Immune Mediated Hemolytic Anemia (IMHA) in Dogs & Cats

September 25, 2014 (published)

Your pet's immune system is an intricate machine with dozens of types of cells and hundreds of chemical messengers that allow it to determine what to fight off. It doesn’t always work the way it is supposed to. In some cases, an immune system decides that something harmless is dangerous and it goes into action, as it does with allergies. In other cases, your pet's immune system gets even more confused and decides that something normal in the body is harmful and tries to fight it off. These are autoimmune diseases, such as rheumatoid arthritis, lupus, and diabetes.

One of the more common autoimmune diseases that veterinarians see is immune-mediated hemolytic anemia, or IMHA, in which the immune system decides that red blood cells are harmful and starts destroying them. In most cases, the trigger is never known. The ‘hemolytic’ in the name refers to the process of popping, or lysing, red blood cells. Incidentally, onions, mothballs, and the zinc found in pennies minted after 1982 can all cause the same thing.

Some of the signs of IMHA include:

  • Pale or yellow gums
  • Extreme weakness and lethargy
  • Loss of appetite
  • Jaundice (yellow discoloration) of the whites of the eyes.

It can happen with remarkable speed – one day your dog is happily snoozing by the fire, the next day your veterinarian is telling you that he’s in a 50:50 battle for his life. The speed with which this occurs is breathtakingly rapid. An immune system reacts efficiently even to dangers that are not real.

IMHA and its close cousin Evan’s syndrome (in which red blood cells and platelets are both consumed) are not easily treated. The odds of survival are around 50:50. Doing battle with the immune system is not for the faint of heart or lean of wallet.  Many days in the hospital, multiple transfusions and, in some cases, medications costing thousands can all take their toll and many owners elect not to go ahead with therapy.

One of the worst consequences of IMHA is a blood clot that can travel to the lungs and is often fatal: a pulmonary thromboembolism, or PTE. In some cases that seem to be headed towards survival, involving blood transfusion after transfusion, a massive PTE can still occur.

Pets who survive are on medication for months (steroids such as prednisone are the cornerstone of therapy) and about one dog in five will relapse at some point.  Cats are comparatively resistant to IMHA; they can still get it, but it's common to see hundreds of canine cases and only a handful of cats.

There is an association between recent vaccination and the onset of IMHA, but that does not infer that vaccines cause IMHA.  It’s just not that simple. The association between the two is tenuous and somewhat unproven, and vaccines are still one of the most important tools available against dreaded diseases like parvovirus and distemper. We do, however, usually recommend that patients who have had IMHA not receive vaccinations ever again in the future, out of an abundance of caution.

Since most cases of IMHA are seen in middle-aged to older pets (females are slightly more likely to get autoimmune diseases in general; Cocker spaniels are the poster dog for IMHA) and vaccines have the greatest protective effects in the young, this recommendation does not usually mean that skipping a vaccine will open the door to potential diseases. Whatever you do, don’t skip vaccines in the hopes of avoiding IMHA, especially in a younger dog. Parvo has killed millions more dogs than IMHA and the vaccines are proven and effective in preventing it.

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