Riverina Equine Vet



Placentitis in the Mare

Placentitis can be a devastating problem in the horse leading to abortion, stillbirths or weak or premature foals.  It is the single most common 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 for transfer of oxygen, nutrients and waste products between the foal and the mare.  A healthy placenta is therefore necessary to allow the pregnancy to be maintained.

The foetus and placenta are protected from bacteria/bugs and foreign material by the vulva, vagina, and closed cervix, which act like a physical barrier.  There is also a thick mucus plug that forms within the cervix to further prevent any contamination getting to the placenta and foetus.


Causes of placentitis

The most common cause is a bacterial infection of the uterus that enters via the vagina and manages to get through the barrier at the cervix. Sucking of air and faeces into the vagina can occur in mares with poor conformation or injury of the vulva.  Abnormalities of the shape and normal function of the vulva can occur in any age but is commonly seen in older or underweight mares where the anus becomes sunken. Injury to the cervix and vagina from previous foalings may allow infection past these structures more easily.

Infection causes inflammation of the placenta which can eventually lead to premature labour.

Infection in the placenta can lead to reduced oxygen and nutrient transfer to the foal, thereby affecting its growth or leading to its death.  Infection can also travel across the placenta to the foal resulting in abortion or, if the infection is low grade, the birth of a weak, infected foal.


Common signs of placentitis

Clinical signs can be inconsistent and can often go unnoticed. 

Commonly reported signs are early 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.  Sometimes mares which have bagged up too early due to placentitis will also run milk, which can be seen leaking down the mare’s legs. This also means that she will have leaked out all her colostrum (‘first milk’). 

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.  Diagnosing a specific cause for the placentitis can often be even more difficult. 

Usually, 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. Unfortunately ultrasounding the placenta in small ponies is not possible.



Early treatment of placentitis is critical in improving the survival rate of foals.

Management of placentitis is aimed at fighting the infection, reducing the inflammation, supporting the pregnancy and treating any underlying illnesses. Treatment usually involves a combination of antibiotics, anti-inflammatory drugs and progesterone until the birth of the foal.  TRIMIDINE is a good initial antibiotic because of its broad-spectrum activity against a range of bacteria and their ability to cross the placenta.  Anti-inflammatory drugs such as BUTE can help to reduce inflammation and decrease the production of prostaglandin (a chemical involved in abortion). ALTRENOGEST, a synthetic progesterone, also helps to assist in decreasing the effects of prostaglandins and therefore help to maintain pregnancy.