What is Anemia in Horses?
Anemia is a common disease process in the horse and there are many causes that may lead to it. The most common type of anemia seen in horses is anemia of chronic inflammation.
Anemia occurs when there is a decrease in the number of red blood cells (RBCs). RBCs are one of three cell types that are in the blood, and are responsible for carrying oxygen to the body’s tissues and organs. A decrease in these cells leads to a decrease in the ability for the blood to transport oxygen to the body’s tissues to maintain vital functions.
There are breed, gender, and age related differences in the amount of RBCs horses have in their bloodstream. Healthy draft horses and ponies have lower percentages of RBCs than Thoroughbreds and Arabians. Males tend to have larger amounts of RBCs while foals tend to have smaller RBCs.
Symptoms of Anemia in Horses
Anemia in horses may show clinical signs by:
Elevated heart rate
Elevated respiratory rate
Reduced exercise tolerance
Icteric (yellow) gums
Low blood pressure
Loss of energy
Loss of appetite
Causes of Anemia in Horses
There are three main types of anemia:
Blood loss (hemorrhage)
RBC destruction (hemolysis)
Lack of production of RBCs
Hemorrhage and hemolysis are considered “regenerative anemias,” meaning the body will try to replace the lack of RBCs. Lack of production of RBCs is “non-regenerative,” meaning the body is not able to replace the RBCs. Anemia from blood loss may be acute (sudden), for example, after an injury or trauma. Alternatively, it may be from chronic loss of blood (slow over time).
Examples of chronic blood loss conditions include:
Gastric squamous cell carcinoma
Parasites including either intestinal (worms) or external (ticks, lice)
Hemolysis is when the body destroys its own RBCs. This occurs with immune-mediated diseases–when the body attacks its own RBCs because it either doesn’t recognize them as its own, they contain a parasite like Ehrlichia or Babesia, or there is damage to the RBCs from a toxicity, like onion or red maple leaf ingestion.
Neonatal Isoerythrolysis is an uncommon cause of hemolytic anemia seen in only one to two percent of the equine population. It occurs in foals between one to seven days of age. The disease is caused by antibodies in the mare’s first milk that the foal ingests, causing clumping and destruction of the foal’s RBCs.
Equine infectious anemia is a disease caused by a virus in the bloodstream that is transmitted by biting flies. It causes a hemolytic anemia as well as fever, lethargy, and weakness. It can be fatal.
The most common non-regenerative anemia seen in horses is anemia of chronic inflammation. Inflammatory molecules in the body prevent iron from being used to make new RBCs. This is seen with chronic inflammatory diseases like abscesses, pneumonia, and cancer. Non-regenerative anemia is also seen with chronic kidney disease and problems within the bone marrow.
How Veterinarians Diagnose Anemia in Horses
The first step in diagnosing anemia in horses is a complete physical examination. Anemic horses often have an elevated heart rate and respiratory rate. Occasionally, a heart murmur may be heard. Their gum color will be pale or in a case of hemolytic anemia, yellow (icteric).
The next step is a thorough history. Your veterinarian will inquire about:
Any potential access to toxicities, medications, or vaccinations it recently received
Most recent Coggins test results
Previous medical history including pregnancies, illness, or injuries may be helpful too.
Bloodwork is needed to confirm a diagnosis of anemia and help differentiate the type of anemia. Your veterinarian will take blood from your horse for a complete blood count (CBC), a chemistry panel, and a blood smear. The CBC will help determine the severity of the anemia. The blood smear will allow your veterinarian or a clinical pathologist (a veterinarian specialized in laboratory analysis of disease processes) to look at the RBCs under a microscope. In some cases, the cause of the anemia may be diagnosed from this; for example, if parasites that live in RBCs are seen or RBCs appear damaged indicating a toxicity, or if the cells are clumping together, raising suspicions of an immune mediated problem. Special blood tests can be ordered to help confirm some of these diseases.
A Coombs’ test is used to diagnose an immune-mediated hemolytic anemia and is used to diagnose equine infectious anemia. If iron deficiency is suspected, a total serum iron level can be performed. If a chronic bleed, like from an ulcer or stomach cancer is suspected, an endoscopy or “scoping the stomach” should be performed. Horses that suffer from exercise-induced pulmonary hemorrhage can have their respiratory track (or airway) scoped.
When looking for evidence of chronic inflammation, an abdominal ultrasound can be used to look for abdominal abscess or cancer and an ultrasound of the thorax can be done to search for lung abscess, pneumonia, or chronic pleuritis (inflammation of the lung tissue). If the source of the anemia still cannot be determined, a bone marrow test is necessary to help identify why new red blood cells are not being produced.
Treatment of Anemia in Horses
There are many reasons for a horse to be anemic, and treatment of anemia depends on the cause of the disease.
For acute blood loss due to trauma or injury, the source of the bleed needs to be identified and stopped. If you can identify the source of the bleed, try to apply pressure with a bandage and seek medical care immediately. Loss of 1/3 of the horse’s total blood volume can cause shock and death. These horses often need a blood transfusion and intravenous fluid therapy immediately to keep their blood pressure in the normal range and prevent them from going into shock.
Some hemolytic diseases may be treated with appropriate antiprotozoals. If anemia is due to a toxicity, withdrawal from that toxicity and supportive care are needed; however, even with treatment, some horses will not make a full recovery.
Unfortunately, there is no treatment for horses with equine infectious anemia and no vaccination to prevent it. Because it is a communicable disease amongst horses, federal regulations require that horses tested positive for it be maintained in a lifelong quarantine or euthanized.
For anemia due to chronic inflammation, the source of inflammation must be identified and treated accordingly. A diagnosis of pneumonia may require hospitalization and an extended course of antibiotics. Abscesses may need surgery for removal.
Recovery and Management of Anemia in Horses
Anemia due to an acute problem that has been identified and addressed will have a restricted recovery time and your horse may gradually return to normal work. However, if the source of anemia is a chronic problem, it will be important to monitor your horse closely. Rechecks of bloodwork with your veterinarian should be performed. These horses often benefit from a reduced to non-existent workload. In severe cases, anemia may even progress to death.
Anemia in Horses FAQs
How common is anemia in horses?
Anemia in horses is a relatively common disease, with anemia of chronic inflammatory diseases being the most prevalent type.
Can worms cause anemia in horses?
Intestinal parasites, like strongyles, can cause anemia, in addition to external parasites like ticks and lice. It is important to work with your veterinarian on a routine parasite prevention program.
Allen, Julie. Veterinary Information Network. Horsing Around with Clinical Pathology: Blood Work and Urinalysis. May 2020.
Marks, Steven. MSD Veterinary Manual. Anemia in Horses. October 2022.
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