Jaundice in Mares

By ATBC
Following a letter from Coolmore re a mare that had been shown as a possible jaundice carrier, Dianne Lanham asked Greg Mitchell (resident vet) to explain the condition. The following is his response:

“Anaemia

There are multiple causes of anaemia in the neo-nate (foal). Part of the anaemic process may be a jaundice presentation (yellow discolouration of the mucus membranes etc). Of interest in this discussion is a condition known as neo-natal isoerythrolysis as one of the primary clinical presenting signs for this condition is anaemia and jaundice.

Neo-natal isoerythrolysis (NI foal)

This condition occurs when a mare produces antibodies against her own foal’s red blood cells. The antibodies present in the colostrum of the mare are ingested and absorbed by the foal hence initiating the process. This condition parallels the “blue baby” condition as described in humans when a mother and father have incompatibility with regard to blood groups.

Why does this occur

A foal can inherit it’s blood groups from either of its parents. This condition of significance oc curs when the foal inherits a blood group antigen from the sire that is not present on the mare’s red cells. The most common blood groups of significance in this condition are the Aa and Qa groups, although other groups have been implicated, statistically they are suggested to be of less significance. If the antigen is exposed to the red cells of the mare she reacts by producing antibodies in response. Basically the mare recognizes theses antigens as foreign and mounts an immune response against them.

Factors.

There are 32 blood group antigens identified in the horse. The most common blood groups in volved in this condition are the Aa and Qa groups. That is, a mare with blood type of (Aa- Qa+) or (Aa+ Qa-) or (Aa- Qa-) mated with a stallion that is Aa+ or Qa+ or (Aa+ Qa+) has a percentage chance of producing a foal that has inherited either the Aa+ factor, the Qa+ factor or both and hence has a chance the mare will produce antigens against theses factors which could potentially result in the syndrome known a NI in foals.

Prevalence

Approximately 22% of the thoroughbred mare population is Aa- or Qa- and hence able to pro duce the antigen. Reality suggests that only 50% of these mares do.

Clinical Signs.

This condition rarely occurs during the first pregnancy as it requires some exposure between the mare’s and foal’s red blood cells. Although this can occur during the foaling the actual immune re sponse mounted by the mare may take some nine days and hence not affect her first foal. Signs consistent with NI vary usually from about 8 to 96 hours post parturition. Severity of signs de pends on the amount of anti red blood cells antibodies absorbed by the foal. Signs noted include weakness and lethargy, icterus (jaundice/discolouration, particularly of the sclera of the eye and mucous membranes) elevated heart and respiration rate, there may be red tinged urine and if anaemia is severe enough, seizures and death may occur.

Recommendations To Prevent NI

Mares are tested for specifically the presence of Aa and Qa genes. If she lacks one or both of these, she is considered to be a risk of producing NI causing antibodies. Mated with a stallion that is either Aa+ Qa+ or both places her firmly in the high risk category. High risk mares are noted and at foaling their foals are prevented from nursing initially whilst a test is performed to confirm or negate the presence of this condition. The test involves a cross match between the mare’s an tigens and the foal’s red cells (this is a Coomb’s test). Following a negative Coomb’s test, the foal is allowed to nurse normally with little to no risk. Following a positive Coomb’s test the mare’s co lostrum is withheld from the foal and the foal is supplemented for up to 48 hours with non-reactive colostrum. The mare’s colostrum is milked and discarded.