Infertility in the mare
172-176, 179-185

Infertility can be divided into categories  

  • 1) mares that fail to cycle
  • 2) mares that cycle normally but don't conceive and
  • 3) mares that cycle normally and conceive but then suffer early embryonic death (EED)
The non-cycling mare
  • First, always rule out the possibility of pregnancy! 
  • Other causes of failure to cycle include winter anestrus, transition, prolonged diestrus, behavioral anestrus, chromosomal anomalies, and ovarian tumors. 
  • The topics of winter anestrus, transition and behavioral anestrus have been covered previously.
  • A prolonged luteal phase, also called prolonged diestrus, persistent CL or pseudopregnancy can be caused by a variety of factors. 
    • Possible causes include failure of PGF release, insensitivity to the PGF released (e.g. diestrous ovulation), embryonic death after maternal recognition of pregnancy, etc. 
    • If untreated, it can last for 60-90 d. 
    • This was more of a problem in the pre-ultrasound days, but now with ultrasound and the ability to diagnose pregnancy at 14 days, prolonged luteal phase in the nonpregnant mare can be easily diagnosed and treated. 
    • Treatment is Prostagandin.

Chromosomal anomalies
  • Abnormalities of karyotype are being found more commonly as our ability to look for them increases.
  • Gonadal dysgenesis mares appear phenotypically female. Their external genitalia are normal. They are generally small in size (often short with a big head). Regarding sexual behavior, in response to teasing they are passive or occasionally exhibit irregular estrous behavior. On palpation per rectum, their ovaries are very small, smooth and firm. The uterus is small and flaccid (a thin band). The cervix is small and flaccid, but may appear normal on speculum exam. Karyotyping is needed for definitive diagnosis. These mares are sterile.

Molecular Cytogenetics Laboratory
Room 318 B
Bldg 1197
Department of Veterinary Integrative Biosciences
Texas A&M University
College Station, TX 77843-4458

Telephone: 979-458-0520
Fax: 979-845-9972

Please call in advance. Laboratory led by Dr. Bhanu Chowdhary

  • XY sex reversal, XO and XX pseudohermaphrodite mares have been reported. XX pseudohermaphrodite mares are phenotypically "female" yet have testes. In some cases this has been attributed to a translocation of the testes determining factor to another chromosome.

