IMHA/AIHAWhat is IMHA/AIHA?
Immune-mediated haemolytic anaemia (IMHA) or autoimmune haemolitic anaemia (AIHA) is a disease in which the body's immune system attacks its own red blood cells. A dog with IMHA will have a lower than normal number of red blood cells within the blood, i.e. the dog suffers from anaemia. The percentage of the red blood cells to the whole blood is called haematocrit, and normally ranges from 37-55%, depending on the breed. We talk about a primary immune-mediated disease (sometimes also called AIHA - auto-immune haemolytic anaemia) when no underlying cause can be found after an exhaustive clinical laboratory and imaging examination. A secondary IMHA refers to all anaemias that occur when the immune system inadvertently destroys its own blood cells due to an immune attack directed against an underlying condition such as cancer, endocarditis (inflammation of the inner layer of the heart), heartworm, medication etc.
What are the Symptoms of IMHA?
When a large percentage of red blood cells are affected and/or they are destroyed very fast, the dog shows external signs of the disease.
The clinical signs of IMHA are usually gradual and progressive, but occasionally an apparently healthy dog suddenly collapses in an acute haemolytic crisis. The clinical signs basically reflect the lack of oxygen supply as the haemoglobin in the red blood cells is the primary carrier of oxygen in the blood. Signs include weakness, anorexia, and an increase in the heart rate and respiration. During the clinical examination heart murmurs, pale mucous membranes (gums, tongue and conjunctivas), rapid heart rate and obscure pulse may be noticed. More severe cases also have a fever and jaundice (icterus) - a yellow discoloration of the gums, the white of the eyes and skin. This is due to a build up of bilirubin, one of the breakdown products of haemoglobin.
The diagnosis of IMHA is usually made on these clinical signs as well as additional examinations. The complete blood count usually shows a regenerative anaemia with spherocytes. Spherocytes are a special type of red blood cells that develop only in IMHA. The blood samples may also auto-agglutinate (clump). A Coomb's test (evidence of antibodies against the red blood cells) can support the diagnosis.
Why My Dog?
Unfortunately no one knows why an individual dog develops IMHA. Certain breeds such as cocker spaniels and poodles are at a higher risk than other breeds. Typically, middle-aged bitches are affected. However immune-mediated haemolytic anaemia may occur in any breed at anytime, including mixed breeds.
The veterinary medical field has not yet discovered why an individual dog gets IMHA. However, the literature suggests that recent vaccinations may be associated with a higher incidence of IMHA and so does the administration of certain medications like sulfonamide antibiotics. Dogs with serious infections or cancers in their body may also develop IMHA (secondary IMHA). The thought for the underlying cause is that something, e.g. vaccine and cancer cells, triggers the immune system to react and to create antibodies. Accidentally the antibodies also destroy the red blood cells and sometimes also the platelets (in this case these cells are called "innocent bystanders") causing the reaction as described above.
Is There a Cure?
IMHA is better thought of as a disease that can be treated but not always cured. Medications are used to decrease the hyperactivity of the immune system and suppress the abnormal immune response directed against the red blood cells. Treatments may need to be given indefinitely, but most dogs are on medications for at least 4-6 months. Dogs that have had IMHA once are more likely to get it again, and therefore recurrent vaccinations are not recommended for these animals.
What are the Treatments?
The treatment is based on the suppression of the increased immune response of the body. Several drugs are available, but the initial drug is always cortisone (steroid). They work quickly, are cheap and very effective - the standard therapy. Other immunosuppressives such as azathioprine, cyclophosphamide and cyclosporine can also be used.
Patients with IMHA may also need a blood transfusion in order to stabilise their clinical state until the immunosuppressive treatment starts to work. Usually whole blood or packed red blood cells are used.
One of the most dreaded complications with IMHA is pulmonary thromboembolism (blood clots in the lungs) which primarily arises from the activation of the clotting system due to inflammation and the destruction of the red blood cells. In dogs with IMHA, several other factors, including IV catheters and administration of cortisone, contribute to the increased clotting tendency of the blood. Unfortunately the prognosis for these patients is very poor.
Side Effects from the Drugs
Prednisone and other cortisone medications are catabolic substances; the dog loses muscle mass and strength. Additionally, these drugs cause increased thirst and urinations by affecting kidney concentrating ability. These drugs are potentially irritating to the gastrointestinal tract and can cause vomiting/diarrhoea. Sometimes they can also cause gastrointestinal tract bleeding and pancreatitis (inflammation of the pancreas).
IMHA is a life threatening immune disease and owners of dogs with IMHA face a guarded to poor prognosis for the pet at the time of diagnosis. The chances for a complete recovery are about 50-70%.
Dr. Kamil Tomsa, Dipl. ECVIM-CA
Ennetseeklinik für Kleintiere AG
(Adapted a text written by Dr. Robert Murtaugh and Dr. Elizabeth Rozanski, USA).