Anemia occurs when there is a decrease in the number of red blood cells. It can develop from loss, destruction, or lack of production of red blood cells. Anemia is classified as regenerative or nonregenerative. In a regenerative anemia, the bone marrow responds appropriately to the decreased number of red blood cells by increasing red blood cell production. In a nonregenerative anemia, the bone marrow responds inadequately to the increased need for red blood cells. Anemias due to bleeding or the destruction of existing red blood cells are usually regenerative. Anemias that are caused by a decrease in the hormone that stimulates red blood cell production or an abnormality in the bone marrow are nonregenerative (see Blood Disorders of Dogs: Anemia in Dogs).
Regenerative anemias include blood loss anemia (see Blood Disorders of Dogs: Blood Loss Anemia) and hemolytic anemia (see Blood Disorders of Dogs: Hemolytic Anemia). Hemolytic anemias may be due to immune system dysfunction, diseases of the small blood vessels, metabolic disorders, toxins, infections, and genetic diseases.
Many classes of drugs can cause anemia if they are ingested accidentally or if their prescribed use is not closely monitored. These include common human and animal drugs such as acetaminophen, aspirin, naproxen, penicillin, and many other antibiotic and antiparasitic agents. Other anemia-causing toxins include plants such as oak, red maple, and bracken fern; foods such as fava beans and onions; chemicals; and heavy metals such as copper, lead, and zinc. It is always important to give as complete a history as possible to your veterinarian when anemia is suspected, in order to help pinpoint the cause.
Many infections—caused by bacteria, viruses, or other organisms—can lead to anemia, by direct damage to red blood cells (leading to their destruction) or by effects on the elements that produce red blood cells in the bone marrow. Some infections that can cause anemia in cats include feline leukemia virus and feline immunodeficiency virus.
Inherited red blood cell disorders can also cause anemia in cats. Deficiencies of an enzyme called pyruvate kinase are seen in Abyssinian and Somali cats. Affected cats have hemolytic anemia that waxes and wanes over a long time. Signs may improve if the spleen is removed or if corticosteroids are given. A hereditary blood disorder, porphyria (see Metabolic Disorders of Cats: Congenital Erythropoietic Porphyria in Cats), which leads to a build-up of porphyrins in the body occurs in cats, people, and other species. Porphyrins are proteins that become part of the hemoglobin molecule in red blood cells. Your veterinarian can check your cat for these conditions.
Neonatal isoerythrolysis is an immunologic disease seen in newborn cats. It occurs when kittens nurse from a mother whose colostrum (the yellowish fluid rich in antibodies and minerals that is produced after giving birth and before producing true milk) contains antibodies to the newborns' red blood cells. This can be caused by exposure of the mother to another blood type during a previous pregnancy or an unmatched blood transfusion. Cats with blood type B also have naturally occurring antibodies to blood type A.
The kittens get the antibodies when they first begin nursing. Once absorbed, the antibodies enter the bloodstream where they attach to red blood cells and cause them to rupture. Newborns with neonatal isoerythrolysis are normal at birth but develop severe hemolytic anemia within 2 to 3 days. A veterinarian can perform tests to confirm the diagnosis. Treatment consists of stopping any colostrum while giving supportive care with transfusions. Neonatal isoerythrolysis can be avoided by withholding colostrum from the kittens' own mother and giving colostrum free of the antibodies. A veterinarian can perform a test to check for alloimmune hemolysis before the newborn is allowed to receive maternal colostrum.
Anemia may be noted in cats with hypophosphatemia, a deficiency of phosphates in the blood. Hypophosphatemia occurs in cats with diabetes or fatty degeneration of the liver. It may also occur as a complication of refeeding syndrome, which is a shift in the concentration of several minerals in the blood that happens during recovery from a period of fasting.
Nonregenerative anemias include anemias caused by poor diet, chronic diseases, kidney disease, and disorders of the bone marrow (see Blood Disorders of Dogs: Nonregenerative Anemias).
