horse fetus“The single most important cause of premature delivery is placentitis.”
(Photo : Tim Flach)

annet beckerAnnet Becker Summerhill Broodmare and Foal Care ManagerImproved diagnostic techniques and advances in the understanding of equine reproductive physiology and pathology have resulted in increased pregnancy rates in mares. In contrast, the incidence of early pregnancy loss has remained fairly constant at a rate of 10-15 %. Pregnancy losses during late gestation (> 5 months) represent an even greater problem for the equine breeding industry. Affected mares will not only fail to produce a foal, but will often have a lower conception rate during the next breeding season.

Pregnancy losses during late gestation could be the result of fetal illness, placental dysfunction, or maternal illness. Monitoring of maternal health and preventive treatments of the pregnant mare against diseases that may cause abortion have been performed routinely for decades. However, monitoring of the placenta during late gestation has only recently gained recognition in equine veterinary medicine.

Premature delivery of a weak or dead foal is devastating to horse owners. Even if they receive the best neonatal care, most of these foals, if they live, never have productive performance careers. The single most important cause of premature delivery is placentitis. It accounts for nearly one-third of late-term abortions and fetal mortality in the first day of life. Placentitis is most commonly caused by bacteria that ascend through the vagina and breach the cervical barrier.

An abnormal thickness and partial separation of the allantochorion from the endometrium has been observed in mares with clinical signs of ascending placentitis based upon transrectal ultrasonography. In advanced stages, the space between the uterus and the placenta is filled with hyperechoic fluid. In a field study on Thoroughbred mares at commercial stud farms, it was concluded that an increased CTUP (combined thickness of the uterus and the placenta) during mid and late gestation, indicates placental failure and pending abortion. None of the mares with normal thickness of the placenta lost their pregnancies, and all mares that aborted had a marked increase of the CTUP or placental detachment. Under practical conditions it was suggested that a CTUP >8 mm between day 120 and 300, >10 mm between day 301 and 330, and >12 mm after day 330 suggests placental failure and pending abortion.

For a while now, we have been measuring the CTUP of all the mares at Summerhill on a bi-monthly basis. Any mares with an increased CTUP are placed on daily antibiotics and are rechecked by the vet a month later to see if there is a decrease in the CTUP. If it has decreased, we suspend her antibiotic treatments, but if it increases she will continue on antibiotics for another month. In the previous years we have done this with all our ‘high risk’ mares with great success – all of them producing live, viable foals. As there is a 10-15% pregnancy loss in normal mares, we feel by checking their placental thickness on a regular basis, we see more mares carry to term and produce live foals.

While transrectal and transabdominal ultrasonographic examination of the placenta is very useful in detecting early signs of some placental pathology, it is important to keep in mind that placental changes resulting in periparturient problems can sometimes be subtle, and may not readily be detected on ultrasonographic examination.