Neonatal Isoerythrolysis
History. You are called to examine a mare, 7 months pregnant, that has a previous history of having conceived and delivered a normal foal after her first pregnancy; the foal was subsequently suckled and was sold as a weanling.
The mare had no trouble conceiving and carrying the next two pregnancies, but the foals died within 2 to 4 days of birth, even though they were healthy and vigorous at birth and the mare had colostrum and milk. The previous owner became discouraged because of these deaths and sold the mare to the current owner at a bargain price.Clinical Examination. You perform a general physical examination of the mare and find all organ systems to be normal. Palpaliuii of the uterus per rectum reveals the presence of a viable fetus that appears to be of the correct size for a pregnancy of the purported duration. Both the external genitalia and the mammary glands are normal in appearance.
Comment. From the history, and because the mare appears to be undergoing a normal pregnancy, you conclude that there is nothing wrong with the reproductive process per se. The fact that the previous two foals were healthy at birth and yet weakened rapidly and died within 4 days indicates that something likely happened to them after delivery. If the deaths were caused by an issue associated with the mare, the most likely cause of these deaths would be neonatal isoerythrolysis. In this situation the mare becomes exposed to the red blood cells (RBCs, erythrocytes) of the fetus during pregnancy, or the mare could have been exposed to RBCs from a stallion, whose erythrocytes are recognized as foreign. If the mare is exposed to the RBCs from the stallion or the fetus, those RBCs enter the circulation of the dam. She responds by making antibodies to the RBCs because of the presence of foreign antigen on the fetal erythrocytes that were inherited from the sire.
In the mare, these antibodies do not pass through the placental barrier, so the fetus is protected from these antibodies during pregnancy. The antibodies do pass into the colostrum and are concentrated during the process of colostrum formation. Therefore, when the foal suckles the colostrum, it acquires the antibodies that will react to its own RBCs. The foal develops a type II hypersensitivity reaction, in which the antibodies destroy the foal’s RBCs through different mechanisms.Treatment. The foal needs to be prevented from suckling the mare for the first 2 to 3 days of life. During the first 1 to 2 days, the foal is able to absorb large protein molecules, including the important immunoglobulins that enable the foal to ward off infections, as well as, in this case, antibodies against fetal RBC antigens. The gut epithelium closes to the passage of large protein molecules by 36 to 48 hours of life; at this time, or shortly thereafter, the foal can be allowed to suckle without risk of absorbing the antibodies. The key is to prevent the foal from suckling during the first 2 to 3 days of life to prevent absorption of the antibodies that will react to its own RBCs. The mare needs to be monitored closely before parturition so that the foal can be muzzled shortly after delivery. The foal does need nourishment during the first
2 to 3 days of life; thus it is important that the foal be fed colostrum obtained from other mares (usually maintained frozen). If the foal does receive the mares colostrum containing antibodies that will react to its own RBCs, the foal can still be treated. The foal should not nurse the mare for the first
3 to 5 days of life, and the foal can be given packed RBCs from a donor if necessary. To anticipate the potential risk, blood typing of stallions and mares is now available.
SECTION Vl Reproduction and Lactation