is an individual. How a patient responds to treatment may be a better indicator of whether the patient has a chance at a favorable outcome, rather than the severity of clinical and laboratory signs at presentation. REFERENCES PATHOPHYSIOLOGY IMHA is a type II hypersensitivity reaction in which RBCs are destroyed by the body’s own immune system. The process typically involves a breakdown of immune self-tolerance and the production of anti-erythrocyte antibodies, IgG, or IgM. These antibodies are often recognizing and reacting to RBC membrane glycoproteins.7 Erythrocyte destruction 1. Scott MA. Immune-mediated thrombocytopenia. In: Feldman BF, Zinkl JG, Jain NC, eds. Schalm’s veterinary hematology. 5th ed. Philadelphia, Pa: Lippincott, Williams & Wilkins, 2000;478-486. 2. Desnoyers M. Anemias associated with Heinz bodies. In: Feldman BF, Zinkl JG, Jain NC, eds. Schalm’s veterinary hematology. 5th ed. Philadelphia, Pa: Lippincott, Williams & Wilkins, 2000; 78-184. VETERINARY MEDICINE December 2008 663

Table of Contents Feed for the Digital Edition of Veterinary Medicine - December 2008

Veterinary Medicine - December 2008 Contents Letters Thanking Our Valued Reviewers Practical Matters Idea Exchange Why Do Dogs and Cats Eat Grass? Managing Atypical and Critical Cases of Primary Hypoadrenocorticism in Dogs Behaviors Suggestive of Postoperative Pain in Cats What to Consider When Considering Digital Radiography Avoid Common Bandaging Mistakes in Dogs and Cats IMHA: Diagnosing and Treating a Complex Disease CE Form/Advertiser Index Marketplace/Classifieds Mind Over Miller

Veterinary Medicine - December 2008

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