Immune-Mediated Anemia in Dogs

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Immune-mediated hemolytic anemia (IMHA) is a condition during which the body attacks and destroys or removes red blood cells from circulation abnormally. The immune system is not working properly.

Normal Red Blood Cell Lifecycle

Normally, a red blood cell is produced in the bone marrow and released into circulation to carry oxygen to all the body's tissues. As the cell ages, it gathers antibodies onto its surface which eventually mark it for destruction by the body's immune system in the spleen, liver, or bone marrow when it is old and no longer functions properly.

Once broken down, the iron within the red blood cell goes to the liver to be used in the formation of bilirubin, which is a substance that helps with the digestive process.

If parasites, bacteria, or viruses attach themselves to red blood cells, it also marks them for removal.

But when too many cells are being marked for removal for some reason, anemia and other serious health issues begin to occur. The dog become weak, has too much bilirubin production, so that the substance becomes toxic to the body, and the abnormal blood cells can become sticky and form life-threatening clots.

Causes of Canine Immune-Mediated Anemia

So why would the body start marking and destroying too many red blood cells (IMHA)? There are several processes that can trigger this. Immune-mediated hemolytic anemia can be primary or secondary.

During primary IMHA, the red blood cells themselves are normal and have been targeted by the body in error. Causes of primary IMHA include:

• Autoimmune hemolytic anemia
• Systemic lupus erythematous
• Dysregulated immune system
• Idiopathic (unknown cause—in dogs, this accounts for up to 80% of IMHA cases)

During secondary IMHA, the dog's red blood cells are abnormal for some reason, and that is what triggers the body to attack them. Causes of secondary IMHA include:

• Infectious agents
• Heartworm disease, GI cancer, DIC
• Cancer
• Drug reactions
• Allergic reactions

Cocker spaniels and miniature Schnauzers are diagnosed with IMHA more often than other dog breeds.

Signs of IMHA in Dogs

Signs that a dog may be suffering from IMHA can include:

• Weakness
• Lethargy
• Decreased or absent appetite
• Difficulty breathing and/or fast breathing
• Exercise intolerance
• Collapse
• Vomiting
• Diarrhea
• Tarry, dark stools or "coffee ground" stool
• Pale mucous membranes
• Yellow tinge to skin and/or urine
• Heart murmur
• Dark red spots or splotches in the gums or on the skin

Diagnosis of Immune-Mediated Anemia in Dogs

When a dog has anemia, the veterinarian must determine whether it has been caused by IMHA or something else. For instance, blood loss (externally or internally) or conditions that affect red blood cell production in the bone marrow can also cause anemia, or low red blood cell count.

To determine whether an anemia is being caused by IMHA, lab testing will be done on the red blood cell count to determine whether the body is responding to the anemia by producing more red blood cells. That's done by looking at a sampling of the cells under the microscope and determining whether an overabundance of them are very young cells (reticulocytes).

The veterinarian will also look at a blood smear to search for blood parasites that could be causing the IMHA.

Blood chemistry tests are done to determine the dog's concurrent kidney, liver, and pancreatic function. High bilirubin values in the blood and urine lend support for a diagnosis of IMHA.

A Coombs' test can be done to determine whether there are antibodies on the surfaces of the red blood cells that are marking them for destruction by the body. This is not a 100% accurate test for IMHA, but it does aid in the diagnosis.

X-rays may show evidence of other conditions going on or enlarged spleen and liver during IMHA.

A bone marrow sample can be taken to determine whether there is overproduction of young blood cells.

Treatment of Canine Immune-Mediated Anemia

A dog with severe anemia from IMHA may need to be hospitalized in a veterinary clinic for acute, rapid care. That care can include treatment for DIC if it's present, medications for blood clots if present, treatment for heart failure if it's occurring, and blood transfusions.

If an underlying infection or blood parasite is found to be causing the IMHA, appropriate medications are necessary. Corticosteroids are usually given to decrease the immune system's response. Cyclophosphamide, an immunosuppressant drug, may also be used. Steroids and cyclophosphamide are given over a period of one to months or longer, with serial blood work done to monitor the anemia's response.

Depending on the underlying cause of the IMHA, the condition carries up to an 80% mortality rate, and aggressive treatment is necessary.

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