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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
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
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,
1. Acland HM. Abortion in mares. In: McKinnon
AO, Voss JL, eds., Equine reproduction. Philadelphia: Lea and Fe-biger,
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;203:1170–1175.
3. Prickett ME. Abortion and placental lesions
in the mare. J Am Vet Med Assoc 1970;157:1465–1470.
4. Sertich PL. Clinical anatomy and evaluation
of equine fetal membranes, in Proceedings. Annu Meeting Soc
5. Adams-Brendemuehl C, Pipers FS. Antepartum
evaluations of the equine fetus. J Reprod Fertil Suppl
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
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
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.
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
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.
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