Hemolytic anemia can result from infection, toxin/drug exposure, hypophosphatemia, hereditary deficiencies, and immune-mediated processes. Immune-mediated hemolytic anemia (IMHA) is a type-II immune reaction to red blood cell (RBC) surface antigen that activates the complement cascade, antibody-dependent cytotoxicity, and/or facilitated phagocytosis by the spleen and liver. Destruction of the RBC by the spleen and liver is IgG-mediated, and termed extravascular hemolysis. Destruction of RBC within the vascular space is IgM-mediated and termed intravascular hemolysis. IMHA often occurs in conjunction with a thrombocytopenia.

The underlying cause of the immune activation is not usually identified; however, it has been associated with secondary causes such as infection, neoplasia, drugs, vaccines, incompatible transfusions, and neonatal isoerythrolysis, or primary (auto-immune) causes. IMHA is diagnosed more commonly in dogs than cats, and cats more commonly have secondary IMHA.

Significant anemia results in decreased oxygenation of the tissues and organ dysfunction and acute clinical signs related to hypoxemia, including weakness, pale mucous membrane color, prolonged capillary refill, increased heart rate, bounding pulses, increased breathing rate or effort and fever. Hemolysis may also result in jaundice and orange/red colored urine. Occasionally, vomiting is described, and might represent gastrointestinal hypoxemia. If a thrombocytopenia accompanies hemolysis, petechia/ecchymosis, hematemesis and/or blood in the stool may also be seen.

Supplemental oxygen should be administered, a peripheral IV catheter placed and blood collected for an emergency laboratory database (packed cell volume [PCV], total protein [TPr], serum color, electrolyte panel, venous blood gas and lactate level). A hemolytic syndrome is suspected if there exists an anemia with a normal TS, and yellow (icteric) or pink-red (hemolysed) serum color. It is essential that a hemolytic anemia without an immune-mediated cause be ruled out before making a diagnosis of IMHA and instituting immunosuppressive treatment.

Additional laboratory analysis that supports an IMHA diagnosis includes evaluation of a blood smear (red blood cell destruction and regeneration, spherocytes [dog], or ghost cells [cat]) and a saline agglutination test. Macro­ and microagglutination requires a saline-agglutination microscopic exam. Cells may stick together without an immune-mediated process and cause the appearance of false macroagglutination. A drop of blood directly from phlebotomy is mixed with one drop of saline, and the sample examined microscopically. Rouleaux is the configuration where RBC stick together appearing like a 'stack of coins' but disperse when mixed with saline. True microagglutination occurs when the RBCs cluster together like grapes that do not disperse when mixed with saline.

Blood samples are submitted for a complete blood count, reticulocyte count, serum biochemical profile, tick-related and other infectious screening (e.g., feline leukemia virus, feline coronavirus, haemoplasmosis), and thyroid panel. Thoracic radiographs and abdominal ultrasound are used to evaluate for neoplastic lesions, or other systemic diseases that may be stimulating the immune system.

Reticulocytosis may or may not be present depending on when RBC destruction was activated. Outside of the ER setting, a positive direct antiglobulin test (DAT; Coombs test), differentiating Coombs test, or RBC surface-associated immunoglobulin test may help support a diagnosis when autoagglutination is not easily identified, but these tests do not differentiate primary from secondary IMHA, and they can produce positive results from non-lMHA-related causes (e.g., corticosteroid use). Additional diagnostic evidence for IMHA includes erythrophagocytosis on splenic or bone marrow samples.

Successful treatment of severe cases may require 3–5 days of hospitalization before the overactive immune system is sufficiently suppressed after starting immunosuppressive medication. The patient may benefit from fluid replacement, transfusion, anticoagulation, and nutritional support. Prior to making the decision to treat, the client must be made aware of the need for chronic medication (up to or greater than 6 months), their potential side effects, and follow-up care.

In most cases, intravenous fluid therapy is indicated to replace preexisting or ongoing fluid losses and to promote rheology. Judicious fluid support is provided if the patient is anemic but euvolemic. Packed red blood cell transfusion may be necessary if clinical signs of severe anemia exist, including tachypnea, tachycardia, weakness, etc. The absolute PCV a patient will show clinical signs is variable and the need for transfusion determined on a case by case basis. In general, once the PCV falls below 15%, a transfusion is administered. It is ideal to confirm blood type of the patient using immunochromatographic methods (Abaxis/DMS) and transfuse with the same type blood. Erroneous results can be obtained with card blood typing methods in extremely anemic animals (PCV<10%). It is recommended that all cat s have a crossmatch, and dogs having had a previous transfusion have a crossmatch to identify the potential for a transfusion reaction. However, autoagglutination will mask a true allogenic reaction and may not be reliably assessed. Fluid administration rates depend on the severity of clinical signs related to the anemia and/or the presence of hypovolemia. In general, the amount of blood administered is enough to increase the PCV to an acceptable level for reestablishing adequate tissue oxygenation without causing fluid overload, usually around 20–25%, understanding that ongoing destruction may require additional infusion.