Nutritional deficiencies may lead to anemia if the nutrients needed for red blood cell formation are not present in adequate amounts in the diet. Iron deficiency, for example, occurs in some cats.
Anemia of chronic disease is usually classified as mild to moderate and nonregenerative. It is the most common form of anemia seen in animals. The anemia can occur after a long-term inflammation or infection, a tumor, liver disease, or hormonal disorders such as hyper- or hypo-adrenocorticism (disorders of the adrenal gland) or hypothyroidism (an underactive thyroid gland). Proteins called cytokines, which are produced by inflammatory cells, decrease iron availability, red blood cell survival, and the bone marrow's ability to regenerate, resulting in anemia. Treatment of the underlying disease leads to correction of the anemia.
Aplastic anemia is a disorder of the bone marrow, in which the ability of bone marrow to grow new blood cells is reduced. It has been reported in cats with a condition in which too few red blood cells, white blood cells, and blood platelets are found in the blood and with bone marrow that is underdeveloped and replaced by fat. Most cases have no known cause, but some are caused by infections (including feline leukemia virus and Ehrlichia bacteria), drug treatment, toxins, and radiation therapy. To treat the condition, the underlying cause must be determined and eliminated. Supportive care such as antibiotics and transfusions may also be needed. Drugs that stimulate the bone marrow can be used until the marrow recovers. If the disease has no known cause or if marrow recovery is unlikely, bone marrow transplantation is helpful if a suitable donor is available.
Inpure red cell aplasia, only the red blood cells are affected. In this nonregenerative anemia, there is a severe reduction of the elements that produce red blood cells in the bone marrow. It has been reported in cats, including some with feline leukemia.
Primary leukemias are a type of cancer in which abnormal white blood cells displace normal blood cells. This leads to anemia and a lack of normal white blood cells and platelets. Primary leukemias are uncommon, but they have been reported in cats. Retroviruses are a cause in some cats. Leukemias are classified as acute (sudden and often severe) or chronic (longterm). Acute leukemias, in which the marrow is filled with immature blood cells, generally respond poorly to chemotherapy. In animals that do respond, remission times are usually short. Chronic leukemias, in which there is greatly increased production of one blood cell line, are less likely to cause anemia and are more responsive to treatment.
Myelodysplasia (also called myelodysplastic syndrome) is a bone marrow disorder in which growth and maturation of blood-forming cells in the bone marrow is defective. This leads to nonregenerative anemia or shortages of white blood cells or platelets. It is considered a preleukemic syndrome (occurring before leukemia fully develops). Myelodysplasia occurs in cats, dogs, and humans. The disease can occur as the result of mutations in stem cells or be caused by tumors in other organs or drug therapy. Some cats respond to treatment with synthetic hormones and steroids. Supportive care with blood transfusions may be helpful. Survival rates vary because myelodysplasia can progress to leukemia. Many animals with this condition are put to sleep or die of infection, bleeding, or anemia.
Myelofibrosis is a progressive disease leading to anemia and enlargement of the spleen and liver. It brings on bone marrow failure after it causes normal marrow elements to be replaced with fibrous tissue. It occurs in cats and several other species. Myelofibrosis may arise on its own or as the result of cancer, immune-mediated hemolytic anemia (see Blood Disorders of Dogs: Hemolytic Anemia), radiation therapy, or hereditary causes. A diagnosis can be made by bone marrow biopsy, a procedure requiring anesthesia and often an overnight stay at the veterinary hospital. Treatment depends on the underlying causes but usually involves suppressing the immune system. Because immune system suppression increases the chances that your pet will catch other diseases, carefully follow your veterinarian's recommendations for controlling exposure to disease-causing agents.
Last full review/revision July 2011 by Peter H. Holmes, BVMS, PhD, Dr HC, FRCVS, FRSE, OBE; David J. Waltisbuhl, BASc, MSc; Michael Bernstein, DVM, DACVIM; Karen L. Campbell, MS, DVM, DACVIM, DACVD; Nemi C. Jain, MVSc, PhD; Wayne K. Jorgensen, BSc, PhD; Sarah E. Payne, DVM, DACVIM