Anemia in Newborn Foals

Courtnee Morton, DVM
By Courtnee Morton, DVM on May 23, 2023
Mare and foal

In This Article


What Is Anemia in Newborn Foals?

Anemia, or an abnormally low red blood cell count, can affect horses of any age. Anemia occurs due to decreased/abnormal red blood cell production, or the destruction of red blood cells. This compromises the blood’s oxygen-carrying capacity, which means that tissues and organs throughout the horse’s body does not receive adequate oxygen. Anemia in foals leads to weakness, lethargy, poor growth, and other negative side effects.

Symptoms of Anemia in Newborn Foals

Symptoms of anemia in newborn foals can be somewhat vague but include:

  • Low energy and weakness

  • Not nursing, or nursing less than usual

  • Lethargy

  • Increased heart rate or respiratory rate

  • Yellowing of the eyes or mucous membranes (icterus)

  • Dark urine

Because foals have an immature immune system, illness can set in quickly and cause a rapid decline. If you notice any of the above in your newborn foal, have them examined by a veterinarian right away.

Causes of Anemia in Newborn Foals

Neonatal isoerythrolysis (NI) is one of the most common causes of anemia in newborn foals, usually diagnosed in the first few days of life.

NI is caused by an incompatibility of the mare and foal’s blood types. The mare produces antibodies against the foal’s blood cells, which are then ingested by the foal through colostrum (the mare’s first milk, high in antibodies) in the first 6-8 hours of life. These antibodies then attack the foal’s red blood cells and cause them to rupture, leading to severe anemia which can become life-threatening.

NI signs begin within the first week of life, with an average between six hours and three days after birth.

NI is relatively rare, only occurring in 1–2% of all foals, but up to 10% of mules. In horses, Thoroughbreds seem to be the breed most affected. NI can occur with maiden (first-time pregnancy) mares, but there is a higher risk for mares that have previously had a blood transfusion. Other risk factors include those who may have exposure to fetal blood types before giving birth, such as in cases of placentitis, infection, or a difficult birth. If a mare has a foal with NI once, the condition can become more common with additional pregnancies, especially if bred to the same stallion.

Other causes of anemia in newborn foals include:

  • Blood loss or trauma

  • Inherited disorders or conditions that cause abnormal or insufficient red blood cell production

  • Infection

Diagnosis of Anemia in Newborn Foals

The simplest way to diagnose and prepare for NI in foals is a screening test conducted two weeks before a mare’s due date. A small amount of blood is pulled and checked for antibodies against other blood types. If this is not done before birth and your foal begins to show signs of anemia, your veterinarian will perform multiple tests along with a basic physical examination, blood work, and base IgG antibody testing. These might include:

  • CBC with blood smear: will detect a low PCV (packed cell volume) which is an indicator of anemia. The blood smear will allow veterinarians to look at the cells under the microscope and evaluate for signs of a breakdown of those red blood cells.

  • Crossmatch testing: a simple test that can be done at a clinic, or sent out to a lab. This will check to see if red blood cells are destroyed when the foal’s and mare’s blood are mixed. This test is commonly used before emergent blood transfusions to check for likelihood of a reaction.

  • Bacterial cultures, viral PCR, or blood samples sent out to the lab, depending on suspected other differentials and history if the above diagnostics aren’t decisive.

Treatment of Anemia in Newborn Foals

If your mare has tested positive for NI before giving birth, there will be several precautions that will be taken as soon as your foal is born.

It’s important to following the following guidelines and additional recommendations from your veterinarian:

  • Do not allow the foal to nurse after birth. Use a foal muzzle right away to prevent foals from ingesting the harmful colostrum until further information can be gathered.

  • Blood will be pulled for a Jaundice Foal Agglutination Test (JFA) which will determine if the mare’s colostrum causes lysis of the foal’s red blood cells.

  • Until the results of the JFA are in, the foal might be given frozen colostrum from a NI negative mare, then started on a milk replacer such as Mare’s Match.

  • Foals will be allowed to nurse as soon as a JFA result allows, or at 48 hours after birth, at which point there won’t be any more colostrum produced that can be harmful to the foal.

  • In some cases, the NI-positive mare can be started on a medication called domperidone before giving birth to encourage colostrum and milk production early. Mares then must be milked out frequently before the foal arrives to nurse on their own. This ensures minimal possible antibody ingestion by the foal and allow them to nurse naturally sooner.

If NI is confirmed in a newborn after clinical signs have begun with a CBC and blood smear, treatment may include:

  • Transfusion if anemia is severe

  • Not allowing foal to nurse further until colostrum isn’t being produced or can’t be absorbed (this happens when the gut “closes” at about 48 hours of age)

  • Steroids, NSAIDs, oxygen therapy, or IV fluids as needed for support depending on severity of symptoms

  • Nutritional support (bucket, bottle, or feeding via nasogastric tube) with frozen colostrum and milk replacer

  • Hospitalization with close monitoring and repeat PCV’s and blood work as needed

Recovery and Management of Anemia in Newborn Foals

If caught early and if aggressive treatment is started right away, newborn foals suffering from anemia can go on to lead normal, healthy lives. If NI was severe, or if other complications arise, sepsis, infection, and other secondary diseases may develop. Some cases can be fatal if there is a severe reaction to the mare’s antibodies or if treatment is not instituted in time, so it is imperative that any foal with concerning signs be evaluated by a veterinarian immediately.

Anemia in Newborn Foals FAQs

How common is anemia in newborn foals?

Anemia is relatively rare in foals, with NI affecting under 2% of horse foals, but up to 10% in newborn mules.

How do you treat NI in foals?

NI is treated by preventing the foal from nursing, feeding using frozen colostrum and milk replacer, and blood transfusion or other supportive care if necessary.

Can worms cause anemia in newborn foals?

Very few adult worms are ever found in foals less than two months of age, so worms are unlikely to cause anemia in newborns.

Featured Image: Popa


Jackie, Snyder. Hagyard. Neonatal Isoerythrolysis.

UC Davis. Neonatal Isoerythrolysis in Horse and Mule Foals.


Courtnee Morton, DVM


Courtnee Morton, DVM


Dr. Courtnee Morton is a 2017 Ross University School of Veterinary Medicine graduate. Since graduation, she completed an equine internship...

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