The amount of reconstituted pRBC that is needed can be calculated as follows:
2 x Desired PCV(%) x Body weight (kg)

Definitive treatment of IMHA requires treating any underlying disease processes and suppression of the immune system. The most common immunosuppressive medication used is prednisone (2 mg/kg q24h, up to 40 mg/dog; 2–4 mg/kg q24h in the cat). Dexamethasone (0.3–0.5 mg/kg IV) can be substituted for prednisone as an injectable corticosteroid during induction of immunosuppression and if the patient is unable to tolerate oral medication. Common side effects of this drug include increased water intake, urination and increased appetite as well as excessive panting. Occasionally, gastrointestinal ulcer formation can occur. If the side effects of prednisone become intolerable or severe gastrointestinal ulceration occurs, alterations in the dose and frequency may be made necessary. Corticosteroids should never be acutely discontinued if treatment has been long enough to cause adrenal suppression.

Other immunosuppressive medications can be used to allow earlier reduction of prednisone dose, and potentially improve controlled immunosuppression. In our practice, we combine prednisone with azathioprine (2 mg/kg q24h) in all canine cases of IMHA, and add cyclosporine A (5 mg/kg IV or PO q24h) if repeated transfusion necessary after 48 hours. In smaller patients, it may be easier to dose cyclosporine A than azathioprine. In rare cases, mycophenolate (10 mg/ kg PO q8h) is used when azathioprine and cyclosporine have not provided adequate immunosuppression after 5–7 days. Vomiting and bone marrow suppression are potential side effects in addition to increased risk for infection. In cats, prednisolone is usually effective without the addition of cytotoxic drugs.

Results of infectious disease testing may not be readily available; therefore, it is reasonable to treat for regional tick-borne infections with doxycycline, pending results. Concurrent long-term therapy with antimicrobials is not indicated without evidence of infection, as this practice promotes antimicrobial resistance.

RBC destruction also results in liberation of cell membrane phospholipid and additional microparticles, which are potent initiators of endothelial damage, inflammation and coagulation. This can result in both arterial and venous thrombosis, problems that more commonly manifest after identification of IMHA and initiation of treatment. Anticoagulation using platelet inhibitors (e.g., clopidogrel 1–5 mg/kg q24h PO and/or aspirin [0.5 mg/kg q24h PO]) and coagulation factor inhibitors such as unfractionated heparin (UFH), or low molecular weight heparin (LMWH; enoxaparin, dalteparin) may also be considered. Fixed doses of UFH (150 IU/kg SC q6–8h) in dogs with IMHA appear to convey variable and inadequate therapeutic levels to sufficiently suppress Factor Xa activity. Individual adjusted dosing based on weekly monitoring of Factor Xa activity appears to provide more reliable plasma heparin levels, and may be associated with reduced mortality. LMWH have not been yet shown to be reliable and easy to dose in ill dogs and cats and/or are relatively expensive. Rivaroxaban, an oral, direct Factor Xa inhibitor, may prove to be useful in the future. Factor Xa analysis is only available through a single lab, Cornell University Comparative Coagulation Lab. Platelet inhibitors and heparin should be avoided if a thrombocytopenia is already present. Treatment is generally required for several months, and medication is slowly tapered based on laboratory test results.

Although the risk of immunosuppressive treatment is less than the effects of IMHA, life-threatening side effects of the medication can occur. Splenectomy in the case of refractory IMHA has been described in a small number of dogs. Human IVIG has also been used in cases of canine IMHA, but has not been proven to be more effective than immunosuppression alone in the small number of cases reported. This contrasts with its use in presumed immune-mediated thrombocytopenia, where there may be a faster recovery, decreased need for transfusion, and decreased hospital stay with human IVIG administration.

Prognosis of IMHA is variable and unpredictable. Fatalities are reported in 20–70% of cases, most occurring within the first 2 weeks of initiating treatment. Thrombosis has been identified in up to 80% of cases at postmortem. Success of treatment depends on the underlying disease process, organ dysfunction, response to therapy, or side effects to treatment, and rapid identification and institution of therapy in the ER may make a difference.

References

References are available on request.