Immune-Mediated Hemolytic Anemia

66 Immune-Mediated Hemolytic Anemia



Astrid Nielssen



1. What is immune-mediated hemolytic anemia (IMHA)?

IMHA is an antibody-mediated disease that results in the premature destruction of red blood cells. In most cases, the red blood cell destruction occurs primarily in the mononuclear phagocytic system within the spleen and liver, a process referred to as extravascular hemolysis. In some cases, when the antibody is capable of directly fixing complement, intravascular hemolysis can also occur.


2. What is the difference between primary and secondary IMHA?

As with immune-mediated thrombocytopenia, IMHA can be categorized as being either primary or secondary. Primary, or idiopathic, IMHA (to be referred to as autoimmune hemolytic anemia, AIHA, in the rest of this section) does not have an apparent etiology, is not associated with any underlying conditions, and arises when antibodies develop against red blood cell autoantigens. At its most basic level, AIHA is the result of the loss or dysfunction of mechanisms of normal immunologic tolerance. Secondary IMHA arises when an underlying associated condition or therapy triggers the immune-mediated red blood cell destruction. This can occur as a result of antigenic agents adsorbed to the red blood cell membrane (drugs, infectious agents, immune complexes). It can also arise when a condition or disease process alters the red blood cell membrane, either exposing new antigens or altering existing ones, so as to result in an immune response directed against those epitopes. Infection with agents that carry epitopes similar to those on the red blood cell membrane may also result in the production of an antibody that reacts with self-antigen (antigenic mimicry).


3. What are some of the causes of secondary IMHA?

Secondary IMHA can be seen as a reaction to certain drugs (penicillins, cephalosporins, trimethoprim-sulfadiazine, procainamide), toxins, infectious agents (ehrlichia, other bacterial infections), parasites (heartworm), neoplasia, and systemic autoimmune disease.


Neonatal isoerythrolysis, another form of IMHA, is rarely recognized in the dog, likely as the result of the absence of significant naturally occurring pathogenic red blood cell antigen antibodies. Neonatal isoerythrolysis results from the presence of passively transfused maternal antibodies with specificity for paternally determined red blood cell membrane epitopes. For this complication to arise in pups, the bitch would have to have been previously sensitized by a mismatched blood transfusion or antigen exposure during an earlier pregnancy or labor. It is because of the possible presence of red blood cell alloantibodies that bitches that have had litters, or dogs that have received prior blood transfusions, should not be used as blood donors.


4. Are certain dogs predisposed to AIHA?

A genetic influence predisposing to the development of this disease is strongly suspected. An increased incidence is reported in several breeds including Cocker Spaniels, English Springer Spaniels, Lhasa Apsos, Old English Sheepdogs, Poodles, and Shih Tzus. The Cocker Spaniel has been most consistently overrepresented in the literature. There is suspected to be an increased incidence in female dogs, and although the disease can happen in dogs of any age, the mean age at presentation is approximately 6 years.


5. What are the most common clinical signs of IMHA?

Clinical signs of IMHA can include weakness, lethargy, anorexia, pale or icteric mucous membranes, tachycardia, tachypnea, splenomegaly, and possibly fever. Owners may report having noticed discolored urine (from either bilirubinuria or hemoglobinuria) or stool (as a result of increased bilirubin excretion).


6. What abnormalities are typically noted on the minimum database (complete blood count serum, biochemistry profile, and urinalysis) from an animal with IMHA?

Typical hematologic findings include a regenerative anemia with reticulocytosis, spherocytosis, anisocytosis, polychromasia, and a reactive leukocytosis with a neutrophilia and a left shift. Thrombocytopenia is also commonly seen and can be due to concurrent immune-mediated thrombocytopenia or to increased platelet consumption due to thrombosis or disseminated intravascular coagulation. Abnormalities commonly noted on the serum biochemistry profile include elevations in the total bilirubin and liver enzyme levels. The urinalysis typically reveals bilirubinuria. Other abnormalities associated with either underlying disease conditions or clinical conditions may also be noted.

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Jul 31, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Immune-Mediated Hemolytic Anemia

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