Placentitis is the single most prevalent cause of premature delivery of a foal and accounts for nearly one-third of late-term abortions and foal death in the first day of life.

Placentitis is an inflammation of the placenta, the temporary organ that forms to support the foal while it is in the mare during pregnancy.

Normal anatomy of the placenta and reproductive tract

Throughout pregnancy, the placenta functions to allow selective exchange of gases, nutrients and waste products between the foal and the mare. A healthy placenta is therefore necessary to allow the pregnancy to be maintained.

In the normal mare the uterus provides a sterile environment for the foal to develop. The placenta is derived from the embryo and is made up of two membranes. The amnion is the thin membrane that surrounds the foal during pregnancy. The thicker chorioallantois is the vascular membrane that is covered in villi (microscopic finger-like projections) that attach to the lining of the uterus (endometrium).

The chorioallantois is diffusely attached to the endometrium except at an area that lies adjacent to the cervix. This area is known as the cervical star and is the area of the placenta that the foal breaks through at birth.

It is important to examine the placenta after birth for signs of defects or missing portions. The placenta is placed in an F-shaped arrangement so that the grayish white surface with blood vessels, and the attachment of the umbilical cord and amniotic sac, is outer-most and the red velvety surface that was attached to the uterus is on the inside.

The cervical star is at the bottom of the F and the part that occupied the uterine body forms the vertical part of the F. The non-pregnant horn, which is narrower and thinner, forms the lower arm while the pregnant horn, which is wider and thicker forms, the upper arm of the F.

The foetus and placenta are protected from micro-organisms and foreign material by the normal cervix and lower reproductive tract, which act like a physical barrier. The thick muscular cervix lies between the vagina and uterus and remains closed during the normal pregnancy. Cells within the cervix and lower reproductive tract produce mucous which, under the influence of progesterone during pregnancy, is viscous and impermeable to bacteria and foreign material. A thick mucous plug forms in the cervical canal and projects into the vagina.

Causes of placentitis

A variety of causes for placentitis exist, however, by far the most common cause is a bacterial infection of the uterus that enters via the vagina and breaches the cervical barrier. Aspiration of air and faeces into the vagina can occur in mares with poor conformation or injury of the vulvar opening.

In some mares, the vulvar opening is not vertical and in opposition. This can occur in any age but is commonly seen in older or underweight mares where the anus becomes sunken, pulling the top of the vulvar into a horizontal position.

Injury to the cervix and vagina from previous foalings may allow penetration of infection past these structures. Another proposed mechanism for the development of placentitis occurs when abnormal bacteria in the tract degrade the mucous lining the vagina and cervix allowing bacteria access to the placenta.

Infection normally concentrates at the cervical star although it can spread throughout the placenta. The migration of the bacteria leads to inflammation of the placenta, stimulating the tissue to produce prostaglandin, a hormone that may cause the uterus to contract, leading to premature labour.

As more of the placenta becomes inflamed and thickened it begins to pull away from the uterine lining. This leads to a decrease in oxygen and nutrients to the foal, thereby retarding its growth or leading to its death. Infection can also extend across the placenta to the foal resulting in abortion or, if the infection is of a low intensity, the birth of a weak, infected foal.

Separation of the placenta from the uterus also has implications for birth. In normal pregnancy, the placenta remains attached to the uterus allowing the foal to rupture the membranes at the cervical star. If the placenta separates prematurely in the area adjacent to the cervix, the membranes may not rupture but will be delivered intact around the foal, a ‘red-bag’ delivery. This premature placental separation, if unattended, will result in a stillbirth due to suffocation of the foal.

While most cases of placentitis are caused from an ascending bacterial infection, the placenta can also become infected with fungi via blood borne organisms, or from a residual infection of the uterus.

Fungal infections are rare but usually result in abortion or a foal that rapidly dies after birth. Infections can also arrive at the placenta via the mare’s blood. An acute severe disease of the mare such as colic or diarrhoea can lead to infection and toxins crossing the maternal-foetal blood barrier thereby affecting the placenta. All common organisms that produce uterine infections in mares are capable of producing placentitis. A low-grade infection acquired at or before the time of breeding can lie latent for a period of time before extending to the placenta.

Equine herpes virus (EHV) is the most common viral cause of placentitis and abortion in Australia. While the overall occurrence of this disease is low, it is highly contagious and can cause abortion storms on some studs. There are 5 herpes viruses in Australia with type 1, and occasionally type 4, causing outbreaks of abortion and respiratory disease in horses.

Abortion usually occurs between 7 and 11 months of gestation with the mare seldom showing evidence of respiratory infection or other illness. Abortion occurs suddenly and, if late in the pregnancy, may produce a live, weak foal that invariably dies.

