Each year a small percentage of foals are born to mares whose immune systems, in an odd twist of nature, are prepared to battle their own foals. Antibodies from the mare, delivered to the foal in the colostrum it drinks soon after birth, attack the foal's red blood cells.

The condition that results is neonatal isoerythrolysis, more commonly known as jaundice. The occurence of NI is relatively rare. It happens in only about 1% of Thoroughbred foals and in about 2% of Standardbred foals. The relative infrequence, though, is offset by the possible severity of the condition.

Because the red blood cells in the foal are destroyed, the foal can become severely anemic and, in some cases, die. William V. Bernard, DVM, Diplomate ACVIM, of Rood and Riddle Equine Hospital in Lexington, said about 50% of the NI foals he sees require transfusions to recover. An NI-stricken foal must also be stabled to restrict activity until it has recovered. The low red blood cell count, or anemia, that results from the condition makes the foal at risk if it becomes active. With fewer red blood cells to carry oxygen, the foal can collapse and die if it tries, for example, to keep up with its' dam in a paddock or pasture. In some cases, a foal with NI has died as it exercised in a field or paddock.

The condition can be prevented if a screening test performed in advance indicates the mare has produced the antibodies. If that is the case, the foal is then kept from the mare and her colostrum is stripped.

The condition comes about when there are certain incompatabilities in the genetic makeup of the blood of the dam and sire. While that is common, most blood groups do not give rise to the severe antibody reaction that results in NI. Almost all cases of NI result in what are called the Aa factor of the A blood group and the Qa factor of the Q blood group.

For the condition to result, several things must be present. Among them:

  • The dam must be negative for the blood group in question. Since the condition is most commonly associated with the Aa and Qa groups, dams that are either Aa negative or Qa negative are considered to be at risk of producing a jaundiced foal.
  • The dam must have been exposed previously to the antigen that causes her blood to produce the antibodies. Bernard said that immune systems are thought to have, in effect, memories. As a result, an initial exposure to an irritant, or antigen, can produce only a mild response or no evident response. However, since the system remembers the antigen, a full-blown response can occur the next time it is exposed. It is because of this factor that NI rarely occurs in first foals. Those mares usually have not had the initial exposure to the antigen that would result in the full-blown response on a second or later exposure. This condition can occasionally happen in a first foal if the mare had been exposed to the antigens through a blood transfusion or other means.
  • It is also necessary that the foal inherit the blood characteristics of the sire that produce the reaction in the mare. If the foal has the same blood characteristics as the mare, the reaction would not be induced. However, if the foal's blood contains the antigens that produce the reaction in the mare, NI could result. If that is the case, then the mare, exposed to those antigens through either leakage through the placenta during the pregnancy or leakage at birth, can produce the antibodies that will be passed in her colostrum.

Recognizing And Treating Neonatal Isoerythrolysis

Foals are not born with NI. A foal must ingest the antibodies from the mare's colostrum before the condition occurs, so it is often 24 to 36 hours after the first nursing episode that the signs of NI will occur.

Early signs are weakness and lethargy, which tend to become more severe as the hours pass. A paleness in the eyes and mucous membranes, the result of the anemia, can often precede the yellowing or jaundice in those membrances. The foal's breathing will change, becoming shallow and, later, difficult or labored as the condition worsens.

The progress of the disease can be rapid in a foal who was born, to all appearances, perfectly healthy.

"It can happen quickly," Bernard warned.

Sometimes, the onset does not occur until four or five days after birth. However, Bernard said that when jaundice and/or anemia turns up at that point, "you wonder if it is something else."

The second-most-common cause of jaundice, he said, is liver disease, but that is rarely found in newborn foals.

If NI is present, the red blood cell indicators will decrease (due to the anemia) and bilirubin will increase as a result. Packed red bood values can drop below 20%, and sometimes be as low as 5%. For a definitive diagnosis, tests are necessary. If antibodies are found attached to the foal's red blood cells, there is a definite diagnosis of NI.

At that point, the antibodies present in the mare's milk might already have diminished and the foal's intestinal tract could have become less absorptive. As a result, taking the foal off the mare's milk probably is of little help.

Instead, attention is given to relieving the symptoms rather than eliminating the cause once the foal's symptoms have arisen and the diagnosis confirmed. First, as indicated above, it is important to limit the activity of the foal, weakened as it is by the anemia. Additional fluids, administered intravenously, are usually indicated to assist the kidneys in handling the unusual quantities of hemoglobin that are a result of the condition.

As Bernard indicated, blood transfusions can be part of the treatment because of the anemia that is present in NI. Bernard said he often uses red blood cells from the mare, washed with a saline solution, as a transfusion for the foal. It would not be uncommon to have a transfusion of two liters for an NI foal, he said.

Treatment, though, is only part of the solution, and not the best part, Bernard said. "I preach preventive medicine," he said.

In this case, preventive medicine means testing the mare for the presence of antibodies before the foal is allowed to nurse and ingest colostrum. If antibodies are found, then the foal should not be allowed to nurse from the mare during the first couple of days. The mare should be milked to eliminate the colostrum containing the antibodies. Once the foal's intestinal track has matured and the colostrum with antibodies has been depleted from the mare, the foal can begin normal nursing. The foal can be fed colostrum from a donor.

Jaundice screens, as the tests are known, cost about $15 to $20. While the cost could mount up over a large group of mares, Bernard endorses it. "If you have horses of any value at all, it's cheap insurance," he said.

One final note of warning. A jaundice screen will find the A and Q antibodies that can attack a foal's red blood cells, but it might not turn up every antibody that could attack. Bernard said two NI foals died last year. Even after extensive testing, he said, laboratories could not identify the antibodies that had attacked the foals.

About the Author

Jacalyn Carfagno

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