Early pregnancy loss (Early Embryonic Death - EED)
  • Definition: loss of the conceptus before organogenesis is complete. 
  • Caution must be used in the diagnosis of this condition. 
    • Diagnosis by observation of failure to return to estrus and an assumption of pregnancy will erroneously overestimate or possibly underestimate EED. 
    • Diagnosis by palpation may underestimate the incidence of EED. 
    • Diagnosis by hormonal assay can also lead to errors. A single low value of progesterone is not necessarily indicative of impending loss. ECG will remain elevated even after embryonic death once the endometrial cups are established. 
    • Ultrasonography is the best way to detect EED. With ultrasound, pregnancy can be confirmed and loss can be verified.
There are numerous causes of EED.
  • Decreased pregnancy rates have been observed with significant uterine pathology.
  • It is hypothesized that fibrosis is associated with uterine gland failure.
  • Glandular secretions (histotrophe) are important for support of the embryo due to delayed placental attachment in the mare.
  • If breed association regulations allow it, embryo transfer is a treatment option for these mares.
  • Progesterone supplementation has been used in an attempt to increase pregnancy rates but results are equivocal. 
    • Progesterone deficiency is often thought to be a cause of early loss. 
    • However, primary luteal insufficiency has not been documented as a cause of embryonic loss.
    • Secondary luteal insufficiency, on the other hand, is possible due to any number of causes that result in endogenous PGF release and it is possible to rescue a pregnancy with exogenous progestins. 
    • Progesterone is essential for pregnancy maintenance. Decreased progesterone causes EED, and conversely EED causes decreased progesterone. 
    • There is considerable variation in progesterone levels between mares and between samples in a mare during early pregnancy.
      • A single low value, even <2 ng, is not indicative of progesterone deficiency. 
      • Progesterone must be low for 2 consecutive days to document progesterone deficiency. 
    • No adverse effects are reported from the use of exogenous P4 during pregnancy. On the other hand, progesterone supplementation has not increased pregnancy rates in normal mares. 
    • If progestin supplementation is to be used, give adequate amounts at proper intervals. 
      • Many described regimens do not significantly alter plasma P4. 
      • Ovariectomy at d 35 required exogenous P4 sufficient to maintain plasma levels >4 (or at least >2) ng/ml to maintain pregnancy. 
      • Products and dosages which have been shown capable of maintaining pregnancy are:
        • progesterone in oil - 150 mg for the first 10-14 d then 200 mg every other day;
        • repositol progesterone - 1000 mg every 4 d; 
        • BioRelease P4 LA 150 ("progesterone in biorelease technology vehicle") from BET Pharm - 1500 mg, IM every 7 d
        • altrenogest (Regu-Mate), an oral progestin, - 1 ml/ 50 kg, PO, SID. 
      • It is generally recommended to maintain supplementation for the first 105 -120 d of gestation. 
      • If using progestin therapy, the viability of the conceptus should be periodically reaffirmed.
  • EED is associated with luteolysis or ovariectomy before 50 d of gestation. 
  • Between 50 -100 d of gestation, pregnancy loss is variable after ovariectomy. 
  • After 120 d of gestation, ovariectomy has no effect. 
  • Inappropriate release of PGF, such as from an extra-uterine source can cause luteolysis and pregnancy loss. 
    • It is possible to prevent pregnancy loss with PGF inhibitors such as flunixin meglumine but they must be administered within a very short time frame, i.e. they are not practical in most clinical situations. 
    • Pregnancy loss does not occur immediately after luteolysis and therefore time enough exists to rescue the pregnancy with progestin supplementation. 
    • Therefore, administration of P4 or altrenogest is more practical in most situations.

Endometrial cysts are not associated with pregnancy loss once pregnancy is established. Whether they can interfere with the maternal recognition of pregnancy if they are sufficiently large or numerous is unclear.

Failure of maternal recognition of pregnancy:
  • The embryo suppresses endometrial PGF. This is a transient characteristic of the embryo that is observed in vitro with d 12,13,14 embryos. This points out the importance of the embryo-uterus synchrony that is necessary to establish pregnancy.
  • Retarded development of the embryo may result in pregnancy loss.
  • Mobility of the conceptus is required to prevent luteolysis. The embryo travels all around the uterus for approximately 10 d after it enters the uterus, ceasing movement on about d 16 of gestation. Experimental restriction of this movement results in embryonic loss and the pregnancy can be rescued with progestins.

