Anaemia

Anaemia occurs when there is a reduction of red blood cells or haemoglobin (or both) in the blood stream. The hemoglobin is a protein in red blood cells that transports the oxygen into the body and carbon dioxide out and is responsible for the red colour of blood. Anaemia is a condition that is often a symptom of a disease or a different condition.

The red blood cells are formed in the bone marrow, they circulate for 12 weeks before the liver breaks them down and they are replaced.

Some of the more common causes include:

The symptoms of anaemia in dogs

  • Pale gums
  • Fatigue, tires easily
  • Bruising may appear
  • Dark, tar like stools or dark blood passed within stool
  • Dark blood in vomit or 'coffee beans' (partially digested blood)

If your dog has any of these symptoms, they should be taken to the vet as soon as you can and they will investigate further. As mentioned above, anaemia is usually a symptom of another disease or condition and some of these are very serious.

There are two classifications of anaemia:

  • Regenerative anaemia - bone marrow is actively trying to correct the anaemia by releasing immature red blood cells (leukocytes). Anemias due to bleeding or the destruction of existing red blood cells are usually regenerative.
  • Nonregenerative anaemia - bone marrow is not responding normally to try to correct the anaemia. Anaemias 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.

Blood Loss Anaemia

Sudden and severe blood loss can lead to shock and could be fatal if total blood loss exceeds 30 to 40% of the total blood volume. It is a medical emergency and if the reason is not apparant, it may be a problem with blood clotting, bleeding tumors, stomach ulcers, or parasites. Internal parasites, such as hookworms, can lead to severe blood loss, especially in young dogs.

Hemolytic Anaemia

Hemolytic anaemias occur when red blood cells are destroyed. They are usually regenerative. Toxins, red blood cell trauma, infection, immune system defects, and defects of the red blood cell membrane may lead to hemolytic anaemias.

Immune-mediated Hemolytic Anaemia (IMHA)

In dogs, the most common cause of hemolytic anaemia is immune mediated. This type of anaemia can occur on its own or as a result of tumors, infection, drugs, or vaccinations. The body attacks the red blood cells as it no longer regonises them as self.

Dogs with IMHA are usually jaundiced, they may have a fever, and could have an enlarged spleen. Onset can be slow and the dog may not appear to be in pain or they can become ill very quickly. Immune-mediated hemolytic anaemia is a serious medical condition that causes death in 20-75% of affected animals (dependant on symptoms).

Alloimmune Hemolysis

This is an uncommon condition in dogs where their antibodies attack another animals red blood cells. This could be following a blood transusion, where the blood was not correctly matched. Puppies can also be at risk from the mothers antibodies through the mothers colostrum (yellowish fluid rich in antibodies and minerals that is produced before true milk).

Microangiopathic Hemolysis

Microangiopathic hemolysis occurs when red blood cells are damaged due to turbulent flow through abnormal blood vessels. It can be seen in dogs with severe heartworm infections, blood vessel tumors, twisting of the spleen, and disseminated intravascular coagulation, a condition in which small blood clots develop throughout the bloodstream.

Metabolic causes of Hemolysis

Deficiency of phosphorus in the blood leads to destruction of red blood cells, is seen in dogs with diabetes, hepatic lipidosis (a disorder of fat metabolism in the liver), and refeeding syndrome (chemical and fluid abnormalities that occur during recovery from fasting or starvation).

Toxins (Drugs, Plants, Chemicals)

Anaemia can be cased by incorrect use or accidental ingestion of many human and animal drugs. Plants that have anaemia causing toxins include oak, red maple, and bracken fern. Foods such as fava beans and onions; chemicals; and heavy metals such as copper, lead, selenium, and zinc.

Infections

Infections due to bacteria, viruses, or other organisms may lead to anemia, either by direct damage to red blood cells or by affecting red blood cell production in the bone marrow.

Inherited Diseases

Several inherited red blood cell disorders cause anaemia.

Nonregenerative Anaemias

Nonregenerative anaemias can be caused by nutritional deficiencies, chronic disease, kidney disease, or bone marrow diseases

Nutritional Deficiencies

Red blood cells need adequate amounts of iron, copper, vitamin B and vitamin E to form. A deficiency in any of these nutrients can gradually lead to anaemia. Initially it may be regenerative, but ultimately becomes nonregenerative. Starvation causes anaemia by a combination of vitamin and mineral deficiencies as well as a negative energy and protein balance.

Iron deficiency is the most common deficiency seen in dogs. It is most commonly occurs following blood loss and is rarely a nutritional deficiency. B vitamin deficiencies are rare but some drugs can cause anaemia through interfering with Vitamin B metabolism.

Anaemia of Chronic Disease

Anaemia that is caused by a chronic (longterm) disease is usually nonregenerative and 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. Proteins (cytokines) are produced by inflammatory cells and these lead to reduced iron availability, red blood cell survival, and the bone marrow’s ability to regenerate, resulting in anaemia. Treatment of the underlying disease leads to correction of the anaemia.

Kidney Disease

Longterm kidney disease is a common cause of nonregenerative anemia in animals. They produce less of a kidney hormone called erythropoietin that stimulates development of red blood cells in the bone marrow, leading to anaemia.

Bone Marrow Diseases

Bone marrow disease or failure from any cause can lead to nonregenerative anaemia and a reduction in the number of all types of blood cells. With widespread marrow involvement, white blood cells are affected first, followed by platelets, and then red blood cells.

Aplastic Anaemia 

In aplastic anaemia the ability of bone marrow to generate all blood cells is reduced. It can be caused by infection, drug therapy, toxins or exposure to full body. The immune system may also be involved in the development of this disease.

In pure red cell aplasia, only the red blood cells or the elements that produce the red blood cells are affected. It is a nonregenerative anaemia with severe reduction of the elements that produce the red blood cells in the bone marrow.

Primary leukemia 

Leukemia is blood cancer in which normal white blood cells are replaced by abnormal blood cells. This leads to anaemia and a lack of normal white blood cells and platelets. Primary leukemias are uncommon, but they have been reported in dogs. Leukemias can develop in bone marrow or the lymphatic system and are classified as acute (sudden and often severe) or chronic (long-lasting, with signs that are generally less severe).

Myelodysplasia 

This is a bone marrow disorder in which growth and maturation of blood-forming cells in the bone marrow is defective. This leads to nonregenerative anaemia or shortages of white blood cells or platelets. It is considered a preleukemic syndrome (occurring before leukemia fully develops).

Myelofibrosis 

This is a bone marrow disorder in which normal bone marrow elements are replaced with fibrous (scar) tissue. It can be the initial disease or may occur as a result of cancer, IMHA, whole body radiation exposure, or hereditary anaemias.

(Merck, 2020)

References

Merck, 2020 Merck Veterinary Manual - Blood Disorders of Dogs

AKC, 2020 AKC American Kennel Club - Anemia in Dogs


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