Immune Mediated Hemolytic Anemia & AutoImmune Hemolytic Anemia

Immune Mediated Hemolytic Anemia is defined as accelerated red blood cell destruction due to the presence of antibodies attached to red blood cell membranes. Often described as one of the most common immunohematologic disorders in dogs, IHMA occurs when the immune system recognizes red blood cells as foreign due to antigens attached to their membranes. These "sheep in wolves clothing" trigger an immune response that results in the production of antibodies to destroy the antigens. Therefore the red blood cells inadvertently get killed off and are just "innocent bystanders" since the attachment of the antibody/immune complexes to the red blood cell membrane destroys the antigen and red blood cell alike.

There are two major categories of Immune Mediated Hemolytic Anemia: With primary IMHA the antibody is directed against unaltered red blood cells, while with secondary IHMA the antibody is directed against red blood cells that have been antigenically altered by reaction with chemicals or microorganisms.

• Primary IMHA, aka AutoImmune Hemolytic Anemia (AIHA), occurs when an antibody is bound to unaltered internal red blood cell membrane antigens. AIHA occurs when self antigens are recognized and destroyed by the immune system.
• Secondary IMHA occurs when antibodies are bound to external antigens adherent to red blood cell membranes. Secondary IMHA can coexist or be triggered by another disease. Some secondary causes of IMHA are neoplasms, infections and parasites caused by ticks, sand flies and mosquitoes, and the administration of pharmaceutical agents such as drugs & vaccines.

The mechanism that triggers the immune system to react and create antibodies resulting in the premature destruction of red blood cells is poorly understood since every red blood cell undergoes changes that must trigger an immune response at the end of its life.

It is interesting to note that IMHA has been observed in dogs with increased antibody titers to viral antigens, which seems to indicate that recent vaccinations or viral infections may be implicated in those cases. An association between IMHA and recent vaccination (<30days) has been established, but the cause and effect relationship between IMHA and recent vaccinations hasn't been widely accepted by the veterinary community.

The seasonal incidence of many cases of IMHA commencing in the spring opens up the question of whether it may be correlated to seasonal environmental exposures such as fertilizer, weed killer, etc. or iatrogenic manipulations such as vaccinations and preventative medications. A study even concluded that bee sting envenomation can result in IMHA. Though seasonal incidence supposedly isn't a consistent finding and nothing has been proven, family members of both deceased and survivors have their suspicions.

While IMHA refers to cases of anemia occurring from immune based red blood cell destruction, AIHA refers to cases of IMHA when the underlying cause is unknown, aka idiopathic. Since the specific etiology of IMHA usually goes unrecognized, most cases are and will be classified as idiopathic until detailed studies of red blood cell membrane's antigens and antibodies against these antigens are performed. For this reason, IMHA and AIHA are often used interchangeably. Lilli was a true case of AIHA, so that is the term we will use in the remainder of our discussion.

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