Placentitis in Horses

Jennifer Rice, DVM
By Jennifer Rice, DVM on Mar. 28, 2023
Pregnant paint horse

In This Article


What is Placentitis in Horses?

Placentitis is defined as inflammation of the placenta. Problems with the placenta are the most common cause of mid to late-term abortions, premature delivery, and neonatal death in the first 24 hours of life in horses. The most common cause of placentitis in horses is due to an infectious agent, such as bacteria. Placentitis has become the main cause of reproductive loss in the equine breeding industry with a considerable economic impact.

Placental Anatomy

The anatomy of the placenta includes two parts, amnion and chorioallentois.

The amnion surrounds the fetus and the chorioallentois is attached to the endometrium (uterine lining). Both of these structures play parts in protecting the fetus and provide gas and nutrient exchange which allows the foal to grow.

Placentitis usually affects the chorioallentois causing compromise to the attachment of the placenta to the endometrium or causing inflammation and infection which are all harmful to the foal.

Symptoms of Placentitis in Horses

Common symptoms of placentitis include:

  • Vaginal discharge

  • Softening of the cervix (seen on exam with your veterinarian)

  • Premature mammary gland enlargement

  • Premature lactation

Causes of Placentitis in Horses

Placentitis can occur in mares of any age, but more often occurs in underweight and older mares in which the shape of the vulva allows bacteria to ascend through the cervix. Previous cervical injury can also allow bacteria to enter the cervical barrier and increase the potential for placentitis.

The most common cause of placentitis is due to an infection most commonly bacterial or fungal. Bacteria use one of three ways to access to the placenta and potentially the foal:

  • Ascending infection: when the bacteria gets past the vulva and enters the cervix

  • Hematogenous infection: when the mare is systemically (entire body) sick and the bacteria gets within the blood supply of the uterus/placenta and foal

  • Unidentified infection: it is unknown how this bacteria gains access to the uterus and placenta

The infection generally collects near the cervical star (area of the placenta that is near the cervix) but can also penetrate further and invade the entire placental structure. Once inflammation and infection have spread, the mare’s body will produce prostaglandin (hormone-like body substance that is involved in uterine contraction). Prostaglandin can then lead to uterine contractions and abortion of the foal prematurely.

Inflammation can also cause thickening of the placental tissue and cause the placenta to pull away from the uterine lining. This decreases the available nutrients and oxygen to the foal. Premature separation of the placenta from the uterus can lead to a “red bag” delivery. This is considered a medical emergency since the  emerging foal becomes wrapped in the placental membranes and often causes suffocation of the foal and stillbirth.

How Veterinarians Diagnose Placentitis in Horses

Veterinarians can diagnose placentitis through physical exam and ultrasound. Both transrectal and transabdominal ultrasound may be used in order to examine the placenta. During ultrasound, the veterinarian will look for thickened placenta tissue, separation of the placenta, and changes in the fluid within the uterus.

During the ultrasound exam, fetal activity and heart rate can be observed. If there is any vaginal discharge, a sample may be taken and sent to a lab to identify the infective organism and provide antibiotic sensitivity which allows for the veterinarian to choose the best antibiotic for treatment.

Another diagnostic test includes the measurement of Serum Amyloid A protein concentration. The level of this inflammatory protein commonly increases if an infection is present.  

Lastly, the veterinarian may recommend testing maternal progesterone hormone (steroid hormone that tells the body it is pregnant) levels (indication of pregnancy status) and total estrogen levels to help determine that the placenta and fetus are intact and healthy. If progesterone levels are low, the veterinarian may prescribe a synthetic progesterone such as altrenogest (Regu-Mate) to increase the level of progesterone in the horse's body to help the uterus to stay pregnant.

Treatment of Placentitis in Horses

Placentitis can be treated successfully when diagnosed and treated early in the course of the disease. Veterinarians have developed treatment plans that are aimed at resolving infection, decreasing inflammation and uterine contraction that cause a mare to abort the foal.

These systemic treatment plans often include the following medications:

  • Antibiotics such as Trimethoprim sulfamethoxazole

  • Exogenous (outside source) progestogens such as altrenogest (Regu-mate)

  • Anti-inflammatories such as Banamine

  • Tocolytic agents (medications to decrease uterine contractions)

  • Medications that improve uterine perfusion (blood flow) such as pentoxifylline

Recovery and Management of Placentitis in Horses

With early recognition and treatment, mares with placentitis can maintain their pregnancy and deliver a healthy foal.

Mares that are considered high-risk for placentitis may have an increased amount of veterinary care and check-ups during the course of their pregnancy. Veterinarians can detect placental and/or fetal problems early in pregnancy through routine ultrasound exams and hormone evaluations.

Mares that may be considered high risk include the following:

  • History placental compromise

  • Cervical incompetence or lacerations

  • Chronic disease

  • Old age

  • Poor reproductive conformation

Placentitis can and often leads to abortion if left untreated or undiagnosed but can also lead to other serious diseases such as:

  • Laminitis of the mare

  • Systemic illness/toxemia of the mare

  • Weak or premature foal

  • Stillborn foal

  • Death of mare and foal

Prevention of placentitis requires ensuring your mare is in good overall health as well as reproductive health. If your mare is pregnant for the first time or this mare is new to you, it is always good to have a reproductive soundness exam performed by your primary veterinarian. Your primary veterinarian can also give you guidelines on how to maintain your mare's health throughout her entire pregnancy, from what to feed her, to the vaccines she needs during pregnancy.

Placentitis in Horses FAQs

How does placentitis affect a foal?

Placentitis often leads to the foal being restricted on nutrients and oxygen leading to a premature, weak, and sick foal or abortion of the foal and stillbirth.

How long can a placenta stay in for a horse?

After 3 hours of birth of the foal, all placental and fetal membranes (afterbirth) should have passed from the mare. If it has not, it is important to call your primary veterinarian right away. The longer the placenta stays in the mare the higher the chance for an infection.


1. Kane E. DVM 360. Equine placentitis is on the rise. February 2013.

2. Kentucky Equine Research. Placentitis in Mares. April 2014.

3. Oke S MSc DVM. University of Kentucky. New Test Helps Vets Diagnose Placentitis in Pregnant Mares. May 2013.

4. Wolfsdorf K DVM DACT. AAEP. Placentitis.

Featured Image:


Jennifer Rice, DVM


Jennifer Rice, DVM


Dr. Jennifer Rice is a 2017 graduate from Purdue College of Veterinary Medicine where she specialized in Equine medicine. Since graduating...

Help us make PetMD better

Was this article helpful?

Get Instant Vet Help Via Chat or Video. Connect with a Vet. Chewy Health