More info about an animal
species: click on a picture:


Abortion in horses

Written by:

Dr. Bob Wright – Veterinarian/Disease Prevention/Equine and
Alternate Species/OMAFRA; Dr. Dan Kenney –
Diplomate/A.C.V.I.M./Ontario Veterinary College/University
of Guelph.
Ministry of Agriculture, Food and Rural

You can find more about
horses here


Abortion in horses may result from a variety of causes.
Infectious agents, such as bacteria, viruses or fungi, may
attack the fetus or its membranes, resulting in fetal death
and expulsion. Other factors attributable to the mare,
fetus or external forces may also cause loss of the embryo
or fetus. These factors include twinning, hormonal
deficiencies, congenital anomalies, ergot alkaloid toxicity
or ingestion of tent caterpillar setae. Some of the more
common causes are discussed below.



Middle: aborted foal

Right: abortion due to twinning

• Gestation length: 330 to 340 days

• Umbilical cord length: 36 to 84 cm

• Placental weight (thoroughbred): 5.7 ± 0.08 kg (about
12.5 lb) or 11% of the foal’s body weight

Causes of Abortion

Over the past six years, infectious abortions have
accounted for approximately one-third of all abortions
diagnosed. Equine herpesvirus (EHV-1) is the major
infectious agent, accounting for 18% of all abortion


Equine herpesvirus (EHV-1 abortion
virus) is most often associated with abortions in mares,
while herpesvirus EHV-4 (rhinopneumonitis virus) is usually
associated with respiratory disease in young horses. Both
subtypes have the potential to cause respiratory disease
and abortion. Vaccines are available as aids to prevent
abortion due to EHV-1 infections. The herpes family of
viruses has the capacity to persist in the body of its host
in a dormant state as an inapparent carrier after the
primary infection. Months or years after the primary
infection, the latent herpesvirus may again become manifest
with renewed replication and with the potential for
initiating new outbreaks of disease in its host as well as
susceptible stable mates. Therefore, it is the existence of
these latently infected carrier horses, from which the
virus is re-activated by stress-induced circumstances and
shed into the environment to infect other individuals, that
initiates a new outbreak of the disease.

A fetus may become infected 20 to 90 days after its mother
contacts the disease. Most abortions due to this virus
occur between 8 and 11 months of gestation, although they
may occur as early as 5 months. In some cases, the foal
will be born alive at term and will die shortly after birth
due to infection by the virus. The abortion rate may
approach 100% in a herd of susceptible mares. The vaccines
that are currently available are the best we have but are
of questionable value in preventing abortion.

Equine arteritis virus
(EAV) has the potential to cause abortion as well as the
more commonly observed contagious respiratory disease and
semen-shedding state in infected carrier stallions.


Several species of bacteria have been incriminated as
causative agents of equine abortion and sterility. The most
common cause of bacterial abortion at present is organisms
of the Streptococci
oup. Other bacteria frequently identified from
aborted feti include Leptospira, Nocardia, Klebsiella and Staphylococcal species.

These agents gain entrance to the reproductive tract and
travel to the uterus, causing infection of the fetal
membranes, resulting in abortion. Retention of the placenta
is often a sequel to bacterial abortion as is infection of
the uterus (endometritis and/or metritis). Treatment of the
mare is often required before she can be rebred
successfully. It is a good practice to swab mares before
rebreeding to determine if harmful bacteria are present in
the uterus. Your veterinarian can provide advice on the
need to lavage your mare’s uterus with or without an


Leptospira infection in horses is manifested as either
abortion or recurrent uveitis (moon blindness) in the
horse. Leptospires are motile bacteria called spirochetes.
Leptospires are very common in domestic and wild animals,
and can also infect humans. In particular regions, there
will be one or more maintenance hosts that serve as the
reservoir for the infection. Leptospires will be
transmitted to incidental hosts by shedding into the
environment from a maintenance host. They can invade the
mucous membranes and/or damaged skin and migrate to various
body organs of an incidental host. The maintenance hosts
can be wildlife such as deer, raccoons and rodents.

The diagnosis of leptospirosis will often be one of

the Infectious Agent

When trying to diagnose an infectious agent, veterinarians
will collect paired sera taken three weeks apart as an
essential for historical evidence of a recent infection. A
fourfold change in titre to an organism in the paired sera
would provide a presumptive diagnosis.

Demonstration of the organism in the placenta or antibodies
to the organism in fresh placental tissue, fetal stomach
contents, fetal serum, fetal aqueous humour or fetal
viscera by fluorescent antibody techniques, silver staining
or dark field microscopy for leptospirosis would provide
more conclusive evidence.


Primary umbilical torsion is the most commonly diagnosed
condition of the non-infectious causes of abortion.

Abortion due to

It is well known that the birth of healthy twin foals is
unusual. It is generally accepted that the inability of a
mare to successfully carry twin foals to term is due to
placental insufficiency. In other words, insufficient fetal
membranes are produced to accommodate and provide nutrition
to two developing feti. There is a greater possibility of
twin conception towards the end of the breeding season and
the incidence of twins is higher in young mares. With the
common use of ultrasound in equine practice, twin
pregnancies are now commonly detected early and dealt with.

