Page 56 - November 2015

Blood-type incompatibility that results in the mare’s antibodies attacking the foal’s red blood cells is called neonatal isoerythrolysis.
by Heather Smith Thomas
Blood-type incompatibility that results in the mare’s antibodies attacking the foal’s red blood cells is called neonatal isoerythrolysis (NI). The
word neonatal means newborn, iso means “self”, erythro is a term for red blood cells, and lysis means breaking them up. Since this condition is similar to Rh sensitivity in human infants, some people called these Rh foals. Another term is foal jaundice. The yellow color of gums and other mucous membranes (such as whites of the eye) is due to breakdown of red blood cells, with pigment ending up in the tissues.
This can create a lethal situation for the newborn foal. If he inherits certain blood components from the sire that are not part of the mare’s blood make-up, her immune system recognizes the foal’s red blood cells as “foreign” if any of them get into her bloodstream, and she creates antibodies against them. Since there may be some blood transfer between the dam and fetus, the mare is exposed to the foal’s red blood
cell antigens and this triggers her immune system to attack and destroy them. The antibodies she creates do not affect the fetus, however, because they are too large to slip through the placental barrier. The foal is safe until he is born. He is at risk as soon as he drinks her colostrum since it contains a representation of all her antibodies.
Other ways a mare might have become exposed to a blood type different than her own - stimulating her to create antibodies against it - is if she was ever treated with blood or blood-based products, accord- ing to Kristina Lu, VMD, a reproductive special-
ist and Associate Veterinarian at Hagyard Equine Medical Institute. “If the mare received a blood transfusion at some point in time, for instance, this could be a way she could become exposed. There’s also a theory that placental leakage could expose her to the foal’s blood,” says Lu.
In some instances, the mare may become exposed to a foal’s different blood type while
Kristina Lu, VMD,
a reproductive specialist and Associate Veterinarian at Hagyard Equine Medical Institute
Nathan Slovis, DVM, DACVIM of Hagyard Equine Medical Institute
NeoNatal IsoerythrolysIs (NI) IN Foals
giving birth if blood from the foal/placenta has access to her bloodstream (for example, trauma during birth). This would not affect that particular foal, but the mare might then develop antibodies in her bloodstream that could affect her next foal. “It’s usually not the first exposure that endangers her foal,” explains Lu.
“Many people think NI can’t happen with a mare’s first foal, but occasionally it does. A possible cause could be hemorrhage across the placenta earlier in gestation. Another possibility is that sometime in her life there was exposure to blood. Horses often change hands and you don’t know their entire medi- cal history. Here in Kentucky, most people test every broodmare, including the maiden mares, to be safe. This disease is easy to prevent, but can be expensive to treat,” she says.
Nathan Slovis, DVM, DACVIM of Hagyard Equine Medical Institute, says Thoroughbreds seem to be more affected than some of the other breeds, along with Standardbreds, Paints, and donkeys.
Prevention
“Since NI is easy to avoid, we encourage people to try to prevent it. We remind our clients to do NI screens ahead of foaling,” says Lu.
“Another option less commonly used is blood typing mares and stallions. Textbooks tell you to not breed your mare to a stallion with a blood type that will lead to this disease,
but blood types are generally not known on the stallion. And, many people want to breed to a certain stallion even if he has an incom- patible blood type. I don’t know of anyone who chooses a stallion based on blood
type. There are other things that are more important,” she says.
54 SPEEDHORSE, November 2015
equine health