Bit & Bridle

Equine Uterine Infections

Other Names

  • Endometritis
  • Pyometra
  • Metritis
  • Chronic Endometritis

What are Equine Uterine Infections?

Uterine infections in mares encompass a range of inflammatory and infectious conditions affecting the uterus, from acute bacterial contamination following breeding or foaling to chronic persistent endometritis that can significantly impair fertility. These conditions are among the most common causes of subfertility in broodmares and represent a major economic concern in the equine breeding industry.

Endometritis — inflammation of the uterine lining — is the most frequently diagnosed form. It may be transient (normal and self-resolving following breeding) or persistent, where the mare fails to clear the inflammation effectively. Pyometra, an accumulation of pus within the uterus, is a more severe presentation that requires urgent veterinary intervention. Metritis refers to more widespread uterine wall inflammation, most often seen following foaling complications.

Mares that are susceptible to persistent endometritis often have anatomical or immunological deficiencies that impair normal uterine clearance. Older mares, particularly those that have had multiple foals, are at greatest risk, though younger mares with poor perineal conformation may also be susceptible.

Causes

Uterine infections in horses arise through several mechanisms:

  • Bacterial contamination during breeding: Natural mating or artificial insemination can introduce bacteria into the uterus. Common culprits include Streptococcus equi subsp. zooepidemicus, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa.
  • Contagious equine metritis (CEM): Caused by Taylorella equigenitalis, a highly contagious venereal pathogen that is a notifiable disease in the UK and many other countries. It can be carried by apparently healthy stallions and mares.
  • Post-foaling contamination: Retained placenta, difficult foaling, or foaling in unhygienic conditions increases the risk of bacterial invasion.
  • Faecal contamination: Poor perineal conformation — where the anus sits forward of the vulva — allows faecal material to fall into and contaminate the reproductive tract.
  • Uterine cysts and age-related changes: Older mares develop endometrial fibrosis and cysts that impair the uterus’s ability to clear fluid and bacteria after mating.

Symptoms and Signs

  • Vaginal discharge: Mucopurulent (cloudy to yellow-green) discharge from the vulva is often the most obvious sign. Discharge may be most noticeable after exercise or when the horse urinates.
  • Failure to hold pregnancy: Repeated early embryonic loss, failure to conceive despite multiple coverings, or return to oestrus shortly after mating.
  • Uterine fluid accumulation: Detected on ultrasound examination, fluid pooling within the uterine lumen is a hallmark of active endometritis.
  • Systemic illness: In severe cases of metritis or pyometra, the mare may show fever, inappetence, lethargy, and signs of toxaemia.
  • Enlarged uterus: In pyometra, the uterus may be visibly distended with accumulated pus.

Diagnosis

  • Rectal palpation and ultrasonography: Transrectal ultrasound allows assessment of uterine fluid accumulation, wall thickness, cysts, and overall uterine architecture. It is an essential first step in any reproductive evaluation.
  • Endometrial swabs and cultures: Swabs taken from the uterine lumen identify the causative organism and guide antibiotic selection. Low-volume lavage cultures improve sensitivity.
  • Endometrial biopsy: Provides a histological assessment of the uterine tissue, grading the degree of fibrosis and inflammation. This gives the most accurate prognosis for future fertility.
  • Cytology: Examination of uterine cells from a cytobrush or flush sample can detect inflammatory cells (neutrophils) indicative of active infection even when cultures are negative.

Treatment

  • Uterine lavage: Large-volume saline flushing of the uterus removes inflammatory debris, bacteria, and exudate. This is often the most important immediate treatment. Lavage is typically repeated on consecutive days until the retrieved fluid runs clear.
  • Intrauterine antibiotics: Infusion of antibiotics directly into the uterine lumen, guided by culture and sensitivity results. Common choices include amikacin, ticarcillin, or gentamicin.
  • Systemic antibiotics: Required for severe infections, metritis, or pyometra, and for CEM (which requires systemic treatment of both mare and stallion).
  • Oxytocin: Administered to promote uterine contractions and improve drainage of accumulated fluid.
  • Surgical correction: Mares with poor perineal conformation benefit from a Caslick’s procedure — surgical closure of the upper portion of the vulva to prevent faecal contamination. This simple procedure can dramatically improve fertility in susceptible mares.
  • Immunomodulatory treatments: Platelet-rich plasma (PRP) or other novel treatments are being explored for mares with persistent endometritis that does not respond to conventional therapy.

Prevention

  • Ensure good perineal conformation before breeding — consult your vet about a Caslick’s procedure if necessary.
  • Practise rigorous hygiene during breeding, AI procedures, and foaling.
  • Test stallions and mares for CEM before each breeding season, particularly in the UK where it is a notifiable disease.
  • Perform a pre-breeding examination — including ultrasound and uterine culture — on broodmares each season, especially older mares.
  • After foaling, monitor the mare closely for retained placenta, abnormal discharge, or signs of systemic illness.
  • Maintain clean foaling facilities and change bedding frequently.

Prognosis

Prognosis varies considerably depending on the cause, severity, and duration of the infection. Mares with acute, uncomplicated endometritis caused by susceptible bacteria generally respond well to treatment and can successfully carry a foal the same season. Mares with persistent endometritis, significant uterine fibrosis (evidenced by biopsy), or pyometra have a more guarded prognosis for future fertility. In advanced cases, the mare may be retired from breeding. Early detection and treatment significantly improve outcomes.

When to Call the Vet

Contact your veterinarian promptly if you observe vaginal discharge at any point outside of oestrus, if your mare repeatedly fails to hold pregnancy, if you suspect retained placenta after foaling, or if your mare shows signs of systemic illness such as fever or inappetence. Early investigation through a BEVA-registered equine veterinarian is key to preserving long-term fertility. For related reading, see our entries on Equine Reproductive Loss Syndrome and equine reproductive health.

Frequently Asked Questions

Can a mare with endometritis still get pregnant?

Yes, many mares with endometritis can conceive and carry a foal successfully, particularly if the infection is identified and treated before or immediately after breeding. Mares with mild, acute endometritis often respond well to a single course of uterine lavage and antibiotics. However, mares with severe or long-standing infection, significant uterine fibrosis, or pyometra may have substantially reduced fertility, and some will not be able to carry a foal to term.

Is Contagious Equine Metritis (CEM) a serious concern in the UK?

Yes. CEM is a notifiable disease in the UK, meaning it must be reported to the Animal and Plant Health Agency (APHA) if diagnosed. It can spread rapidly through a breeding operation if screening protocols are not in place. Both stallions and mares must be tested before breeding under the Horserace Betting Levy Board (HBLB) Code of Practice. Affected animals must be treated and test negative before being used for breeding.

How long after treatment can a mare be bred?

This depends on the severity of the infection and the mare’s response to treatment. A mare that has cleared a mild endometritis may be ready to breed within the same season. Your veterinarian will typically confirm resolution through a follow-up ultrasound and uterine cytology before recommending breeding. Mares that required intensive treatment may benefit from waiting until the following breeding season to allow full recovery.

What is a Caslick’s procedure and when is it needed?

A Caslick’s procedure involves suturing the upper portion of the vulval lips together, leaving only a small opening for urination. It is recommended for mares whose vulval conformation allows air and faecal material to be drawn into the vagina, predisposing them to uterine infections. It is a minor procedure performed under local anaesthetic and is very effective at preventing contamination. The sutures must be opened before foaling to prevent tearing.