Neonatal Isoerythrolysis



Neonatal isoerythrolysis is a condition in which the mare creates antibodies against the foal’s red blood cells, and then passes these antibodies to the foal via the colostrum.  Once the foal absorbs these antibodies, they result in lysis of the foal’s red blood cells within 24  - 36 hours after birth.  This red blood cell destruction is widespread throughout the foal’s body and can lead to life-threatening anemia and/or jaundice.  (This is similar to the human Rhesus factor where a woman that is Rh-negative gives birth to her second or subsequent child that is Rh-positive, resulting in destruction of the newborn’s red blood cells).  The mare must have had previous exposure to blood that contained these particular antibodies to the foal’s blood (such as through a blood transfusion or a previous foaling) in order for this situation to occur. 

Signs of neonatal isoerythrolysis depend upon the rate and severity of red blood cell destruction.  Affected foals are born healthy, and then typically develop signs within 24 – 36 hours (as stated above).  In severe cases, the signs of NI may be evident within 12 – 24 hours.  Whereas, in mild cases, signs may not be present until

3 – 4 days of age.  NI foals will develop progressive anemia, thus leading to depression, anorexia, collapse, and death.  These foals may also develop pale mucous membranes that later become yellow, or jaundiced.

If neonatal isoerythrolysis is suspected in a foal that is less than 24 hours old, further ingestion of the mare’s colostrum must be prohibited.  This foal should be seen by a veterinarian and fed an alternative source of colostrum via a nasogastric tube.  Foals that are over 24 hours old when suspected to be suffering from neonatal isoerythrolysis need to be seen by a veterinarian immediately for evaluation and treatment.  These foals may need to undergo a blood transfusion in order to save their life.

Prevention of neonatal isoerythrolysis is more effective than treatment.  Prevention begins with identifying mares at risk for producing a foal with NI.  These are mares that have previously had an NI foal or are positive for antibodies to the Aa and/or Qa blood antigens.  (A veterinarian will need to draw a blood sample from the mare in order to detect the presence of these blood antigens).  Two common breeds that express an increased incidence of NI are Standardbreds and Thoroughbreds.  Individuals who are involved with the breeding of these horses should be particularly aware of this life-threatening condition. 

Colostrum from any NI-producing mare should never be used or saved.  Alternative sources of colostrum should be acquired prior to parturition for cases where an NI foal is highly likely.




The National Colostrum Network was founded at the

University of Minnesota-College of Veterinary Medicine.