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Disclaimer: The information on this page is to be used as a guideline.  Some information comes from SHG Members, while other information comes from internet sources.  You are advised to always contact your veterinarian regarding any health issuesThe officers and members of the SmallestHorse Group are not responsible for the content.

Late Gestation Abortion

Submitted by:  Judy Zachary - Zachary Farms

The incidence of abortion after the fourth month is very low. Maybe as low as 5 percent of all pregnancies abort after the fourth month. Most causes of late term abortion are caused by infectious disease, some of which are contagious. This means that abortions can occur in storms on some farms with great economic loss. The best prevention is maintaining small stable herds of pregnant mares with no movement of new horses into or around the herd. Sometimes late term abortions are caused by infection that is secondary to a damaged cervix.

If the mare's cervix does not form a good seal, abortion can result in late pregnancy due to a placentitis. Scientific Reports Transrectal Ultrasonography of the Placenta in Normal Mares and Mares with Pending Abortion: A Field Study Mats H.T. Troedsson, DVM, PhD; Catherine D. Renaudin, DVM; Walter W. Zent, DVM; and John V. Steiner, DVM Am Assoc Equine Pract PROCEEDINGS 1997

Using transrectal ultrasonography, we have recently established the normal range for the combined thickness of the uterus and the placenta (CTUP) at an area immediately cranial to the cervix during midgestation and late gestation in normal pregnant mares.

Results from this study suggest that an increased CTUP during middle 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. All mares with normal pregnancies after 271 days of gestation had a CTUP within 2 SD of the control. However, more than half of these mares had a CTUP above 2 SD of the controls between 150 and 270 days. The conditions of examination may explain the differences between the CTUP in control mares and mares examined at farms. The hospital conditions, under which the normal range was established in control mares, six differ from field conditions in time spent per examination, restraining arrangements, and lighting conditions.

Measurement of the CTUP below 5 mm may be difficult to measure accurately at farms, and the present study suggests that a CTUP of up to 7 mm prior to day 300 of gestation may be considered normal under field conditions.

A CTUP above 2 SD of the established normal range after 271 days of gestation suggests placental failure and pending abortion (8 mm between days 271 and 300; 10 mm between days 301 and 330; and 12 mm after day 330).

A CTUP above 10 SD of the normal range prior to 270 days of gestation also indicates pending abortion. A CTUP slightly above normal but below that for placentitis should be monitored closely until placental disease can be ruled out, based on return to a normal CTUP or absence of any clinical evidence of disease.
It was concluded from this study that transrectal ultrasonographic examinations of the CTUP can be used under field conditions to monitor early signs of placental failure and pending abortion. Mares with an abnormal combined thickness of the CTUP should be considered to be at risk of abortion. The importance of early diagnosis to prevent abortion in mares with placental failure and the efficacy of different treatment regimes requires further investigation. This study was supported by AGRIA Insurance Company, Sweden.

1. Acland HM. Abortion in mares. In: McKinnon AO, Voss JL, eds., Equine reproduction. Philadelphia: Lea and Fe-biger, 1993;554562.
2. Giles RC, Donahue JM, Hong CB, et al. Causes of abortion, stillbirth, and perinatal death in horses: 3527 cases (1986 1991). J Am Vet Med Assoc 1993;203:11701175.
3. Prickett ME. Abortion and placental lesions in the mare. J Am Vet Med Assoc 1970;157:14651470.
4. Sertich PL. Clinical anatomy and evaluation of equine fetal membranes, in Proceedings. Annu Meeting Soc Theriogenol-ogy 1993;178184.
5. Adams-Brendemuehl C, Pipers FS. Antepartum evaluations of the equine fetus. J Reprod Fertil Suppl 1987;35:565573.
6. Renaudin CD, Troedsson MHT, Gillis CL, et al. Ultrasono-graphic evaluation of the equine placenta by transrectal and transabdominal approach in the normal pregnant mare. The-riogenology 1997;47:559573.

Though the cause is infectious, this is not a contagious problem and the infection is usually normal vaginal inhabitants. It is a frequent problem of mares with a history of a difficult birth. A careful appraisal of a mare with a history of a difficult birth, difficulty settling, or abortion includes a careful exam of the cervix which should include visual inspection, manual palpation, and ultrasound. You are looking for tears that have not healed well or scar tissue which may prevent proper sealing during pregnancy.

Recent work has tried to characterize ultrasound findings and the possibility of late term abortion and has found some predictive indicators for abortion. However we do not know yet know if this information can be used to prevent abortion with appropriate treatment like antibiotics and progesterone.

Viral Abortion
There are a number of infectious agents that can cause abortion. Equine Herpes Virus 1 is the most common. This virus becomes a threat in the last half of pregnancy and the mare frequently shows no symptoms before or after the abortion. The fetus is expelled dead with no outward lesions, though commonly edema and small hemorrhages will be found in the lungs and small white spots on the liver.

Another virus which can cause abortion is Equine Viral Arteritis. Unlike herpes virus abortion, EVA causes the mare to develop flu like symptoms with conjunctivitis and peripheral edema. Abortion follows in a week or two. Vaccination can reduce the incidence of viral abortion, for more information.

Bacterial Abortion
Bacteria and fungi can be infrequent causes of late abortion and most gain entrance to the fetus through the cervix. The diagnosis is not hard as the fetal membranes will show changes indicative of inflammation and the organism can usually be isolated from the foal's lungs. Cervical trauma during a difficult birth may result in a cervix that allows bacteria in during the last trimester.

Leptospirosis is a bacteria that can gain access to the fetus by way of the blood stream. The bacteria is hard to isolate from the fetus, but the mare will show a rising titer in paired blood samples.

Endotoxemia can be a cause of abortion and may be due to colic, colitis or grain overload. Recent research by Dr. Peter Daels shows Banamine (Reg TM) to be ineffective at preventing abortion following an endotoxic episode. On the other hand Regu-mate (Reg TM) was very effective at preventing endotoxin induced abortion in the two? to four month range of pregnancy, for more information.

Twins will cause late term abortion, though some mares will carry one or both live to term. Prevention involves early recognition and appropriate treatment. Recent ultrasound studies have found ways to make a diagnosis late in pregnancy, usually a difficult time to detect twins, for more information.

Certain drugs will cause abortion. Prostaglandins and possibly corticosteroids and organophosphates will cause abortion.

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