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. Anemias due to bleeding or the destruction of red blood cells are usually regenerative. In a nonregenerative anemia, the bone marrow responds inadequately to the increased need for red blood cells. Anemias that are caused by a decrease in the hormone that stimulates red blood cell production or by an abnormality in the bone marrow are nonregenerative.
Some infections, including equine infectious anemia, babesiosis, and trypanosomiasis, can cause anemia in horses (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.
Neonatal isoerythrolysis is an immune disorder in which red blood cells are destroyed in newborn foals. It occurs when foals 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. In horses, the antigens usually involved are A, C, and Q. Neonatal isoerythrolysis is most commonly seen in Thoroughbreds. The foals 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 foal's own mother and giving colostrum free of the antibodies. A veterinarian can perform a test to check for this potential disorder before the newborn is allowed to receive maternal colostrum.
Nonregenerative anemias include anemias caused by deficiencies in vitamins or minerals (such as iron) needed for the formation of red blood cells, as well as by chronic disease, kidney disease, and primary bone marrow diseases (see Blood Disorders of Dogs: Nonregenerative Anemias).
Aplastic anemia is a severe disorder of the bone marrow, in which the ability of bone marrow to generate blood cells is reduced. It has been reported in horses with pancytopenia (a condition in which there is an abnormal reduction in the number of red blood cells, white blood cells, and platelets in the blood) and with marrow that is underdeveloped and replaced by fat. Most cases have no known cause, but known causes include infections, drugs, 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.
In pure red cell aplasia, the number of both mature and immature red blood cells is decreased. In this nonregenerative anemia, there is a severe reduction of the elements that produce the red blood cells in the bone marrow. The use of recombinant human erythropoietin, used to treat anemia caused by kidney failure, may cause this condition in horses. Discontinuing the drug may lead to recovery in some animals.
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 to rare in most domestic species but have been reported in horses. Leukemias are classified as acute (sudden and often severe) or chronic (long-lasting, with signs that are generally less severe). 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.
Last full review/revision July 2011 by Peter H. Holmes, BVMS, PhD, Dr HC, FRCVS, FRSE, OBE; Nemi C. Jain, MVSc, PhD; Susan M. Cotter, DVM, DACVIM (Small Animal, Oncology); Wayne K. Jorgensen, BSc, PhD; Sarah E. Payne, DVM, DACVIM