Neonatal isoerythrolysis (NI) is an acquired disease of foals caused by the ingestion of incompatible colostrum. The colostrum contains antibodies that target the foal’s red blood cells, causing their destruction. This results in an anaemia and an abundance of red blood cell pigment (bilirubin), causing a jaundiced (yellow) discoloration to mucous membranes and urine.
Mares develop antibodies through exposure to foreign red blood cells from foals during the birthing process or from incompatible blood transfusions. Although uncommon, the exposure and subsequent sensitization of mares can occur during the first pregnancy if there is disease of the placenta (such as placentitis), causing the foal red blood cells to leak across the placental barrier. The antibodies can last for many years in mares and are transferred to foals through ingestion of antibody-rich colostrum.
The blood group system in horses is complex. The are seven systems (EAA, EAC, EAD, EAK, EAP, EAQ, and EAU), 34 factors and 59 alleles. Each factor and allele vary in their ability to evoke an antibody response with Aa and Qa factors being the most important. Mares without Aa or Qa red cell antigens are at greatest risk for having NI-affected foals assuming the foals inherit these red cell factors from the stallion.
Clinical Signs
Foals are born heathy with abnormal signs developing between 1 and 3 days after birth. The foals will become weak and lethargic due to the developing anaemia caused by destruction of their red blood cells. This will cause increases in both heart and respiratory rates. They will have pale and icteric (yellow) mucous membranes and the urine will become golden in colour. Rarely, affected foals may seizure if there is bilirubin pigment laid down in the brain (kernicterus).
Treatment
The combination of anaemia and high levels of bilirubin are consistent with NI. Treatment typically requires giving the foal replacement red blood cells. This can come from a universal donor (horse that is Aa and Qa negative and does not have antibodies), washed red blood cells from the mare (to remove the offending antibodies), or from a gelding (if the options for universal donor or washing mare cells is not available). The gelding has presumably not had a foal and hopefully has not received a blood transfusion. The breed is important as the frequency Qa and Aa negative horses varies. Good choices include Standardbreds (all Standardbreds are Qa negative) and Quarter Horses.
Prevention
Ideally, testing of mares before foaling to confirm blood type and the presence of offending antibodies is performed. This is now very difficult in most parts of the world. Any mare with a history of having foals with NI should be carefully managed after subsequent foalings. The foal should be separated from the mare (or muzzled) and fed an alternative source of colostrum and milk replacer. The mare should be milked out every couple of hours and the colostrum/milk discarded. It should be OK to remove the muzzle at 20-24 hours and allow the foal access to the mare. There is test that can be performed before sucking where the foals blood is mixed with mare colostrum to check for clumping (agglutination). This is called the Jaundiced Foal Test (JFT). The test does require glass tubes and a centrifuge.
Tags: Foals; Haemopoietic