Placentitis in Horses: Causes, Signs, Treatment, and Prevention

Placentitis is one of the most common causes of late-term abortion, stillbirth, and premature birth in mares. It refers to inflammation and infection of the placenta, the critical structure that nourishes and protects the developing foal during pregnancy. This condition can be devastating, as it threatens the health of both mare and foal, often without clear warning until late in gestation.

Fortunately, with early recognition, prompt veterinary intervention, and good breeding management, many mares affected by placentitis can still carry and deliver healthy foals. This article explains what causes placentitis, how to identify it, and what steps owners can take to prevent it.

What Is Placentitis

The placenta is a thin, vascular organ that attaches to the uterine wall, providing oxygen and nutrients to the developing foal and removing waste products. When the placenta becomes inflamed or infected, its ability to perform these functions is impaired. The foetus may become deprived of oxygen and nutrients, leading to abortion, stillbirth, or the birth of a weak, premature foal.

Placentitis typically occurs in the final trimester of pregnancy but can develop at any stage. It accounts for an estimated 20 to 30 percent of late-term pregnancy losses in mares.

Types of Placentitis

Placentitis in horses can be classified into several types based on how the infection develops and where it occurs.

1. Ascending Placentitis

This is the most common form. It occurs when bacteria enters the reproductive tract through the vulva and cervix, ascending into the uterus and infecting the placenta at the cervical star, the area closest to the cervix.

Common bacteria include Streptococcus zooepidemicus, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa.

2. Diffuse (Hematogenous) Placentitis

This form occurs when infectious agents spread through the bloodstream from another part of the body to the placenta. It is often associated with Leptospira interrogans, the same bacteria responsible for leptospirosis and can affect large portions of the placenta. This type may cause abortion or stillbirth without external signs in the mare.

3. Nocardioform Placentitis

A distinct type caused by bacteria such as Nocardioform actinomycetes. It affects the base of the uterine horns rather than the cervical region. Nocardioform placentitis is less common but has been reported in certain geographic regions such as Kentucky and the southern United States.

Causes and Risk Factors

Several factors can increase the risk of placentitis in mares:

• Poor perineal or vulvar conformation, allowing bacteria to enter the reproductive tract
• Cervical injury or incompetence (for example, after previous foaling trauma)
• Contaminated breeding practices or poor hygiene during insemination
• Stress, systemic illness, or hormonal imbalances that reduce immune defence
• Previous history of placentitis or abortion
• Environmental exposure to pathogens such as Leptospira in contaminated water

Signs and Symptoms

Placentitis can develop slowly and often goes unnoticed until abortion occurs. However, careful observation of pregnant mares can reveal subtle early signs:

• Premature development or enlargement of the udder (bagging up)
• Milk or waxy secretions appearing weeks or months before the expected foaling date
• Vaginal discharge, which may be white, yellow, or bloody
• Restlessness or discomfort
• Decreased appetite or mild fever
• Extended gestation beyond expected due date (in chronic infections)

Some mares may show no external signs until foetal loss occurs, underscoring the importance of regular veterinary monitoring during late pregnancy.

Diagnosis

Early diagnosis is key to improving the outcome for the mare and foal. Veterinary evaluation typically includes:

1. Ultrasound Examination

Transrectal and transabdominal ultrasound are used to assess:

• The thickness of the placenta (combined thickness of uterus and placenta, or CTUP)
• The presence of fluid accumulation or debris between the placenta and uterine wall
• The foetal heart rate and movement to assess viability

In ascending placentitis, the placenta often appears thickened at the cervical region.

2. Vaginal Examination

A veterinarian may collect vaginal discharge samples for culture and cytology to identify the bacteria involved.

3. Blood Tests

Elevated hormone levels, particularly progesterone and oestrone sulphate, may indicate foetal distress. Blood samples can also be used to assess inflammation (white blood cell count, fibrinogen, serum amyloid A).

4. Post-Mortem Examination

If abortion or stillbirth occurs, examination of the foetus and placenta helps confirm the diagnosis and identify the infectious agent.

Treatment

The goal of treatment is to stop infection, reduce inflammation, and support the pregnancy long enough for the foal to reach maturity.

1. Antibiotic Therapy

Broad-spectrum antibiotics are used to control bacterial infection. Common choices include trimethoprim-sulfa, penicillin, ceftiofur, or gentamicin. The treatment may be adjusted based on bacterial culture results. Therapy often continues until foaling.

2. Anti-Inflammatory Medication

Non-steroidal anti-inflammatory drugs (NSAIDs) such as flunixin meglumine help reduce inflammation and pain in the uterus and placenta, improving blood flow to the foetus.

3. Hormonal Support

Progestins such as altrenogest (Regumate) are often prescribed to help maintain uterine quiescence and support the pregnancy, particularly if premature contractions are suspected.

4. Tocolytic Agents

Medications that suppress uterine contractions may be used in some cases to prevent premature labour.

5. Supportive Care and Monitoring

Mares with placentitis require close veterinary monitoring with regular ultrasound checks, temperature recording, and observation for changes in discharge or udder development. Stall rest is often recommended to reduce physical stress.

6. Foal Care

If premature birth occurs, the foal may require intensive neonatal care, including antibiotics, plasma transfusions, and supportive feeding.

Prevention

Preventing placentitis requires good management practices that protect pregnant mares from infection and stress throughout gestation.

1. Good Reproductive Hygiene

• Maintain strict hygiene during breeding and foaling.
• Use clean, disinfected equipment for insemination.
• Avoid over-manipulation of the cervix, which can cause microtrauma.

2. Perineal and Vulvar Conformation

• Mares with poor vulvar conformation should undergo a Caslick’s procedure, a minor surgical closure of the vulvar lips to prevent ascending infection.

3. Vaccination and Biosecurity

• Vaccinate mares against equine herpesvirus (EHV-1) to prevent late-term abortion.
• Control leptospirosis in endemic areas through vaccination and by preventing access to contaminated water.
• Isolate pregnant mares from new arrivals or horses showing respiratory symptoms.

4. Environmental and Nutritional Management

• Keep pregnant mares in clean, dry paddocks.
• Avoid overcrowding and minimise stress.
• Provide balanced nutrition and regular deworming.

5. Monitoring Late Pregnancy

• Perform regular ultrasound examinations to detect early signs of placental thickening or fluid accumulation.
• Observe mares daily for abnormal discharge or premature udder development.

Prognosis

The outcome of placentitis depends on how early it is detected and how quickly treatment is started. If the infection is controlled early, many mares will deliver viable, healthy foals. In advanced or undetected cases, abortion, stillbirth, or the birth of weak, premature foals is common.

Even after successful treatment, affected mares may have an increased risk of recurrence in future pregnancies, so preventive care is especially important.

Conclusion

Placentitis is a serious but manageable condition when identified and treated promptly. Because it often develops quietly, regular veterinary monitoring and sound breeding management are the best defences.

By maintaining excellent reproductive hygiene, vaccinating appropriately, and keeping pregnant mares in a clean, stress-free environment, horse owners can greatly reduce the risk of placentitis and improve the chances of delivering healthy foals year after year.