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 stillborn.

Prostaglandin stimulates early maturation of the fetal lungs, resulting in delivery of a foal that is viable but very small. [3] Placentitis can also interfere with the delivery of nutrients and oxygen to the foal, further contributing to poor development.

With both hematogenous and ascending placentitis, the bacterial infection can also spread to the developing fetus via the umbilical cord, resulting directly in fetal death or fetal sepsis. [3] Foals born alive are in critical condition and require intensive treatment to control the infection.

Risk Factors

There has not been thorough research into the risk factors associated with placentitis. Suggested risk factors include:

  • Having multiple previous foals
  • Increasing age of the mare
  • Poor conformation of the perineum [4]
  • Previous injury to the cervix, such as lacerations or scar tissue [1]

Clinical Signs

Inflammation of the placenta triggers hormonal changes like those that initiate the birthing process. [6] Thus, placentitis often results in symptoms similar to a mare entering labor.

Primary clinical signs include:

  • Premature udder development
  • Streaming of milk from the udder
  • Vulvar discharge [3][4][6]

Left untreated, placentitis can result in premature delivery or abortion of the foal. Foals that are born alive have an increased risk of developing sepsis or “dummy foal syndrome”. [3] Affected foals commonly require hospitalization and have poor survival rates.

Placental Examination

After delivery, placentitis can be quickly identified by examining the placenta. To conduct this examination, lay the placenta out in an “F” configuration. Signs of placentitis are most apparent around the cervical star, which is the pale, star-shaped area at the bottom of the “F”.

Signs of placentitis in the placenta include:

  • Thickening of the placenta
  • Areas of discoloration, such as grey/brown areas on the normally bright red chorion
  • Areas of the chorion without the “velvet” texture
  • Exudate on the placental surfaces


Ultrasound is the primary method for diagnosing placentitis and monitoring the health of the growing fetus. Testing hormone levels can also help monitor the overall health of the placenta.


Transrectal ultrasound is the most common diagnostic technique and highlights the cervical star for diagnosis of ascending placentitis. [6]

Increased thickness of the placenta and uterus and changes in the amniotic fluid on ultrasound are the main indicators of placentitis. [6][7] Other identifiable changes include abnormalities in the fetus or umbilical cord. [3]

Transabdominal ultrasound is best for diagnosing hematogenous placentitis. [6] Premature separation of the placenta can be identified and indicates a foal at a high risk of premature delivery or abortion. [6]

Transabdominal ultrasound is also used to monitor fetal heart rate, movement, and growth of the fetus after placentitis is diagnosed. [6][7] Your veterinarian may ultrasound your mare daily or weekly to monitor fetal health if your mare is at high risk of abortion. [6]

Hormone Testing

Progesterone is a hormone produced by the placenta that is responsible for maintaining pregnancy in horses. In placentitis, progesterone levels drop rapidly, which can be identified on a blood test. [6]

Regular progesterone testing is used to monitor treatment efficacy, particularly when combined with repeat ultrasound of the placenta to identify changes in thickness. Decreases in other hormones, such as estrogen and relaxin, are also associated with placentitis and can be identified on blood testing. [6]

Other Diagnostics

Your veterinarian may test mammary secretions from the mare for electrolyte levels and pH balance, which can indicate impending abortion or premature delivery. [7]

Typically, calcium and potassium levels in the mare’s milk rise, while sodium levels decrease before delivery. These changes also occur prior to delivery due to placentitis. [4]

Continued monitoring of electrolyte levels can also show response to treatment. Additionally, testing cervical or vaginal discharge for bacteria or fungal agents can help your veterinarian select the most appropriate medication. [7]


The treatment for placentitis in horses is based on protocols developed for other species. The main goal of treatment is to reduce inflammation and address the infections to prevent premature delivery of the fetus.

A combination of antimicrobial and anti-inflammatory therapies and progesterone supplementation is most effective in treating placentitis. [6][7]

In most cases, treatment continues for the remainder of the pregnancy, as discontinuing treatment can result in reappearance of clinical signs and increased risk for the fetus. [7]

Antimicrobial therapy

After diagnosing your mare with placentitis, your veterinarian will typically start her on an antibiotic or antifungal treatment that covers a wide variety of infectious agents. If initial treatment is unsuccessful, a specific protocol will be designed based on the results of vaginal or cervical discharge testing. [6]

Reducing the infection diminishes inflammation levels, which in turn lowers the production of prostaglandin. This helps to reduce the risk of abortion or premature delivery.

Common antimicrobials used for treating placentitis include:

  • Penicillin
  • Gentamicin
  • Trimethoprim-sulfa
  • Ceftiofur
  • Doxycycline
  • Enrofloxacin [7]

Anti-inflammatory therapy

Inflammation within the placenta causes the release of prostaglandin, which triggers labour. Therefore, anti-inflammatories are common in placentitis treatment protocols to inhibit prostaglandin production.

