Equine anemia is a condition that can significantly impact the health and performance of horses. It is defined as a reduced number of red blood cells in circulation.

Horses with anemia typically experience low energy levels, elevated heart rate, poor coat quality, depression and loss of appetite.

Red blood cells are responsible for transporting oxygen throughout the body. Without enough of these cells circulating, the heart must work harder to oxygenate tissues and the tissues are at risk of hypoxia, or decreased oxygenation.

Anemia can be a sign of an underlying nutritional deficiency, health condition, or disease process. There are three main reasons the red blood cell count may be low – blood loss from wounds or ulcers, insufficient production of red blood cells in the bone marrow, or the destruction of red blood cells.

This condition can also be caused by an infection, such as the equine infectious anemia virus, babesiosis or trypanosomiasis.

Blood tests including packed cell volume (PCV), red blood cell (RBC) count, and hemoglobin concentration are useful for determining if a horse has anemia. Other diagnostic tests may help to determine the cause of anemia and the treatment required.

What is Equine Anemia?

Equine anemia refers to a reduction in the number of red blood cells (erythrocytes) in the body due to blood loss, blood cell destruction, or decreased red blood cell production.

The primary role of red blood cells is to facilitate the transfer of oxygen from the lungs to the rest of the body. These cells also carry carbon dioxide away from the tissues to the lungs where it can be exhaled.

Red blood cells contain hemoglobin, a protein that binds to and transports oxygen from the lungs to the rest of the body.

Maintaining an appropriate level of red blood cells is critical for supporting metabolic functions, including energy production. When the level of red blood cells falls below normal, the body is unable to produce energy as efficiently.

Anemia is particularly problematic for horses in heavy work or training. A deficit of oxygen to the muscles impedes physical capacity, stamina, and the ability to recover from activity.

Hemoglobin and Iron

The trace mineral iron is required to form hemoglobin in red blood cells.

Approximately 60% of the iron in the horse’s body is found in hemoglobin and another 20% is found in myoglobin – a protein that stores oxygen in muscle tissue.

In humans, anemia can often result from iron deficiency in the diet. However, most horses receive adequate iron in their diets and deficiency is not a common cause of anemia.

Red Blood Cell Production

Red blood cells are made in bone marrow – the porous, spongy tissue in the center of bones. Erythropoietin, a hormone produced by the kidneys, controls the rate at which red blood cells are produced.

Levels of erythropoietin increase when blood oxygen levels are low and influence the rate at which red blood cells are produced and released into circulation. Anemia can occur due to kidney failure if erythropoietin production is compromised. [23]

Horses store approximately 50 percent of their red blood cells in their spleen. [1] The spleen releases red blood cells during intense exercise and during the flight response to effectively increase the level of oxygen in the blood. [2]

Types of Anemia

Anemia in horses can be classified as either regenerative or non-regenerative. [16]

Regenerative anemia: The bone marrow responds to a deficit of red blood cells by increasing the production of these cells. Anemia caused by blood loss or the destruction of red blood cells is typically regenerative.

This process results in reticulocytosis, which is defined as increased levels of reticulocytes (immature red blood cells) in the blood. Reticulocytosis indicates that the body is compensating for the drop in red blood cell levels by releasing new blood cells before they have fully matured.

However, horses very rarely release reticulocytes into circulation. This makes it challenging for veterinarians to classify the anemia as either regenerative or nonregenerative.

Nonregenerative anemia: The bone marrow fails to respond to an increased need for red blood cells and is not producing an adequate amount of these cells to sufficiently oxygenate tissue.

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Causes of Anemia in Horses

Equine anemia always occurs due to an underlying health problem. It is critical to understand what is causing anemia in your horse to successfully treat this condition.

The underlying causes of this condition can be divided into three categories:

1. Hemorrhage

Blood loss due to any type of internal or external bleeding increases the risk for anemia because it results in a loss of red blood cells and plasma circulating in the bloodstream.

Although blood plasma is replaced quickly, it takes time for the body to produce new red blood cells in the bone marrow. If large volumes of blood are lost, it can take a month or more to fully replace the lost blood cells.

Causes of blood loss in horses include:

2. Hemolysis

Anemia caused by blood cell destruction is referred to as hemolytic anemia or hemolysis.

When hemolytic anemia occurs, red blood cells are destroyed faster than they can be produced. Causes of hemolytic anemia include:

  • Immune-Mediated Adverse Reactions: Involves an immune response to a substance or pathogen that is foreign to the body.
  • Autoimmune Disease: The immune system attacks the body instead of protecting it. [3]
  • Liver Disease: Inflammation and infection in this organ, impairing normal liver function.
  • Toxin Exposure: Ingestion of poisonous plants or other toxic substances. [4][5]
  • Infectious Agents: certain bacteria[6], viruses, and blood parasites can inflict direct damage to red blood cells.

Conditions associated with hemolytic anemia include the following:

  • Septicemia: Occurs when the body’s response to infection causes damage to tissues.
  • Systemic lupus erythematosus (SLE): An auto-immune disease. [7]
  • Equine Infectious Anemia (EIA): Also known as swamp fever, this is a viral illness that affects the immune system. [8]
  • Neonatal Isoerythrolysis (NI): An immune-mediated disease that occurs in foals. [9]

3. Reduced Production of Red Blood Cells

Anemia can also be caused by a decrease in the production of red blood cells.

  • Inflammation and Infection: Infections and ensuing inflammation can promote anemia by increasing iron transfer into immune cells such as macrophages instead of using iron for making red blood cells. [18] Conditions such as Rhodococcus equi infection in foals could cause anemia. [19]
  • Endocrine conditions: Equine metabolic syndrome (insulin resistance) [20] and PPID [17] are associated with inflammation in the body, which can contribute to anemia.
  • Nutritional deficiencies: Insufficient levels of vitamin B12, iron, cobalt, folic acid, and riboflavin interfere with red blood cell production.
  • Disease: Conditions including bone marrow disease (aplastic anemia), cancer [10], and kidney disease or failure can cause a decrease in red blood cell production.
  • Aging: A combination of less exercise, lower regenerative capacity of bone marrow and presence of chronic diseases may reduce the number of red blood cells produced in senior horses. [21]

Anemia caused by chronic disease, such as PPID and chronic kidney disease, usually results in mild to moderate reductions in red blood cells. It is the most common form of anemia seen in animals. [17]

Signs of Anemia

Common signs of anemia include: [17]

  • Lethargy
  • Poor performance
  • Low stamina
  • Loss of appetite
  • Poor coat condition
  • Pale mucous membranes of the eyes, nostrils, and gums
  • Increased heart rate
  • Depression
  • Weight loss
  • Abnormal urine color if kidney function is impaired

Diagnosis

A veterinarian can provide a diagnosis of anemia based on an assessment of clinical signs in combination with diagnostic testing. The following blood components are ty