Immune-mediated Hemolytic Anemia: Review and Update on Splenectomy as Adjunctive Therapy

Introduction: Immune mediated hemolytic anemia (IMHA) is one of the most common immune mediated conditions in dogs treated in both the primary care and referral settings. Despite decades of knowledge and research on IMHA, it remains to be a frustrating and very deadly disease with reported mortality rates range from 22-70%. Although the highest mortality rates were seen in the earlier studies, the frequency of poor outcomes is still unacceptably high. Most veterinarians are familiar with the foundations of IMHA treatment, mainly immunosuppression with corticosteroids and thromboprophylaxis, however, it is also important for veterinarians to be aware of adjunctive therapies that may need to be utilized in more severe or refractory cases. IMHA results in hemolysis of red blood cells that are targeted by the body’s own immune system. The complexity of this immune mediated process is beyond the scope of this presentation. Simplistically, red blood cells are coated with immunoglobulin, complement, or both, leading to hemolysis. The hemolysis may occur intravascularly or extravascularly, although both routes are occurring simultaneously in most cases. Indicators of intravascular hemolysis include free hemoglobin in the plasma or urine. Clinically these can be identified by finding hemolyzed or red colored plasma after centrifugation, and red colored urine (pigmenturia) that does not separate out with centrifugation. Extravascular hemolysis is mediated by red cell removal via macrophages within spleen and liver. Extravascular hemolysis results in icterus, or bilirubin found in the plasma or urine. IMHA may also be classified as either a primary or secondary process. This talk will mostly focus on primary or idiopathic IMHA, in which no identifiable trigger is found. Certain secondary triggers should always be ruled out including any neoplasia, infections, and drug reactions. The most commonly implicated infections are the tick-borne, including Erlichia spp., Anaplasma spp., and Babesia. Reported drug-related causes of secondary IMHA include cephalosporins, sulphonamides, and carprofen. The discontinuation of these drugs may resolve the IMHA. Vaccination was associated with IMHA in an early study, however, this has not been confirmed by more recent reports.