In-depth Information on Immune Mediated Hemolytic Anemia in Dogs (IMHA)
Immune mediated hemolytic anemia is exactly what the name implies. “Anemia” is a deficiency of red blood cells, and may result from many causes including bleeding, failure to produce enough new red blood cells, or destruction of existing red blood cells. “Hemolysis” refers to the lysis, or destruction, of the red blood cells (“heme” is an essential component of red blood cells that allows them to carry oxygen). The term “immune mediated” simply states that in these cases the process of red blood cell destruction is carried out by the immune system.
The immune system is a complicated network of cells and products that are secreted from cells. In a healthy animal, these cells and their products recognize germs as being foreign, and they attack and destroy those germs. The immune system is designed to recognize the animal’s own cells as being harmless, and to refrain from attacking the animal’s own healthy cells. When an animal develops immune mediated disease, the immune system destroys the animals own cells rather than just germs. Sometimes the attack on the animal’s own cells is accidental, and sometimes it is purposeful. A purposeful attack is said to be an “autoimmune” process. In those cases, the immune system thinks the animal’s own cells are foreign, and attempts to destroy them. This destructive process may be directed against many different cell types, but when the cell type under attack is the red blood cell, immune mediated hemolytic anemia is the result.
The development of immune mediated disease is complicated and poorly understood. In some cases a trigger can be identified that may have precipitated the misdirection of the immune system, but in most cases, such a trigger is never found. IMHA, like most such diseases, occurs more often in females than in males. Young adult to middle aged animals are most likely to be affected, and dogs develop the disease much more frequently than do cats. While any dog may develop IMHA, cocker spaniel, Springer spaniel, miniature poodle, Finnish spitz, Irish setter, bichon frise and Old English sheepdog are more likely than most to be affected.
Immune mediated hemolytic anemia is a rapidly life-threatening disease. With severe anemia of any cause, the tissues are unable to receive adequate oxygen. In cases of IMHA, destruction of red cells results in a sudden, and often very severe, decrease in red blood cell numbers. Although there is usually a substantial increase in the number of new red blood cells produced within the bone marrow, production of new cells cannot keep up with the rapid destruction of cells. Unless the immune system’s attack on the red cells can be curbed, the animal will die. Swift treatment may stop the attack, allowing the newly made red blood cells to replace those that were destroyed. Unfortunately, it is not always a simple matter to stop the immune attack, and there are many potential complications of IMHA. Although many animals treated for IMHA go on to live full lives, even those who receive appropriate therapy may succumb to the disease.
There are different forms or subtypes of IMHA. They are most commonly referred to as primary, secondary, intravascular and extravacular.
Differential Diagnoses (Other Causes of Anemia) in Dogs
It is crucial that the diagnosis of IMHA be confirmed, because there are many causes of anemia other than IMHA. Both treatment and prognosis for these other causes are often quite different that that of IMHA. Other potential cause of anemia include: