Endometritis - Horse

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Endometritis,Copyright RVC 2008

Endometritis is an infection of the uterine endometrium.

Endometritis is a common cause of poor fertility in the mare. It occurs following uterine contamination during covering, artificial insemination, reproductive examination, parturition and as a result of poor conformation.

Types of Endometritis

  1. Venereal Infectious Endometritis
  2. Non-venereal Infectious Endometritis
  3. Persistent Post-mating Endometritis
  4. Chronic Degenerative Endometritis
  5. Chronic Infectious Endometritis

Venereal Infectious Endometritis

Three major pathogens cause venereal endometritis in the mare:

The mare may present with vaginal discharge (ranging from scant to copious) in an acute infection. However she may also present in a carrier state, in which case there may be no outward clinical signs. Stallions are usually sub-clinical carriers of disease. All three bacteria prevent conception.

Equine Viral Arteritis (EAV) and Equine Herpes Virus 3 (EHV-3) are also classified as venereal infections however they do not cause endometritis or prevent conception.

Detailed guidelines on the diagnosis, treatment and prevention of all these infections can be found in the of Practice which are followed by Thoroughbred breeders in the UK.

Non-venereal Infectious Endometritis

This occurs following infection during covering, reproductive examination or foaling. The mare will normally present with a history of infertility or early embryonic death and short cycles. There may also be evidence of vaginal discharge. Infection may be caused by:

Persistent Post-mating Endometritis

This is more common in older and multiparous mares. They present with a history of short cycling and often an vaginal discharge approximately two weeks post-cover.

A transient inflammatory response is normal in the mare post-cover, however a normal immunological response is mounted and the infection cleared before the embryo exits the fallopian tube. In the cases of persistent post-mating endometritis the inflammation persists longer than 72-96 hours so that when the embryo enters the uterus the environment is still unsuitable for embryonic development, resulting in early embryonic death.

Chronic Degenerative Endometritis

Chronic degenerative endometritis aka endometriosis is degenerative change that occurs in older mares or following repeated inflammation of the uterus. If the condition is severe it may result in delayed clearance of the uterus post-cover. Definitive diagnosis can only be achieved by biopsy, which will show degenerative change of the uterus histologically.

Chronic Infectious Endometritis

Normally an underlying conformational condition such as pneumovagina predisposes the mare to chronic infectious endometritis. Definitive diagnosis is again by biopsy which should show infiltration of the endometrium with lymphocytes and plasma cells. Infection may be:

Prognosis is guarded due to the chronic nature of the infection and the anatomical faults predisposing to infection. Surgical correction of the conformational abnormalities may be attempted using Caslicks procedure.


  • Clinical examination may reveal vulval discharge or matted tail hairs.
  • Vaginal examination should identify any discharge and increased vascularity of the tissue. Conformational abnormalities resulting in e.g. pneumovagina and urovagina may be evident.
  • Ultrasound examination of the uterus - more than 2 cm of fluid with abnormal character suggests endometritis.
  • Clitoral and endometrial swabs should be taken for culture and sensitivity. Clitoral swabs should be taken if a chronic venereal infection is suspected. Guarded endometrial swabs should be taken during oestrus to identify either acute venereal infections or the causative organism of other endometrial infections and evidence of inflammation. A high level of neutrophils is indicative of endometritis. The mare should be confirmed as not pregnant before taking an endometrial swab.
  • Uterine flush - the uterus should be flushed with 100mls of fluid which can then be examined cytologically for evidence of inflammatory cells and bacteria.
  • Endometrial endoscopy can be performed to visualise and assess the endometrium.
  • Uterine biopsy - this should be performed after all other tests have failed to reach a diagnosis or to definitively diagnose endometriosis or chronic infectious endometritis. A single sample should be representative if the uterus feels normal on palpation. If an abnormality is detected on examination per rectum then samples should be taken from both normal and abnormal sites.


A combination of multiple therapies should be used to collectively resolve the inflammation within the uterus and treat existing infections:

(1) Uterine lavage with copious amounts of fluid. This is beneficial because it:

removes contaminants such as bacteria and purulent material
stimulates uterine contractions to aid clearance
causes mechanical irritation to the endometrium aiding healthy neutrophil recruitment

2-3 litres of saline or lactated ringers solution should be administered using a uterine flushing catheter and then drained back into the bag and inspected. Dilute Povidone iodine can also be used as a cheap alternative.

(2) Antibiotics (intrauterine or systemic). Antibiotic type should be guided by culture and sensitivity and activity of the drug in the uterus where possible. The length of the treatment should be proportional to the severity of infection.

(3) Administration of ecbolics to stimulate uterine contractility and clearance of infection - oxytocin and prostaglandin analogues

(4) Hormonal therapy- Oestradiol Benzoate and PG can be used to aid the clearance of infection. They are more effective during oestrus.

In the cases of venereal infections additional treatment may be required:

Acute infections: Repeated antibiotic clitoral irrigation and reintroduction of normal flora. A clitorectomy may be considered.
Chronic/carrier infections: Repeated clorhexidine clitoral irrigation and reintroduction of normal flora.

Management of Susceptible Mares

(1) Identify problem mare

(2) Plan a single insemination using a stallion with high fertility rates approximately 1-2 days prior to ovulation. Semen extender may be delivered intra-uterine before cover.

(3) Ultrasound the uterus in the first 12 hours post-cover. The character and volume of fluid present should be assessed.

(4) Remove uterine contaminants via lavage and antibiotic infusion. Administer oxytocin 8 hours after treatment.

(5) Repeat ultrasound examination after 24 hours.

Treatments should not be repeated if possible as this may introduce infection.


  • Employment of strict hygiene measures during breeding, reproductive examinations and parturition
  • Adherence to the Codes of Practice
  • Surgical correction of any existing conformational abnormalities


The prognosis of this condition vary according to the type, severity and chronicity of the infection and the age of the mare.

Endometritis - Horse Learning Resources
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Test your knowledge using flashcard type questions
Equine Reproduction and Stud Medicine Q&A 12


Codes of Practice (2011) Horserace Betting Levy Board (HBLB)

Pycock, JF (1997) Self-Assessment Colour Review Equine Reproduction and Stud Medicine Manson

Pycock, JF (2004) Pre-breeding checks for mares In Practice 2004 26: 78-85

Ricketts, S (1987) Vaginal discharge in the mare In Practice 1987 9: 117-123

RVC staff (2009) Urogenital system RVC Intergrated BVetMed Course, Royal Veterinary College

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