Immune Mediated Hemolytic Anemia (IMA, IMHA)
Immune mediated hemolytic anemia (IMA, IMHA) is the destruction of normal red blood cells (RBC) or destruction of RBC with foreign antigens on surface. IMA or IMHA can occur in any age patient but young adults are most commonly affected. Females are affected more often than males. Cocker spaniels appear to be over represented. IMA may occur alone, or with other immune disorders. When both RBC and platelets are destroyed the disease is called Evans syndrome.
There are several types of IMA. The prognosis may be influenced by the type of hemolysis. The most common type of hemolytic anemia is the extravascular removal of RBC that have antibodies on their surface. The antibody-coated RBCs are removed by the mononuclear phagocytic system in the spleen, bone marrow and liver. The antibody on the RBC may be directed against normal components of the RBC in which the anemia is called primary or the antibody may be directed against a non-RBC antigen "stuck" on the cell surface. The antigen may be a virus, part of a drug or part of a neoplastic cell. This form of hemolytic anemia is called secondary. Most often the antigen on the RBC surface can not be identified. The involvement of foreign antigens is often speculative based on history. Animals with extravascular hemolysis DO NOT have hemoglobinemia or hemoglobinuria but icterus and bilirubinuria can be present depending upon the rate of hemolysis and ability of the liver to handle the bilirubin generated from degraded RBC. Some of these patients will have intravascular agglutination of RBC.
Intravascular hemolysis (The destruction of erythrocytes) occurs when complement-fixing antibodies on RBC cause RBC destruction (lysis) in the blood stream and release of free hemoglobin into the blood stream. The released hemoglobin is bound to a carrier protein, haptoglobin. When all the haptoglobin is bound to hemoglobin, any free (unbound) hemoglobin will cross the glomerulus and lead to hemoglobinuria (The presence of free hemoglobin) in the urine, indicating hemolysis of erythrocytes within the vascular system). Hemoglobinemia (red plasma) and hemoglobinuria (red urine) are observed in patients with intravascular hemolysis. These patients are at high risk for development of DIC, thrombosis and renal failure from cell products released from lysed RBC.
There are two rare conditions in which antibodies against RBC are only active at reduced temperatures. One type results in anemia and skin lesions (ischemia); only in cold weather as the RBC agglutinate in the cool parts of the body (extremities) and impair blood flow. The other form is characterized by anemia and hemoglobinuria in cold weather.
SIGNS
Pale mucous membranes are usually observed. Anemia may cause weakness or depression. Fever is variable. Jaundice can be present in patients with either intravascular or extravascular hemolysis. Enlargement of the liver or spleen are variably present.
DIAGNOSIS
IMA is suspected when a patient is best on signs and clinical tests. Because IMA can be secondary to other diseases (infections, tumors, drug reactions) or may be a part of Systemic lupus erythematosus (SLE), a complete evaluation of the patient is indicated. A complete history may yield clues as to triggering factors such as drug administration or travel to areas where infectious agents such as Babesia or Ehrlichia are present. Evaluation might include complete blood count (CBC), urine tests, biochemical screen, radiographs and abdominal radiographs or ultrasound to assess exposure to infectious agents. If multiple organs appear to be affected, a test is indicated to evaluate for SLE.
TREATMENT
Treatment involves removal of the cause if known, supportive care and the administration of immune suppressive drugs. Supportive care should include fluid therapy if the animal is not eating and drinking. Blood transfusions may be necessary if the animal is showing signs of respiratory distress (hypoxia) at rest.
Corticosteroids , usually prednisone or prednisolone are most commonly used in the treatment of hemolytic anemia. Immunosuppresive doses of prednisone are ~ 1 to 2 mg/kg every 12 hours. Corticosteroids suppress destruction of RBC and may have an effect on antibody production. If the patient responds favorably, the dose of prednisone is gradually tapered when the volume of packed red blood cells (PCV) stabilizes. The entire course of therapy is often several months. Some patients will relapse as the dose of prednisone is reduced and may need to be maintained on the lowest dose required to maintain an acceptable PCV. Failure to respond to prednisone or unacceptable side effects of prednisone may necessitate the use of other immune suppressive drugs such as cyclophosphamide, azathioprine or cyclosporine.
Sources:
1. Good Dogkeeping by Diane Morgan
2. Diet-Related Risk Factors for Gastric Dilatation-Volvulus in Dogs of High-Risk Breeds
Malathi Raghavan, DVM, PhD, Nita Glickman, MS, MPH, George McCabe, PhD, Gary Lantz, DVM and Lawrence T. Glickman, VMD, DrPH
Go Pets America recommends seeking the advice of your local veterinarian for the most appropriate vaccination program and for the diagnosis and treatment of your pet's health problems. For vaccination requirements please contact your state and local licensing authorities.
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