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 to the decreased number of red blood cells by increasing red blood cell production. Anemias due to bleeding or the destruction of existing red blood cells are usually regenerative; however, it takes the bone marrow 3-5 days to respond to an anemia. Anemias that do not show signs of a bone marrow response are termed nonregenerative. 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.
Signs of Anemia in Cats
The signs of anemia in animals depend on the severity, the duration (short or long-term), and the underlying cause of the illness. 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. The cause of the blood loss may be obvious, such as a major injury. If no evidence of external bleeding is found, your veterinarian will look for a source of internal or hidden blood loss, such as a ruptured tumor on the spleen, a bleeding disorder, or parasites. If red blood cells are being destroyed, the cat may appear jaundiced (a yellowish color of the whites of the eyes, skin, or gums).
Cats with long-term anemia have had time to adjust, and their signs are usually slower to develop. These signs 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 enlargement of the spleen or a heart murmur.
Diagnosis of Anemia in Cats
A complete medical history is an important part of diagnosing anemia. Questions a veterinarian may ask include: how long signs have been present; whether there is a history of exposure to toxins such as rodent poisons, heavy metals, or toxic plants; what drug treatments and vaccinations the pet has had; where the pet has traveled; and 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 cat’s feces under a microscope for trace amounts of blood and parasites can be useful. X-rays can help identify hidden disease. Bruising or bleeding may be signs of a disease or condition affecting the blood’s ability to clot and 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 infectious agents may also help define the cause of anemia.
Bone marrow evaluation is typically done for any animal with an unexplained, nonregenerative anemia. Sampling the bone marrow requires sedation or anesthesia.
Regenerative Anemias
Regenerative anemias include blood loss anemia and hemolytic anemia.
Blood Loss Anemia
Sudden and severe blood loss can lead to shock and even death if more than 30 to 40% of the total blood volume is lost and the condition is not treated quickly with intravenous fluids or blood transfusions, or both. The cause of blood loss anemia may be obvious, such as excessive bleeding after a major injury. If the reason for blood loss is not apparent, your veterinarian will look for other causes, such as conditions affecting the blood’s ability to clot, bleeding tumors, stomach ulcers, or parasites. Low-grade, longterm blood loss eventually results in iron-deficiency anemia. This leads to abnormally small red blood cells and a lack of hemoglobin. In kittens, this is often caused by parasites (for example, fleas, lice, or intestinal worms), but in older cats, bleeding from stomach ulcers or tumors is more common.
Hemolytic Anemia
Hemolytic anemias occur when red blood cells are destroyed. They are usually regenerative. Hemolytic anemias may be due to immune system dysfunction, diseases of the small blood vessels, injury to red blood cells, metabolic disorders, toxins, infections, and genetic diseases.
Immune-mediated hemolytic anemia is a life-threatening disease that occurs when the immune system no longer recognizes red blood cells as its own. This causes the immune system to create antibodies against and destroy the body's own red blood cells. In some cases, antibodies are directed against immature red blood cells in the bone marrow, resulting in nonregenerative anemia. This type of anemia can occur on its own or as a result of tumors, infection, drugs, or vaccinations. Animals with immune-mediated hemolytic anemia have signs of anemia (such as weakness and pale gums), are usually jaundiced, and sometimes have a fever or an enlarged spleen. Affected cats can show mild, slow-developing signs and not appear to be in any pain, or they can suddenly be in severe crisis. Treatment involves stopping the destruction of the red blood cells with strong medications that suppress the immune system. Patients also often require blood transfusions. Any underlying conditions also must be treated.
Immune-mediated hemolytic anemia is a serious medical condition that causes death in 20% to 75% of affected animals. The risk of death depends on which signs are seen, but rapid drops in red blood cell counts, moderate to high white blood cell counts, abnormal bruising, and excessive clotting may indicate a higher risk. Your pet's veterinarian may need to refer your cat to an internal medicine specialist.
Toxins (Drugs, Plants, Chemicals)
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, selenium, 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.
Infections
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,feline immunodeficiency virus, Haemobartonella bacterial species, and Cytauxzoon parasites.
Genetic Diseases
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, 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
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.
Hypophosphatemia
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
Nonregenerative anemias include anemias caused by poor diet, chronic diseases, kidney disease, and disorders of the bone marrow.
Nutritional deficiencies may lead to anemia if the nutrients needed for red blood cell formation are not present in adequate amounts in the diet. Anemia develops gradually and may initially be regenerative, but ultimately becomes nonregenerative as the body runs out of the nutrients needed to make red blood cells. Starvation causes anemia by a combination of vitamin and mineral deficiencies as well as inadequate protein, carbohydrate, and fat intake. The deficiencies most likely to cause anemia are iron, copper, vitamin B12, vitamin B6, riboflavin, niacin, and vitamin E. Iron deficiency occurs in some cats, but it is more likely to occur due to blood loss than a nutritional deficiency.
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 hypoadrenocorticism (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.
Chronic kidney disease is a common cause of nonregenerative anemia in animals. The hormone erythropoietin stimulates the development of red blood cells in the bone marrow and is produced in the kidneys. Animals with kidney disease produce less erythropoietin, leading to anemia. A synthetic form of the hormone has been used for treatment. Animals receiving the treatment require supplemental iron to support red blood cell production.
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 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), drug treatment (such as methimazole or some chemotherapy drugs), toxins, and radiation therapy. The immune system may also be involved. Bone marrow tests are needed to diagnose aplastic anemia. To treat the condition, the underlying cause must be determined and eliminated. Supportive care such as antibiotics and transfusions may also be needed. Your veterinarian may also prescribe a drug that suppresses the immune system. 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, 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. Other cases may involve immune system dysfunction and may respond to drugs that suppress the inappropriate immune response. Supportive care, including a blood transfusion, may be necessary if the anemia is severe.
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 (feline leukemia virus and feline immunodeficiency virus) 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 commonly occurs in cats with feline leukemia but can also occur as the result of other tumors, drug therapy, or mutations in stem cells. 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, radiation therapy, or hereditary causes. A diagnosis can be made by bone marrow biopsy, a procedure requiring anesthesia. Treatment depends on the underlying cause 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.
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