Anti-inflammatory drugs can also reduce the effects of toxins produced by the bacteria causing the infection.

The most common anti-inflammatories used are:

  • Flunixin meglumine (Banamine)
  • Phenylbutazone (Bute)
  • Firocoxib (Equioxx)
  • Pentoxifylline [7]

Pentoxifylline may also increase blood flow to the placenta to support the health of the fetus. [3]

Progesterone Supplementation

Progesterone supplements are administered to maintain the pregnancy and replace the progesterone that would normally be produced by a healthy placenta.

Adequate progesterone levels in the mare reduce the risk of premature delivery by decreasing uterine contractions and inhibiting the production of prostaglandin.

Altrenogest (Regumate) is the most common progesterone product used in pregnant mares. [6]


Tocolytics are medications that prevent uterine contractions, helping to delay labor for a short period. In human medicine, tocolytics can prevent labour for up to 48 hours, providing time for steroid treatment to promote faster maturation of the fetus. [6]

Studies in horses have shown variable efficacy, so tocolytics are not common in equine practice at this time. [3][6]

During and After Foaling

Placentitis increases the risk of a “red bag delivery”, due to inflammation causing separation of the placenta from the uterus prior to delivery of the foal.

Mares diagnosed with placentitis require careful monitoring when labour is anticipated, so that the red bag delivery can be corrected immediately by cutting open the fetal membranes. Without correction upon delivery, the foal is unable to breathe and is at a high risk of developing “dummy foal syndrome”.

Foals born to mares affected by placentitis also have an increased risk of sepsis (a systemic inflammatory response) which may likely require hospitalization. Sepsis can either originate from the bacterial placentitis spreading to the fetus, or from poor quality colostrum due to the mare leaking milk prior to delivery of the foal.

Oxygen support, antimicrobials, plasma transfusions, fluids, and tube feeding are common treatments for affected foals. [4] However, foals with sepsis have a poor prognosis, even with intensive care.

Mares may have uterine infections after placentitis, requiring additional care. Uterine lavages with antimicrobials or oral antimicrobials can restore a healthy uterine environment. [4]


Prevention of placentitis primarily focuses on reducing the mare’s exposure to infectious agents. The main preventative measure is a breeding soundness examination; however hygiene and cleanliness of the mare also play a role.

Breeding Soundness Examination

Before breeding, a breeding soundness examination can identify risk factors for placentitis, as well as other reproductive issues affecting the mare’s fertility. During the breeding soundness examination, the mare’s perineum, vagina, and cervix are closely examined.

Poor perineal conformation can lead to fecal contamination of the vulva, introducing large amounts of bacteria into the vagina and subsequently contaminating the cervix. [8]

A Caslick’s procedure, where the upper portion of the mare’s vulva is surgically closed, can help prevent fecal contamination.

Age-related weakening of the cervix, cervical lacerations from prior deliveries, or scar tissue on the cervix can also affect the cervix’s ability to form a tight seal and may allow bacterial entry. [8] Scar tissue or lacerations may require surgical intervention to treat, and extensive damage may prevent maintenance of pregnancy entirely. [8]

Future Pregnancies

Mares previously affected by placentitis are predisposed to developing the condition in future pregnancies, and some veterinarians suggest preventative treatments. Antibiotics, anti-inflammatories, and progesterone supplementation starting at 7 months of gestation are common.

Routine examination of these mares via ultrasound is also prudent for early identification of placentitis and optimal mare and foal health.

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  1. LeBlanc. M. Ascending Placentitis in the Mare: An Update. Reproduction in Domestic Animals. 2010. View Summary
  2. Hong. C. B. et al. Etiology and Pathology of Equine Placentitis. J Vet Diagn Invest. 1993. View Summary
  3. Cummins. C. et al. Ascending Placentitis in the Mare: A Review. Ir Vet J. 2008.
  4. Macpherson. M. L. and Bailey. C. S. A Clinical Approach to Managing the Mare with Placentitis. Theriogenology. 2008. View Summary
  5. Fedorka. C. E. et al. Clinical, Pathologic, and Epidemiologic Features of Nocardioform Placentitis in the Mare. Theriogenology. 2021. View Summary
  6. McKinnon. A. O. et al. Equine Reproduction. Wiley-Blackwell. 2010.
  7. McNaughten. J. W. and Macpherson. M. L. Placentitis, in Equine Reproductive Procedures. John Wiley & Sons, Ltd, 2021.
  8. Lavoie. J.-P., Ed. Blackwell’s five-minute veterinary consult. Equine, Third edition. Hoboken, NJ: Wiley-Blackwell, 2019.