The jaundiced foal agglutination

Wednesday, February 14, 2007

Mare-foal Incompatibility

The mare-foal incompatibility test is a crossmatch procedure that looks for incompatibility between the mare and the foal. Specifically, the test compares whether there are antibodies in mare serum (or colostrum) to foal erythrocytes. The mare-foal incompatibility crossmatch is a test for the confirmation or prevention of neonatal isoerythrolysis (NI). Neonatal isoerythrolysis is a hemolytic anemia that occurs in foals (that inherit their sire's blood group antigens) born to mares of a different blood type to the stallion they were mated to. The mare is sensitized to the blood group antigen of the stallion through previous pregnancies (the most common situation), previous blood or plasma transfusions or vaccination with products containing equine erythrocyte antigens. Sensitization occurs during pregnancy due to a phenomenon called retroplacental bleeding, in which the foal's blood comes into contact with the mare's circulation during the last few weeks of pregnancy. It can naturally occur earlier if there is any placental pathology. Therefore, NI occurs usually in multiparous mares and only after the foal has ingested colostrum. The most common blood types in the horse that produce NI are Aa and Qa antigens, although NI to other antigens (including Pa and Ua) has been reported. When the foal ingests colostrum containing antibodies from the mare against the sire's red cell antigens, the foal (as long as it has inherited the sire's blood type) suffers a hemolytic anemia, which can be severe and result in neonatal mortality. Furthermore, all horses lack an antigen that is unique to donkey red blood cells (so-called donkey factor). Thus, mule pregnancies are at risk of initiating NI.

To determine if NI has occurred in a foal, a crossmatch can be performed on the mare and foal. This requires the submission of erythrocytes from the foal (EDTA blood) and serum from the mare. Alternatively, a direct Coombs test can be done on the foal's blood (this requires an EDTA tube). However, these tests are usually performed by referral laboratories and the results may only be available after several days. There is a kit available, called the jaundice foal agglutination test, that can be performed "stallside" (if you have a centrifuge). This test assesses for mare-foal incompatibility by testing for agglutination between mare colostrum or serum and foal blood. Colostrum or serum is serially diluted and mixed with foal blood in test tubes. The tubes are centrifuged and the button is observed for agglutination. If agglutination is present, the button does not disperse when the tube is flicked. A positive reaction at a titer of > 1:16 is indicative of an incompatibility (at lower titers, false-positive reactions may occur) and indicates that colostrum should be withheld from the foal. This is a useful test to determine if a mare, that has previously had a foal with NI and has been mated to the same stallion that produced the NI foal or another stallion of unknown blood type, has antibodies to the current foal. However, the test does not look for hemolysis and is less sensitive than the routine crossmatch procedure (which assesses for hemolysis and microscopic agglutination).

Neonatal isoerythrolysis can be prevented by testing a susceptible mare before parturition and withholding colostrum. Ideally, the mare should be crossmatched with the stallion (stallion red cells and mare serum), especially in the last few weeks of pregnancy when exposure to the foal's erythrocytes is likely to occur (thus boosting the antibody titer, if there is an incompatibility). Some authors recommend repeat crossmatching every two weeks in the last month of pregnancy to look for a rising titer (often increases from <>1:256 in the last 4 weeks prior to foaling). A rising titer indicates a high likelihood of NI and reveals the need for withholding of colostrum from the foal. Alternatively, the jaundice foal agglutination test can be used at parturition. Note that a single crossmatch between a mare and a stallion may not reveal an incompatibility if the mare is tested before or early in pregnancy (due to low antibody titers that may not produce an incompatible reaction).