Neonatal Isoerythrolysis

Neonatal Isoerythrolysis (NI) is a disease of foals less than a week of age.  The disease causes illness in foals ranging from weakness to death, through destruction of the foal's red blood cells.

NI occurs when a foal inherits a blood type from the stallion which is different to the blood type of the mare.  Some mares produce anti-red cell antibodies (protein produced by the immune system to neutralise or destroy a certain foreign particle) against certain foreign blood types that they have been exposed to in the past.  This occurs either during a previous foaling through exposure to the foal's foreign blood type (there is no transfer of blood across the equine placenta), or if the mare has had a blood transfusion in the past.  Maiden mares are rarely affected.

Foals do not receive antibodies through the placenta like humans.  They rely on the antibody-rich colostrum of the mare, from which they can absorb these specialised proteins for the first 24 hours of life.  It is after they absorb the anti-red cell antibody from the colostrum, which the mare has made in response to previous exposure, that the destruction of red blood cells begins.  This is why most NI foals are born normal and only begin to show signs of disease between 12 hours and 4 days of age.

The severity and onset of the disease depends on the amount of antibody ingested and how destructive the particular antibodies are.  Foals can present lethargic and dull, or in severe cases can be found suddenly dead.

Clinical signs of NI include;

  • Jaundice - a yellow discolouration of the mucous membranes caused by accumulation of bilirubin from red blood cell breakdown. Best seen in the eye and gums.
  • Anaemia - a reduction in the red blood cell volume within the blood. Causes pale mucous membranes.
  • Dullness and depression. Often progressive, can lead to recumbency (inability to rise).
  • Increased heart and respiratory rates.

If NI foals are caught early, treatment can be successful.  Depending on the severity of the anaemia, a blood transfusion is often necessary.  Whole blood, usually collected from a local donor horse is used.  It is best to use a "universal donor" which has been tested negative for the commonly implicated blood types.  If this option is not available, an unrelated standardbred (SB) gelding who has not had a blood transfusion is the safest bet, as SBs are most likely to have the suitable blood type, and geldings are safer as they have never had a foal! Other treatments commonly employed are based around supportive care.

Once a mare has become sensitized, she is likely to produce subsequent NI foals.  It is possible, but seldom practical or desirable, to avoid NI by mating the mare to a stallion with a known blood type lacking the factors to which the mare produces antibodies.  98% of thoroughbreds have the most common blood factor for NI.

Mares can be tested in late pregnancy to see whether they have anti-red cell antibodies present; the test is called a NI titre.  This can be performed on blood collected from the mare. Testing is done at the Massey University Equine Parentage and Genetics lab.  If the titre is found to be over a certain level, preventative steps should be taken.  Any mare that has previously had a NI foal should be tested.  The lab can also blood-type your mare at any time to advise what her risk is for having a NI foal.

The foaling of a potential NI must be attended and the foal muzzled to prevent it from drinking the mare's colostrum for 48-72 hours.  The foal must be supplemented with another source of colostrum, mares' banked colostrum is best.  The dam's colostrum should be stripped and discarded for this period, after it is safe to let it nurse from the mare. 

NI can be a nasty and heartbreaking disease to be faced with.  If you see any abnormal signs in your foal, contact Totally Vets as soon as possible and remember that prevention is better than cure!