Immune mediated hemolytic anemia means the immune system is attacking the red blood cells and causing them to break down.

Neonatal isoerythrolysis is a form of immune mediated hemolytic anemia that effects foals during the first couple of days after birth. In this disease the break down, termed hemolysis, of the red blood cells occurs because they are attacked by specific anti-red blood cell antibodies that are ingested and absorbed from the mare's colostrum. As the red blood cells are broken down the foal becomes jaundiced and weakens and severe cases are life threatening. To treat this disease it is important to rule out other common causes of newborn foal diseases. To prevent this disease you have to understand why some mares form these anti-RBC antibodies. This article explains how it happens, diagnosis, treatment, and prognosis. Also included is a discussion on preventing the problem in "at risk" pregnancies and laboratories that blood type.

Horses have quite a few "blood types" but two almost universal blood types are the Aa and Qa types. For the purpose of this discussion you should think of blood types as little genetically determined molecules on the surface of the red blood cells that the immune system can recognize. We call such molecules "antigens". Once recognized the immune system determines whether this is a "self" antigen or "foreign" antigen. If it is foreign it attacks it.

Almost all horses have the Aa and Qa antigens on their red blood cell. They would be called blood type Aa positive or Qa positive. But ever so often comes along a mare that lacks these blood types and because they are not self antigens if she becomes exposed to them her body will read them as foreign and develop antibodies to them. The first step towards a newborn with NI is the mare being exposed to a foreign blood type and then becoming sensitized to it. There are a number of ways a mare may be exposed but often they become exposed to a foreign blood type giving birth. But there are other possibilities like having a previous blood transfusion. Whatever the source, if the mare is exposed to these blood types which are different than hers, she will develop antibodies to the antigens that define the blood type. Once exposed it takes about a month to develop a large number of circulating antibodies to the antigen. If the mare has a foal during subsequent pregnancies with the foreign blood type he will take in the mare's antibodies when he nurses the colostrum and they will attack his red blood cells.

Let's look at an example. Imagine a maiden mare that is negative for the Qa antigen. She is bred to a Qa positive stallion. He passes this blood type along to the new formed foal, and when the foal is born the mare becomes exposed to the foal's blood when the placenta separates. It is important to note that different than many other mammalian species, the mare's and fetus's blood to not comingle during gestation, So about a month following the birth the mare develops anti-Qa antibodies. Now she is rebred to the same stallion or perhaps another stallion with the same Qa blood type and this is passed onto the developing foal. Because the mare's blood does not come into contact with the fetus, he develops normally and born without complications. However when he first nurses the mare she passes these anti-Qa antibodies to the foal through the colostrum. He absorbs them into his blood stream where they beginning to attack his RBC's. The severity of the disease will depend mainly on how many antibodies the mare passed along to the foal and can be quite variable.

Because donkeys have some unique blood type characteristics almost all mares bred to donkeys are susceptible to developing antibodies to the donkey's blood factors making subsequent pregnancies at risk.