Placentitis is a common cause of foal loss and neonatal sepsis in horses. Mares with multiple previous foals, older mares, and mares with poor perineal conformation are most at risk of developing placentitis.

Most cases of placentitis arise from bacteria contaminating the cervix and entering the uterus, causing infection within the placenta. The infection results in the production of hormones that stimulate labour, leading to premature delivery or abortion of the developing fetus.

Symptoms of placentitis include premature udder development, streaming milk, and vaginal discharge. Ultrasound is the primary method for diagnosis, as it allows the veterinarian to evaluate the thickness of the placenta and the quality of the amniotic fluid. Other tests, such as hormone level testing, mammary secretion testing or vaginal discharge bacterial culture are also common.

Combination therapy with antimicrobials, anti-inflammatories, and supplementation with progesterone is most effective in treating placentitis. Treatment typically continues until the mare delivers the foal, to prevent recurrence of clinical signs and reduce health risks to the developing foal.

A breeding soundness examination helps identify risk factors for placentitis prior to breeding the mare. Procedures to reduce bacterial contamination of the vagina and cervix, such as a Caslick’s procedure, can help prevent placentitis.

Placentitis in Mares

Placentitis refers to inflammation of the placenta, an organ that develops in the uterus during pregnancy and that is responsible for providing oxygen and nutrients to the developing fetus.

Placentitis is estimated to affect 3-7% of pregnant mares, and can cause significant economic consequences for horse breeders. [1]

Mares with placentitis commonly abort their foals, and foals born alive often require hospitalization due to sepsis.

Early recognition, diagnosis and treatment are critical for maintaining the pregnancy and allowing full development of the growing foal.

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Causes of Placentitis

Most cases of placentitis are due to bacterial infections, notably from pathogens such as Streptococcus equi zooepidemicus, Escherichia coli, or Klebsiella. [2][3]

These bacteria are common in the environment, but infection is usually prevented by the tight seal of the cervix during pregnancy. [2]

In placentitis, bacteria contaminate the cervix and degrade the cervical plug, allowing bacterial entry into the otherwise sterile environment of the uterus.

Ascending Placentitis

When bacteria “ascend” the reproductive tract from the vagina and cervix to invade the uterus, this is called ascending placentitis. [2]

Ascending placentitis most commonly causes thickening around the cervical star, the area of the placenta immediately adjacent to the cervix. [4] This form of placentitis is most common in mid-gestation, between 6 and 9 months.

Hematogenous Placentitis

Some forms of bacterial placentitis arise when bacteria from the mare’s bloodstream enter the placenta, leading to infection. This is referred to as hematogenous placentitis. [2]

Maternal infection by Leptospira is a common cause of hematogenous placentitis, and often results in abortion of the fetus between 7 and 9 months of gestation. [2] However, compared to other species, hematogenous placentitis is relatively rare in horses.

Nocardioform Placentitis

A unique form of bacterial placentitis is nocardioform placentitis, caused by Amycolatopsis, Crossiella and Streptomyces species.

The route of infection for nocardioform placentitis is unknown, however increased reports occur when there are hot, dry conditions in autumn. [5] Central Kentucky is primarily affected, however there have been reports worldwide. [5]

Placentas with nocardioform placentitis frequently have cystic lesions on the allantois, however this feature is not exclusive to this type of placentitis. [5]

Fungal Placentitis

Uncommonly, fungal infections can cause placentitis. Fungi associated with placentitis in horses include Aspergillus, Histoplasma and Candida. [3]

Most cases result from ascending infections, and usually occur during late gestation. [2]


Regardless of the cause of placentitis, the infection ultimately results in inflammation within the placenta. Inflammation leads to the production of prostaglandin, the major hormone that stimulates labour and uterine contractions. [3]

In early or mid-gestation, the activation of uterine contractions results in abortion of the developing fetus. With placentitis in late gestation, foals are either born small and undeveloped, or