EHV is throughout Australia and is a common cause of ‘colds’ in young horses. The majority of older horses do not show clinical signs of infection. Once infected, horses carry the virus for life and, when stressed, the virus is reactivated and excreted in body fluids, contaminating the environment for up to 2 weeks. It is rare for a mare to abort again due to EHV1 and following isolation of the mare for 30 days to prevent virus spread, can be bred again.

Common signs of placentitis

Unfortunately, there is not a consistent clinical sign apparent in all mares that have a placentitis. Commonly reported signs are premature udder development or ‘bagging-up’, and vaginal discharge from as early as five months of gestation. Normal mares undergo udder development 2-4 weeks prior to birth. The udder development can be associated with lactation and in severe cases, milk can be seen leaking down a mare’s leg. Softening of the cervix and release of the mucous plug may allow discharge to be released from the uterus.

Depending on the stage of pregnancy the above clinical signs are followed by either abortion or the birth of a small, weak foal that may be infected (septic) or develop sepsis. Alternatively, foals may present as a ‘dummy’ due to chronic oxygen deprivation in the uterus. There are some mares with placentitis that cannot be identified clinically as they do not exhibit premature udder development or a vaginal discharge. These mares may abort or produce a compromised foal.


Diagnosing placentitis in a mare can be difficult in some cases. Arriving at a specific cause for the placentitis often proves to be more challenging. It is important that a thorough history of the mare is obtained. Information including due foaling date, recent illness, previous problem conceptions or pregnancies, in-contact horses, and vaccination status may be relevant to the diagnosis.

Typically, diagnosis of placentitis is based on clinical signs and ultrasonography. An ultrasound via the rectum may reveal an increased placental thickness or placental separation from the uterus, although focal areas of placentitis may be impossible to detect. Some mares with placental infection can abort spontaneously and may not be identified via ultrasound as thickening or separation of the placenta may not have occurred.

A variety of samples for laboratory evaluation can be useful in determining the cause of placentitis. Samples of vaginal discharge may be collected from the mare or from the foal or placenta following foaling or abortion. It is critical that samples are taken from a freshly aborted foetus to prevent contamination with opportunistic infections. Often the placenta is too contaminated with dirt for microbiological samples, however it is important that the placenta is examined for signs of infection.

Early treatment of placentitis is critical in improving the survival rate of foals and should be commenced prior to the results of laboratory tests.


A horse foetus, unlike many other species, matures in the final 5-7 days of pregnancy, and if removed from the mare before this time will usually die. In mares with placentitis, the foal will often respond by maturing more quickly as a result of the stress.

Stressed foals can be born early and survive with minimal care, if the infection develops slowly and premature delivery can be delayed. Therefore, if infected mares can be identified early, and treated to delay delivery, the chances of foal survival can be improved and the need for intensive hospitalization minimised.

Management of placentitis is aimed at fighting the infection, reducing the inflammatory response, supporting the pregnancy and treating any underlying illnesses. Treatment outcomes are optimised by a combined approach of antibiotics, anti-inflammatory drugs and progesterone until the birth of the foal.

Potentiated sulfonamides are a good initial antibiotic because of their broad-spectrum activity against a range of bacteria and their ability to cross the placenta. Anti-inflammatory drugs such as flunixin meglumine and phenylbutazone may help to decrease the production of prostaglandin (a chemical involved in abortion).

Altrenogest, a synthetic progesterone, is thought to assist in decreasing the effects of prostaglandins and therefore help to maintain pregnancy. Unfortunately there is no specific treatment for EHV1 and foals born alive will die within a few days of birth despite intensive treatment.


Identifying mares that are at a higher risk of developing placentitis is advised. This would include mares that have had previous episodes of abortion or infection, or older mares with poor reproductive conformation. These ‘at risk’ mares should be monitored closely through pregnancy for subtle signs of placentitis and treated accordingly. For mares with poor vulvar conformation, a Caslick surgery may be indicated.

Mares should be in good body condition (but not fat) before breeding and throughout pregnancy and lactation. Prevention measures should include the use of clean, minimally contaminated breeding techniques, to reduce the occurrence of uterine infection at mating.

Pregnant mares should be kept in small groups and isolated from horses that are new to the premises. This will help minimize the risk of an infectious outbreak on the stud. Vaccinations for equine herpes virus are available to help control this disease and can be given at the 5th, 7th and 9th months of gestation. While this vaccine does not produce total immunity, it may prevent some abortions and will reduce the spread of the infection.

Placentitis in mares can be a significant problem for horse owners, stud managers, and veterinarians. Abortions and foal death can occur when even the most comprehensive interventions are utilised.

These foal losses represent a considerable emotional and economic cost to the equine industry. Current research is advancing prevention, early detection and treatment methods. However, by implementing correct breeding protocols, horse owners and veterinarians can help reduce the risk of foal death.