Pre-Breeding Evaluation of the Mare

Pre-Breeding Evaluation of the Mare

Initially some understanding of normal expectations of fertility are useful:

For Thoroughbred mares in the UK the pregnancy rate has increased from a low of 61.2% in 1971 to a high of 72.4% in 1989 (date of pregnancy determination not specified). Similarly, the live foal rate has increased from a low of 53% in 1971 to a high of 66% in 1989 (Ricketts and Young, 1990). For the Hunter Improvement Scheme the current live foal rate is around 60%. Ponies tend to have better rates.

Pregnancy rates at the end of the season will depend on: (1) the fertility of the stallion; (2) the fertility of mares; (3) and the management of the pregnant mare.

This last factor is often related to the value of the horses involved, i.e. frequent veterinary attention in cases where it is justified by the potential value of the foal, results in better fertility. Very expensive stallions tend to attract more fertile mares, or the stud may accept only young, fertile mares. Well-managed studs tease mares regularly and individually; this is very time consuming. An experienced stud manager knows, for example, the reasons why some mares fail to exhibit oestrous behaviour. In turn, the length of time that a mare fails to show oestrus before being presented to the veterinary surgeon for examination depends on stud policy and the owner's wishes.

    A clinical protocol for the investigation of a mare prior to breeding.

    Obtain the mare's previous breeding history

    Assess her physical condition, general health and perineal conformation

    Culture swab samples collected from the vestibule, clitoral fossa and sinuses

    Examination per vaginam using a speculum, and collection of endometrial swabs for bacterial culture and stained smear

    Manual vaginal examination

    Examine the reproductive tract by rectal palpation

    Transrectal real-time ultrasound examination of the reproductive tract

    Endometrial biopsy

    Endoscopic examination of the endometrium

    Peripheral venous blood sample for hormone/chromosome analysis

    Clinical examination of the female reproductive tract 


In the normal mare the vulva provides an effective barrier to protect the uterus from ascending infection. If the vulval seal is incompetent, pneumovagina may occur and the reproductive tract can become infected. The initial vaginitis may lead to cervicitis and acute endometritis resulting in subfertility. Caslick (1937) first pointed out the importance of this condition in relation to genital infection. Interestingly, it is most commonly found in Thoroughbreds, and, in the author's experience, is almost unknown in Shires and native ponies. Defective vulval conformation can be (1) congenital, which is very rare or (2) acquired, which is due to (a) vulval stretching following repeated foalings, (b) injury to perineal tissue or (c) poor bodily condition.

Older, pluriparous mares are more commonly affected with pneumovagina. However, young mares that are in work and have little body fat and/or poor vulval conformation, can develop pneumovagina. In some mares, pneumovagina may only occur during oestrus when the perineal tissues are more relaxed. A `Caslick index' has been described in an attempt to determine which mares require treatment (Pascoe, 1979), but its use is not widespread. Some mares make an obvious noise whilst walking, but in other mares the diagnosis may be more difficult. The presence of a frothy exudate in the anterior vagina on examination with a speculum is pathognomic. Rectal palpation of a ballooned vagina or uterus from which air can be expelled confirms the diagnosis. Real-time ultrasound examination of the uterus may reveal the presence of air as hyperechoic (white) foci. Cytological and histological examination of the endometrium may demonstrate significant numbers of neutrophils and eosinophils indicative of an endometritis.

Treatment should be directed at correcting the physical pnemovaginitis and concurrently treating the acute endometritis. The former can be done surgically by Caslick's operation.

However, when the angle of the vulval surface relative to the vertical is the primary deformity, Caslick's operation is ineffective, and perineal resection should be used to achieve a satisfactory vulval conformation (Pouret, 1982). In the author's opinion many mares are subjected to Caslick's operation unnecessarily: the operation should be reserved for mares with a true vulval defect rather than just because the mare has failed to become pregnant.

Clitoral Swabbing

Before the breeding season, swabs may be taken from the clitoral fossa and clitoral sinuses (only the central sinus may be obvious), and the vestibule. The perineal area of the mare should not be cleaned except for the removal of gross contamination of the vulva with faeces using a dry paper towel. A protective disposable glove should be worn by the veterinary surgeon on the hand used to evert the ventral commisure of the vulva and expose the clitoris. The swabs should be placed in transport medium, clearly labelled with the mare's name and sent to a reputable laboratory. It is important to penetrate the clitoral sinus, and therefore a large swab tip should not be used.

Swabs are cultured aerobically on blood and MacConkey agar particularly to screen for the presence of K. pneumoniae and P. aeruginosa. Microaerophilic culture on chocolate blood agar (with and without streptomycin) may also be done for the detection of CEMO.


Vesicovaginal reflux, also known as urovagina and urine pooling, is the retention of incompletely voided urine in the vaginal fornix due to an exaggerated downward cranial slope of the vagina. Pneumovagina from a defective vulval conformation also predisposes to the condition. Transient urine pooling, which is sometimes found in postpartum mares, usually resolves after uterine involution has occurred. Uterine infection with an accumulation of exudate in the vaginal fornix can be confused with the condition. It can be treated surgically by vaginoplasty (perhaps more correctly termed caudal relocation of the transverse fold, as surgical intervention is in the vestibule) (Monin, 1972), urethral extension (McKinnon and Belden, 1988) or perineal resection (Pouret, 1982).

Vaginal bleeding from varicose veins in the remnants of the hymen at the dorsal vestibulovaginal junction is occasionally seen in older mares, particularly during oestrus. Treatment is not usually necessary as the varicose veins normally shrink spontaneously, although diathermy can be used.

Manual vaginal examination of maiden mares often reveals the presence of hymen tissue which generally breaks down with pressure. A complete persistent hymen can also occur which can result in the accumulation of fluid within the vagina and uterus due to impaired natural drainage. Sometimes the hymen may be so tough that it can only be ruptured using a guarded scalpel blade or scissors. The small incision can then be enlarged using the fingers and hand. Rarely, failure of proper fusion of the Mullerian ducts may result in the presence of dorsoventral bands of fibrous tissue in the anterior vagina and fornix. They do not interfere with fertility and are easily broken down manually.


The cervix, whilst forming an important protective physical barrier to protect the uterus, must also relax during oestrus to allow intrauterine ejaculation of semen at coitus and drainage of uterine fluid. A cervicitis is usually associated with endometritis and/or vaginitis.

Fibrosis of the cervix often occurs in older mares, particularly maiden mares. Adhesions of the cervix arise from trauma at parturition or mating; they can be broken down manually, but this must be done daily to prevent recurrence. Artificial insemination has been used successfully in mares with an abnormally narrow cervix. Impaired cervical drainage of uterine fluid can predispose to chronic endometritis.

Cervical lacerations may need surgical repair if severe. Developmental abnormalities of the cervix have been described; these include aplasia and a double cervix.

Endometrial Swabs and Smears for Diagnosis of Endometritis

A diagnosis of endometritis can be made by collection of concurrent endometrial swab and smear samples during early oestrus for bacteriological culture and cytological examination, respectively. This allows time for resolution prior to mating, and maximises the chances of pregnancy. The ideal technique should ensure that the swab enters the uterus and collects bacteria from only the uterine lumen.

Two methods can be used:

A non-guarded endometrial swab on a sterile extension rod is carefully passed via a sterile speculum through the cervix into the uterine body and, after withdrawal, is placed in transport medium. A second swab is taken immediately afterwards for the endometrial smear.

    A guarded swab is passed into the uterine lumen using a sterile speculum or enclosed in a disposable plastic arm-length glove. The swab tip is exposed only when it is in the uterine lumen. A second swab for cytological examination should again be taken. Swabs for culture should be plated on blood and MacConkey agar, and incubated at 37ºC for 48 hours. Cultures should be examined at 24 and 48 hours. An air-dried smear is made by gently rolling the second swab either on a Testsimplet (Boehringer Corporation), which is a pre-stained slide or a clean dry microscope slide. The smear can be differentially stained with a rapid stain such as Diff-Kwik (American Hospital Supplies). The stained smear should then be examined for the presence of inflammatory and endometrial cells the latter confirming contact of the swab with the endometrium. 