Abortion due to Progesterone

Progesterone is the hormone whose function is to prepare
the uterus for the reception and development of the
fertilized egg. In most animals, this hormone is produced
primarily in a structure known as the “corpus luteum of
pregnancy.” This structure forms at the location of the
follicle that ruptured releasing the egg, which was
fertilized at conception. The corpus luteum is composed of
specialized cells that produce progesterone. In the horse,
the corpus luteum of pregnancy produces enough progesterone
to maintain pregnancy for only 40 to 50 days. At about this
time, structures known as “endometrial cups” develop in the
uterus. These structures secrete a hormone known as
gonadotropin that stimulates the ovaries to produce more
eggs, resulting in the formation of more corpora lutea.
These newly formed corpora lutea produce the progesterone
required to maintain pregnancy into the fourth or fifth
month of gestation. At this stage, the placenta takes over
production of progesterone to term. Abortions at the fourth
or fifth month of pregnancy are often attributed to a lack
of sufficient hormones to maintain pregnancy. Whether this
is true is open for debate. Many mares are treated with
synthetic progesterone (Regu-Mate®) to help maintain a

Umbilical Cord Torsion

Torsion, or strangulation of the umbilical cord, is said to
be the cause of fetal deaths and abortions in the later
stages of pregnancy. The cord normally has three clockwise
turns. In a few cases, excessive twisting or wrapping
around the limb of the fetus shuts off the flow of blood in
the cord, resulting in the death of the fetus. Umbilical
torsion was observed in 19% of 515 cases submitted to the
Animal Health Laboratory over six breeding seasons3.

Congenital Defects

In recent years, mares are being diagnosed pregnant with
the ultrasound by day 15 of gestation. Before the common
use of the ultrasound, early embryonic deaths would be
mistaken for failure of conception followed by silent heat.
Many of these early embryonic deaths have genetic or
chromosomal defects that lead to improper development of
the embryo and subsequent rejection by the dam.

 Other Causes

Ergot Alkaloid Toxicity in the Late-Gestation Mare

A number of fungi, both saprophytic and endophyte, can
produce mycotoxins in the class of chemicals called ergot
or, more specifically, ergopeptine alkaloids. Mares are
sensitive to ergopeptine alkaloids at levels as low as
50-100 ppb, while cattle do not show visible signs until
1,000-2,000 ppb. These alkaloids exert toxic effects on the
reproductive tract and mammary gland of the mare and have
been associated with depression of serum prolactin and
progestagens (5 alpha-pregnanes), a prolonged gestation, a
thickened edematous placenta and agalactia. The ergopeptine
alkaloids interfere with the normal rise of progestagens
(mainly 5 alpha-pregnanes) and prolactin in the last 40
days of gestation. The progestagen levels normally increase
from 300 days to birth (4.8 ± 1.5 to 22.7 ± 2.7 ng/mL).
Foals born without the normal increases in maternal
progestagens suffer hypoadrenocortical function and are
small, weak or stillborn.

Fescue Toxicity is the form of ergot alkaloid
toxicity that has been widely seen in the U.S. It is caused
by the endophytic fungus Neotyphodium coenophialum
(formerly called Acremonium coenophialum). It lives inside
the plant and produces ergot alkaloids (primarily
ergovaline), resulting in the disease condition called
fescue toxicity. These chemicals cause dystocia in mares
and deaths of foals. The primary clinical signs of ergot
alkaloid poisoning in the late-gestation mare include:
extended gestation length from 11 to 12 months;

• dystocia, with mares sometimes trying to foal for many

• agalactia (little or no milk) with poor quality colostrum
(low immunoglobulin levels);

• “red bag” placentas from premature separation;

• thick edematous placentas with weights exceeding 6.5 kg
for a thoroughbred mare;

• weak or dead foals with aspiration pneumonia from
struggling to get out through a thickened placenta.

Endophyte-free varieties of fescue have been developed to
get away from the problems caused by these mycotoxins.
Endophyte-infected varieties of fescues are commonly used
for erosion control and golf greens. Occasionally,
endophyte-infected seed will be accidentally sold to horse

Ergotism is the clinical syndrome caused by the genera,
Claviceps. It is a saprophytic fungus that lives on the
plant, utilizing the plant’s nutrients without a
recognizable benefit to the host. Claviceps can live on a
variety of hays and pasture grasses and produce fruiting
bodies on bluegrass and cereal rye. The Claviceps sclerotia
(fruiting bodies) contain a large array of ergopeptine
alkaloids similar to those seen in fescue toxicity. During
the 1999 foaling season in Ontario, seven perinatal foals
died when their dams consumed cereal-rye-straw bedding. The
mares had thickened edematous placentas and agalactia.

Mare Reproductive Loss

Mare reproductive loss syndrome is a term given to the
syndrome that resulted in the loss of hundreds of foals
(approx. 500) and spontaneous abortion in mares in central
Kentucky and parts of Ohio in the spring of 2001.

Evidence points to the setae (hair-like projections) of
Eastern tent caterpillars as the primary cause of the
problem. They can become imbedded in the submucosal lining
of the gastrointestinal tract and cause areas of
inflammation and an entrance for bacteria. These bacteria
can then enter the blood stream and localize in the
placenta and fetus, resulting in abortion.


Nutritional deficiencies have not been associated with
abortion in mares. In general, if mares are in good
condition (body condition of greater than 2 on a scale of 0
to 5, where 5 is very fat), they will carry a foal. Mares
that are too thin, however, will not cycle or conceive.


Certain drugs (e.g., corticosteroids) have been reported to
cause abortion in mares. These reports have seldom been
substantiated. Examine drug labels to see whether
administration to pregnant animals is contraindicated.

In spite of common beliefs, injury seldom causes abortion.
Experimental rough manual manipulation of the pregnant
uterus has not caused abortion or embryonic death.

Only 40% of the equine abortion cases submitted to
veterinary diagnostic laboratories are diagnosed. Aborted
foals and their placentas should be submitted to a
laboratory as quickly as possible to help improve the
success rate of diagnosis. In most cases, little can be
done in the current breeding season, but a course of action
to prevent losses in other mares or in subsequent seasons
can be determined.