Neonatal Isoerythrolysis in Horses

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Key takeaways

Neonatal Isoerythrolysis (NI) describes destruction of red blood cells in a newborn foal.

  • When a foal has a different blood type from its mother, the mare may develop antibodies (part of the immune defense system) against the foal’s red blood cells during pregnancy
  • The antibodies are passed to the foal after birth through the mother’s first milk and attack the foal’s red blood cells
  • Symptoms of NI include lethargy, weakness, yellow skin and gums, and rapid breathing
  • Diagnostics include a physical examination, testing for antibodies, and bloodwork
  • Treatment includes blood transfusions and fluid therapy
  • The prognosis with treatment is good to excellent, barring complications such as kidney or liver failure
  • Blood tests performed late in pregnancy can determine whether a mare has antibodies that are dangerous for its foal as a preventative measure
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A closer look: Neonatal Isoerythrolysis in Horses

Neonatal Isoerythrolysis (NI) is a common and potentially life-threatening condition in foals. Red blood cell breakdown can cause significant organ damage, as the bloodstream is unable to deliver adequate oxygen to the rest of the body.

Any foal presenting with symptoms requires emergency medical attention to prevent organ failure and death.

Risk factors

Any breeding pair with different blood types can potentially produce a foal with a different blood type from its mother.

Mares with foals who have previously had neonatal isoerythrolysis are at a greater risk of having future foals with the condition. Blood testing these mares during pregnancy is particularly prudent for preventing NI.

Possible causes

Foals may have a different blood type than their mother, inherited from their father. During pregnancy, the mare’s immune system is exposed to the foal’s blood type. When the mare’s system detects this ‘foreign’ blood type, her body develops antibodies against it. Mares can also develop antibodies against certain blood types if they have received a blood transfusion previously.

After birth, mares produce colostrum, a type of milk containing proteins, nutrients, and antibodies, which provides basic immune system function to the foal while their own immune system is still developing after birth. NI develops when a foal consumes colostrum containing antibodies against its own blood, as the maternal antibodies in the colostrum target and destroy the foal’s red blood cells.

Main symptoms

Symptoms present within the first week of a foal’s life.

Testing and diagnosis

Diagnostics include:

  • Physical examination
  • Blood work
  • Combining mare colostrum or blood with foal blood to detect destruction of red blood cells
  • Detection of antibodies against red blood cells in a foal blood sample

Steps to Recovery

Treatments include:

  • Blood transfusion
  • Oxygen supplementation
  • Fluid therapy
  • Anti-seizure medications
  • Muzzling the foal to prevent them from drinking colostrum
  • Dietary support: replacing colostrum with milk replacer until the mare produces milk

Prognosis for NI is good to excellent with appropriate and prompt treatment. Once a couple of days have passed, an affected foal can return to feeding from the mare, as the composition of the mare’s milk shifts away from containing antibodies (and is no longer considered colostrum).

Some foals develop complications such as kidney failure, liver failure, septicemia, or neonatal maladjustment syndrome. These cases have a guarded prognosis.


Mares can be tested late in pregnancy to determine whether they have antibodies that are dangerous for their foal. In this test, a sample of mare blood is collected and tested against all known blood types, or against the father’s blood if available. If red blood cell destruction occurs, foals are considered high risk, and ensuring the foal does not feed from the mare for the first 2-3 days after birth prevents NI.

After the foal is born, but prior to the foal nursing for the first time, owners can take a blood sample from the foal and combine it with mare colostrum to identify red blood cell destruction. This simple test can also help prevent NI, although it is not always accurate.

Is Neonatal Isoerythrolysis in Horses common?

NI is common in foals. NI is particularly common in mules, as donkey blood has components not present in horse blood.

Typical Treatment

  • Blood transfusion
  • Dietary support


Heather Smith Thomas - Writing for The Horse
Christina S. Cable, DVM, Dipl. ACVS - Writing for The Horse
No Author - Writing for UC Davis Veterinary Medicine
No Author - Writing for Rossdales

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