IMHA – Immune Mediated Hemolytic Anemia

What is an immune-mediated disease? It is any disease where the clinical signs (fever, pain, etc) are caused by the immune system. The immune system normally acts to protect an animal from viruses, parasites, bacteria, and all sorts of other harmful germs. The immune system acts as a ‘search, attack, and destroy’ army, eliminating the countless germs that we get exposed to every day. Immune cells use some pretty strong ‘weapons’ in order to attack the germs. Sometimes these weapons can harm innocent bystanders- the animal’s own cells. So in an immune-mediated disease, the immune system thinks that it is protecting the animal, but is actually damaging body cells.

Immune-mediated hemolytic anemia (IMHA) is a condition where the immune system kills red blood cells. As explained before, the red blood cells are innocent bystanders. There are three types of IMHA: primary, secondary, and idiopathic. Primary IMHA is when the immune system becomes dysfunctional and begins to recognize red blood cells as ‘foreign’. Therefore, your body begins to think that it is being invaded by a germ (the red blood cells) and it works to attack and destroy these invaders.

Secondary IMHA is when the immune system recognizes something foreign that is in or on the red blood cell. In order to get rid of this germ, the immune system kills the red blood cell. Certain types of cancers, infections, and drugs can all potentially cause IMHA. Idiopathic IMHA simply means that the cause of the disease is unknown. Unfortunately, most cases of IMHA are idiopathic.

IMHA tends to be much more common in dogs than in cats. Some breeds are more predisposed to this condition. An animal with IMHA will show most of the same signs as a dog with any type of anemia (decreased number of red blood cells). Anemic pets will be lethargic and will have trouble exercising. This is because they do not have enough red blood cells to supply oxygen to their tissues. IMHA is not the only cause of anemia. Anemia can be caused by other problems, such as loss of red blood cells with excessive bleeding or decreased production of red blood cells in bone marrow disorders (like leukemia).

IMHA is a very bad condition. About half of IMHA dogs do not survive once they are hospitalized. It is a terminal prognosis- the animal will die if it is not treated and unfortunately treatment is very expensive. The pet will need to be hospitalized for at least a week. It is very common for these cases to be referred to a specialty clinic for blood transfusions. The animal will be on steroids for a few months after, in order to keep the immune system from continuing to attack the red blood cells. Treating your pet for IMHA can possibly cost you a few thousand dollars. However, most dogs that recover from IMHA never get it again. These animals tend to have great long-term survival rates. Cost is very often an issue for owners; treating IMHA is a very tough decision to make. This decision should always be made with your veterinarian.

There are other immune-mediated diseases, for example, immune-mediated thrombocytopenia. Thrombocytopenia means a decreased number of platelets. Platelets are blood components that are needed for clotting, so in this condition the animal can quickly bleed to death if not treated. Immune-mediated thrombocytopenia is also expensive to treat but has a better prognosis than IMHA.

Pets can also get immune-mediated arthritis, where the immune system causes inflammation of the joints. This is similar to rheumatoid arthritis in humans. Inflammatory bowel disease, also found in humans, is a fairly common immune-mediated disease. Another common one is flea allergy dermatitis, where the body over-reacts to the presence of fleas and can cause severe skin problems. There are many other immune-mediated diseases that can affect our pets.

Immune-mediated diseases appear to be getting more common. It could be simply because veterinarians are learning to better recognize these conditions. There are also thoughts that pets are being exposed to more harmful substances in their environment. Or maybe the immune-system is being over-stimulated with vaccinations. Perhaps there is too much inbreeding of purebred dogs. There are many guesses, but no answers.

An animal’s immune system is built to protect it from harm. But sometimes, that system is a little bit too efficient and harms body cells in the process. This is what is known as an immune-mediated disease. IMHA is an example of a life-threatening disease. The cause is often unknown and the treatment is expensive. It is one of many immune-mediated diseases that could potentially affect our pets.
By Ashley O’Driscoll – Pets.ca writer