Anemia occurs when there is a decrease in the number of red blood cells, which can be measured by red blood cell count or hemoglobin concentration. 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 erythropoietin (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.
Signs and Diagnosis of Anemia
The signs of anemia in animals depend on the severity, duration (short or longterm), and underlying cause. Sudden anemia can result in shock and even death if more than a third of the blood volume is lost rapidly and not replaced. After rapid blood loss, the animal usually has an increased heart rate, pale gums, and low blood pressure. If red blood cells are being destroyed, the animal may appear jaundiced (a yellowish color of the whites of the eyes, skin, or gums). Animals with long-term anemia have had time to adjust, and their signs are usually slower to develop. These include loss of energy, weakness, and loss of appetite. Affected animals will have similar physical examination findings, such as pale gums, an increased heart rate, and possibly a heart murmur.
A complete medical history is an important part of diagnosing anemia. Questions a veterinarian may ask include:
How long signs have been present?
Does the horse have a history of exposure to toxins (such as poisonous plants, chemicals, or heavy metals)?
What drug treatments and vaccinations has the horse had?
Where has the horse traveled?
Has the horse had any prior illnesses?
A complete blood count is a blood test your veterinarian will use to provide information on the severity of the anemia, the degree of bone marrow response, and the condition of other types of blood cells. A test should be performed to evaluate red blood cell size and shape and to check for red blood cell parasites.
Additional blood and urine tests can be used to evaluate how well the internal organs are functioning. If blood loss within the stomach or intestines is suspected, an examination of the animal’s feces under a microscope for trace amounts of blood and parasites can be useful. Bruising or bleeding may be signs of a disease or condition affecting the blood’s ability to clot and may indicate the need for a test called a coagulation profile. If hemolytic disease (a condition in which there is destruction of red blood cells) is suspected, other tests can be performed. A blood test for infections may also help define the cause of anemia. Bone marrow evaluation may be necessary for any animal with an unexplained, nonregenerative anemia.
Regenerative Anemias
Regenerative anemias include blood loss anemia and hemolytic anemia.
Sudden and severe blood loss can lead to shock and even death if more than 30–40% of the total blood volume is lost and the condition is not treated quickly with intravenous fluids or blood transfusions, or both. Obvious causes of severe blood loss include a major injury or surgery. If the reason for blood loss is not apparent, your veterinarian will look for a source of internal or hidden bleeding, such as conditions affecting the blood’s ability to clot, or stomach or intestinal ulcers. Low-grade, longterm blood loss eventually results in iron-deficiency anemia, which is nonregenerative.
Hemolytic anemias occur when red blood cells are destroyed. They are usually regenerative. Hemolytic anemias may be due to immune system dysfunction (including neonatal isoerythrolysis, see below), diseases of the small blood vessels, metabolic disorders, toxins, infections (equine infectious anemia and blood parasites), 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 and mules. 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. The anemic foals become weak and jaundiced. A veterinarian can perform tests to confirm the diagnosis. Treatment consists of stopping any colostrum while giving supportive care with transfusions. A veterinarian can perform a test to check for this potential disorder before the newborn is allowed to receive maternal colostrum. If a reaction is likely, neonatal isoerythrolysis can be avoided by withholding colostrum from the foal’s own mother and giving colostrum free of the antibodies.
Nonregenerative Anemias
Nonregenerative anemias include anemias caused by nutritional deficiencies in vitamins or minerals (such as iron), longterm disease, kidney disease, and primary bone marrow diseases.
Nutritional deficiency anemias develop when the nutrients needed for red blood cell formation are not present in adequate amounts. Anemia develops gradually and may initially be regenerative, but ultimately becomes nonregenerative. Starvation causes anemia by a combination of vitamin and mineral deficiencies and inadequate energy and protein intake. Iron deficiency anemia is not common in horses. When it occurs, it is not usually due to insufficient intake of iron in the diet. Instead it occurs due to low-grade, longterm blood loss. Your veterinarian will treat this disorder with iron supplements and by identifying and treating the cause of the blood loss.
Anemia caused by a longterm (chronic) disease usually results in mild to moderate reductions in red blood cells. It is the most common form of anemia seen in animals. The anemia can occur after a longterm inflammation or infection, a tumor, liver disease, or hormonal disorders (such as Cushing disease). Proteins called cytokines, which are produced by inflammatory cells, lead to decreases in iron availability, red blood cell survival, and the bone marrow’s ability to regenerate. Treatment of the underlying disease leads to correction of the anemia.
Longterm kidney disease is a common cause of nonregenerative anemia in animals. The kidney senses how much oxygen the body’s tissues are receiving and also produces erythropoietin, the hormone that stimulates the production of red blood cells in the bone marrow. Impairment of these functions leads to anemia in horses with longterm (chronic) kidney disease.
Bone Marrow Diseases
Bone marrow disease or failure from any cause can lead to nonregenerative anemia and a reduction in the number of all types of blood cells—red, white, and platelets. With widespread marrow involvement, white blood cells are affected first, followed by platelets, and finally red blood cells.
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, radiation therapy, and immune-mediated disease. The condition is diagnosed by sampling the bone marrow. To treat the condition, the underlying cause must be determined and eliminated, if possible. Supportive care such as antibiotics and transfusions may also be needed. Other types of drugs may also be used until the marrow recovers.
In pure red cell aplasia, the number of both mature and immature red blood cells is decreased. Other types of blood cells are not affected. In this nonregenerative anemia, there is a severe reduction of the elements that produce the red blood cells in the bone marrow. Cases that are immune-related often respond to therapy that suppresses the immune system. Supportive treatments, such as blood transfusions, may be indicated for severe cases. Recombinant human erythropoietin, used to treat anemia caused by kidney failure or used illegally to boost performance in race horses, may cause this condition. 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.
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