Lets define the terms of the disease one at a time. "Autoimmune" literally means the immunity against the self."Hemolytic" is the destruction of red blood cells. "Anemia" is a clinical sign, not a disease. Anemia is defined as a decrease in the number of red blood cells (RBC's) or the amount of hemoglobin, resulting in a decrease in the oxygen- carrying capacity of the blood.
In AIHA, markers called antibodies, stick to the red blood cells and cause the body to believe the red blood cells are a "foreign invader". This causes the immune system to "kick in", attack the red blood cells and destroy them. The mechanism by which the immune system mistakes the red blood cells for a "foreign invader" varies somewhat according to the cause. It usually involves adherence of the offending agent (parasite, drug or toxin) to the surface of the red blood cells. The immune system wishes to attack the offending agent, but manages to injure the red blood cells as well. When the spleen and the rest of the immune system is working to rid the body of the old, diseased or damaged red blood cells, it is doing its job properly. When a large percentage of the cells are affected, and they are removed faster then they are replaced, AIHA results. The destruction of red blood cells often leaves recognizable cellular debris in the blood stream. In particular, a form of damaged red blood cell known as a spherocyte occurs. Finding spherocytes on a blood smear almost guarantees that some form of hemolytic anemia is occurring. Since this disorder does not stop the production of red blood cells, there are usually immature red blood cells in the bloodstream which can be detected on the blood smears as well.
The symptoms or clinical signs of AIHA can appear suddenly or they may be gradual and progressive. The signs are usually related to the lack of oxygen and manifest themselves in the form of weakness, lethargy, and an increase in the heart and respiration rate. Pale mucous membranes (gums, ears, eyelids) may be observed. The dog also may appear to be jaundice. This is due to a build up of bilirubin, one of the breakdown products of hemoglobin. Vomiting or abdominal pain may be present. Owners may note the presence of blood in the urine or stool Also an increase in temperature may be observed in some dogs. A diagnosis of AIHA is made on the basis of these clinical signs as well as a CBC. A Coomb’s test should be performed to confirm the diagnosis. A small percentage of dogs that have AIHA will test negative on the Coomb’s test.
Most of the time, a specific cause for AIHA is unrecognized. Many theories exist, but the ultimate answer is "We don’t know." Dr. Jean Dodds, a veterinarian studying immunology, states in her article, The Immune System and Disease Resistance, that there are four main causative factors of autoimmune diseases. Those factors are: Genetic predisposition; Hormonal influences ; Infections, especially of viruses and Stress. Dr. Dodds also feels that the multivalent modified-live vaccines overstimulate the immune system. There may be evidence to support this theory because the Merck Veterinary Manual states: " recently, the most typical cases of AIHA in dogs have been marked by a nonregenerative anemia. Some of these cases have occurred after exposure to parvovirus or modified live parvovirus vaccine." Others blame environmental pollutants, or food preservatives such as ethoxyquin, an antioxidant found in most dog foods. There are still others who believe there is a link between AIHA and some drugs. There is strong evidence for a genetic factor in the development of AIHA. Although all breeds can be affected, the Basenji, West Highland White terrier, English springer spaniel, Alaskan malamute, poodle, and beagle breeds can be congentially predisposed to this condition due to defects in enzymes (such as pyruvate kinase). Females are affected with AIHA 3-4 times more frequently then males.