Interpretation: A positive culture result, with no evidence of inflammatory cells in the smear (usually neutrophils), is likely to be due to contamination during collection. Diagnosis is based on the presence or absence of significant numbers of neutrophils in the smear. Very rarely, neutrophils can be detected, usually at the `foal heat' or the first oestrus of the breeding season in maiden mares, although there is no endometritis.

Endometrial Biopsy

In some cases, endometrial biopsy may be a useful diagnostic aid. For detailed reviews of the clinical application and pathological findings in acute and chronic endometritis and endometrosis readers should consult Kenney (1978) and Ricketts (1978). The technique involves the insertion of a biopsy instrument through the cervix and into the uterus. With the biopsy instrument in the uterine lumen, a gloved hand is inserted into the rectum to allow manipulation of the instrument into the desired position. The sample is taken by closing the jaws of the instrument and tugging sharply. To avoid damage, the tissue is carefully transferred into a fixative solution by dislodging it from the jaws of the punch with a fine hypodermic needle. The instrument most commonly used today is the Yeoman (basket-jawed) biopsy forceps, ideally 60cm to 70cm in length, with which tissue specimens 2 x 1cm (about 0.2% of the whole endometrial surface) are obtained. If the uterus appears normal on palpation, the sample should be taken from one of the areas of embryo fixation, i.e. the uterine horn--body junction on either side. Single samples are usually representative of the entire endometrium. If the uterus is abnormal on palpation per rectum, biopsy samples should be taken from both the affected area and a normal area. Biopsy specimens should be fixed in Bouin's fluid followed by sectioning and staining with haematoxylin and eosin. The endometrial biopsy sample should be sent to a laboratory that is experienced in evaluating samples.

Detection of Intraluminal Uterine Fluid Using Transrectal Ultrasound Imaging.

Transrectal ultrasonography provides a non-invasive method of assessment of the uterus. In a study (Pycock and Newcombe 1996) involving the ultrasonic examination and cytological and bacteriological sampling of the uterus in 380 broodmares premating, it was concluded that:

    If no free fluid is detected during oestrus, then acute endometritis as detected in cytology is absent in 99% of cases.

    Free fluid does not indicate inflammation.

    Endometrial cytology and culture fails to detect sterile fluid accumulations. 


References on Pre-Breeding Evaluation

Caslick, E. A. (1937) Cornell Vet., 27, 178.

Kenney, R. M. (1978) J. Amer. Vet. Med. Assn, 172, 241.

McKinnon, A. O. and Belden, J. O. (1988) J. Amer. Vet. Med. Assn, 192, 647.

Monin, T. (1972) Proc. 18th Ann. Conv. Amer. Assn Equine Pract., p. 99.

Pascoe, R. R. (1979) J. Reprod. Fertil. Suppl., 27, 229.

Pouret, E. J. (1982) Equine Vet. J., 14, 249.

Pycock, J. F. and Newcombe, J. R. N. (1996) Equine Practice 18, 19-22.

Ricketts, S. W. (1978) Fellowship Thesis, Royal College of Veterinary Surgeons.

Ricketts, S. W. and Young, A. (1990) Vet. Rec., 126, 68.

This article was produced by Dr. Jonathan F Pycock, B.Vet.Med., Ph.D., D.E.S.M., M.R.C.V.S. of Equine Reproductive Services who provide a wide variety of services, including:

Routine Gynaecological Monitoring of Mares

Treatment of Problem Breeding Mares

Evaluation of Stallion Fertility

Barren Mare Clinics

Artificial Insemination (Chilled and Frozen Semen)

Management of Older Breeding Mares

Behavioural Problems in Mares 

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