Immune-mediated hemolytic anemia (IMHA) strikes without warning, causing death in a matter of days. It only rarely produces a mild illness. Any dog can suffer from this disease, so it’s important to keep it in mind when signs of anemia start to develop.

What causes this disease to erupt is largely unknown. We know that, in a small number, it is tied to a co-existing infection or administration of drugs. Isolated families of dogs have also been affected, but a genetic basis has not been established. Studies repeatedly show that females have a higher incidence of IMHA than males, and the mean age of affected dogs is five to six years (the range is one to 13 years).

The clinical signs associated with IMHA are caused by the anemia it creates. There are an insufficient number of red blood cells (RBCs) in the bloodstream to transport oxygen and the tissues starve for this vital nutrient. Dogs are lethargic, inappetant (do not want to eat) and weak. They cannot tolerate exercise and tend to breathe heavily and fast, even at rest. Heart rate is markedly elevated. Some collapse with the slightest exertion.

Pale gums is the first sign that would make you suspect IMHA. This is the hallmark of anemia, often referred to as “thin blood.” The only other condition that causes pale gums is heart failure. In some dogs with IMHA, the gums have a yellow tinge from the breakdown products from hemoglobin.

When a veterinarian suspects IMHA, she does tests to diagnose the disease and determine its severity. This begins with a quick in-hospital test called a packed cell volume (PCV). A blood sam-ple is put into a tiny tube and spun to pack the red blood cells (RBCs) at one end. The space taken up by the RBCs at the bottom of the tube is expressed as a percentage. Normal is between 40 and 50. In IMHA, the number drops to dangerous levels. Twenty is a grave concern; 15 is critical; 10 is life-threatening. Untreated PCV this low is fatal.

Another test veterinarians do is a complete blood count (CBC) to look at the cells in the blood. This tells us if the bone marrow is responding to the anemia by pumping out more RBCs. Veterinarians can sometimes spot clumped masses of RBCs on a blood smear. These are caused by the antibodies forming bridges between the cells. Their presence strongly suggests IMHA.

Further tests are often done once IMHA is discovered. This is to look for infections that may have triggered the disease, and any other complicating conditions.

The treatment used in IMHA is individualized for each dog and depends on the severity of the condition. If the disease is caught early (the PCV is not too low), the dog can be managed as an out-patient. However, many dogs with IMHA are critically sick and need intensive hospital treatment.

The aim of IMHA treatment – stopping the immune reaction – involves immunosuppressive therapy. First, corticosteroids such as Dexamethasone or prednisone are used. These medications are thought to prevent destruction of the RBCs by reducing the amount of antibodies produced. Unfortunately, side effects are common, including excessive drinking and urination, increased appetite, panting, predisposition to stomach ulcers, and an increased susceptibility to infection.

If steroids fail to stop the loss of RBCs, or side effects of the medication become unacceptable, cytotoxic drugs (ones often used in chemotherapy) can be added. These include azathioprine, cyclosporine and cyclophosphamide.

Transfusions are required if the PCV falls low enough and oxygen transport becomes critical. Cross-matched blood is the best, to ensure compatibility and avoid reactions.

However, dogs don’t have pre-existing antibodies in their blood to foreign blood types. This means that a transfusion rarely causes a problem. If the blood is mismatched, the dog will create antibodies against it and a second transfusion of that blood type will create a reaction.

Anemia is not the only concern. Dogs with IMHA can also die from thrombo-embolism – blood clots form in blood vessels, impairing blood flow. Ultra-low-dose ASA has shown promise in preventing this.

The medications – steroids or other drugs – must be continued until the RBC count returns to normal. This can take a significant time (weeks or more). The steroid dose is then reduced about 25 per cent every three to four weeks.

On average, dogs need about three months of therapy to treat IMHA. After medications are finished, blood tests should be repeated quarterly for a year, then twice yearly thereafter.

Somewhere between 12 and 24 per cent of dogs relapse with IMHA. If this happens, medication is reinstituted at high doses and the tapering off is done more slowly.

Mortality from IMHA can be as high as 70 per cent. Most deaths occur with-in the first two weeks of diagnosis. Dogs that survive the first two weeks after diagnosis have a six-month survival rate of 92 per cent.

Much speculation has arisen about the role that vaccinations might play in triggering IMHA. There is a suggestion that a higher than usual percentage of dogs undergo an IMHA crisis within three months of vaccination. However, other studies on IMHA-prevalence fail to show such a connection.

Should dogs that have suffered from IMHA be vaccinated? There is no conclusive data that suggests vaccines should be avoided in these circumstances. It would be prudent to review these situations with your veterinarian if your dog has survived a bout of IMHA.