Abortion on Horses
by R.G. Wright, Animal Industry Branch, Guelph and J.N. Henry
foto: Paula da Silva

INTRODUCTION

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 its expulsion.
Other factors such as twinning, hormonal deficiencies or congenital anomalies may also result in death of the fetus and abortion.
Some of the more common causes are discussed below.

INFECTIOUS CAUSES OF ABORTION

Viral Abortion
The most common cause of virus-induced abortion in mares is equine herpesvirus:

This virus has two subtypes.
Subtype-1 (abortion virus) is most often associated with abortions in mares, while subtype-2 (rhinopneumonitis virus) is usually associated with respiratory disease in young horses.
However, both subtypes have the potential to cause respiratory disease and abortion.

Commercially available vaccines incorporate subtype-1 strains of EHV-1, (the abortive form).

Recent evidence supports the view that the abortigenic EHV-1 infection of pregnant mares in most instances is not, as once believed. The annual outbreak of rhinopneumonitis of young horses that occurs each fall and winter on most farms engaged in the rearing of foals is not the source of infection.

One of the hallmarks of infection by members of the herpesvirus family is the capacity of the virus to persist in the body of its host in a dormant state 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 initiate a new outbreak of the disease.

A fetus may become infected 20 to 90 days after its mother contacts the dise ase. 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 which are available are of questionable value in preventing abortion.

The virus of equine arteritis is reported to cause abortion.

Bacterial Abortion
Several species of bacteria have been incriminated as causative agents of equine abortion and sterility.
Salmonella abortus equi was responsible for abortion storms in the past and still occurs sporadically.
The most common cause of bacterial abortion at present is organisms of the Streptococci group.
Other bacteria frequently cultured from aborted feti include Actinobacillus, Corynebacterium, Klebsiella and Pseudomonas species.

These agents gain entrance to the reproductive tract at foaling or breeding 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 metritis, or inflammation of the uterus. Treatment of the mare is often required before she may be rebred successfully. It is a good practice to swab mares before rebreeding to determine if harmful bacteria are present in the vagina. Breeding of mares on the foal heat (9 days after foaling) should be discouraged. The exception may be the breeding of healthy mares late in the breeding season.

NONINFECTIOUS ABORTION

Abortion due to Twinning
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 fetuses. 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.

Abortion due to Progesterone Deficiency
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 which ruptured releasing the egg which was fertilized at conception. The corpus luteum is composed of specialized cells which 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 which 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 thought to be due in many cases, to a lack of sufficient hormones to maintain pregnancy. Further information on the reproductive system and hormonal controls can be obtained from Factsheet Agdex No. 460/10 Horse Management: Anatomy and Physiology of Reproduction in the Mare.

Umbilical Cord Torsion
Torsion or strangulation of the umbilical cord is said to be the cause of about one percent of fetal deaths and abortions in the later stages of pregnancy. The cord normally has 3 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 death of the fetus.

Congenital Defects
Early embryonic deaths may be mistaken for failure of conception or silent heats. Genetic or chromosomal defects may result in improper development of the embryo and subsequent rejection by the dam.

MISCELLANEOUS

Nutritional deficiencies have not been associated with abortion in mares.

Certain drugs have been reported to cause abortion in mares but these reports have seldom been substantiated. Examination of labels should be made to ascertain 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.

GENERAL COMMENTS

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 subsequent seasons can be determined.

by R.G. Wright, Animal Industry Branch, Guelph and J.N. Henry
Veterinary Services Laboratory, Guelph
www.gov.on.ca/OMAFRA/english/livestock/horses/facts/89-169.htm

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