Embryonic/oviductal defects
  • It is difficult to separate defects of the embryo or oviduct from problems caused by age of the mare.
  • EED is greater for old mares and it appears this is due, at least in part, to defective oocytes.
  • Embryos from old mares are of poorer quality than those from young mares.
  • Oviductal cells from old mares have a detrimental effect on embryonic development in vitro, compared to cells from young mares.
  • There is a significant difference in protein production between oviductal cells of old and young mares.
  • Pregnancy rates following GIFT are lower using oocytes from old mares compared to young.
  • Similarly, it is difficult to identify what factors may be involved in "Subfertile" mares. Embryo recovery is higher for normal than for subfertile mares.
  • Subfertile mares had more abnormal embryos than did normal mares. There was no difference in pregnancy rates at d 2 after ovulation between normal and subfertile mares, but there was a significant difference at d 14.
  • After transfer of embryos from normal mares to normal and subfertile recipients there were equal pregnancy rates at d 12 and d 28 in one study, but greater EED for subfertile recipients in another study.
  • After transfer of embryos from normal and subfertile mares, collected at d 4, to normal recipients, pregnancy rates were lower with embryos from subfertile mares.
Other causes of EED
  • Nutritional stress has been associated with EED, which could probably be prevented by supplemental feeding. Other types of stress (hauling, weaning, etc.) have been associated with a rise in adrenocorticotropic hormone and transient decrease in progesterone but not with pregnancy loss.
  • Foal heat has no effect on EED. Mares bred on foal heat which are subsequently diagnosed pregnant have a normal likelihood of carrying the foal to term.
Endometritis can be categorized as
  • 1) sexually transmitted diseases
  • 2) chronic infectious
  • 3) persistent mating induced
  • 4) chronic degenerative, sometimes referred to as endometrosis. Chronic degenerative endometrosis, or periglandular fibrosis, has been touched on already and is associated with increased pregnancy loss.
  • When we speak of sexually transmitted endometritis, generally we are referring to Contagious Equine Metritis (CEM).  
    • This disease is due to Taylorella equigenitalis, a Gram negative coccobacillus that grows slowly on chocolate agar in 5% CO2. 
    • It can survive for an extended period on the external genitalia of the stallion and the vagina or clitoris of the mare. When infected, the stallion may show no clinical signs. 
    • The mare, however, shows acute endometritis with a thick, grey, mucoid discharge after breeding and may short cycle due to the inflammation. 
    • The acute signs subside rapidly with some mares remaining as asymptomatic carriers. 
    • To check for the disease, cultures are obtained from the following sites:
      •  in the acute state - endometrial or cervical swab; 
      • in the chronic state the best site to culture in the mare is the medial clitoral sinuses; 
      • in the stallion - urethral fossa, also urethra, pre-ejaculate fraction, folds of sheath, skin of penis. In stallions, repeated negative cultures are needed to consider a stallion negative.
      • Special transport media (Amies with charcoal) is needed to get the sample to the lab.
    • Laboratory tests in addition to culture include serology which in most cases detects a rise in antibodies from 7-45 d post infection in mares but is of little use in detection of the carrier
    • Complement fixation which identifies the chronically infected horse but not carriers, 
    • ELISA, serum agglutination which is good for diagnosis of the acute case, passive hemagglutination, and immunofluorescence. 
    • Control is important. 
      • This is a reportable disease in the US with most measures aimed at quarantine to control spread. 
      • It is generally regarded as not currently in the U.S., however CEM-like organisms were isolated two years ago in California and Kentucky. 
    • Treatment: 
      • Topical treatment of the stallion's penis with chlorhexidine scrub and nitrofurazone seems to be effective. 
      • Most mares recover but up to 20% of mares remain carriers in spite of treatment.
      • Topical treatment of the mare consists of IU antibiotics and flushing the clitoral fossa and sinuses with chlorhexidine then packing them with chlorhexidine or nitrofurazone.
      • Surgical ablation of the clitoral fossa & sinuses is used to treat the carrier mare and is required for export/import by some countries.
  • Certain isolates of Pseudomonas and Klebsiella may be transmitted venereally. 
    • However, Pseudomonas and Klebsiella may also be isolated from the external genitalia of normal stallions. 
    • If these pathogens are suspected to be the result of venereally transmitted disease, serotype the organism obtained from the mare and the stallion and compare them.
  • Dourine a venereal disease of horses, is caused by Trypanosoma equiperdum. It is a serious disease but fortunately not found in the US.
  • Equine coital exanthema is a venereally transmitted disease caused by a herpesvirus (EHV3). It causes infertility as a result of reluctance to breed because of painful lesions on the vulva, penis, and prepuce. As in other herpesvirus diseases, persistent carriers with periodic recrudescence is common.
Chronic endometritis
  • Chronic endometritis results when the barriers to contamination of the uterus are compromised or when the uterus that is susceptible to persistent mating induced endometritis is subjected to repeated insults (i.e. breedings). The anatomic barriers to contamination of the uterus are the vulva, vaginal-vestibular sphincter and cervix. Poor perineal conformation may contribute to the establishment of chronic endometritis.
  • Etiologic agents most commonly found in cases of chronic endometritis are Strep. equi zooepidemicus (the most common), E. coli, Pseudomonas aeruginosa, Klebsiella pneumonia and yeast. Others considered pathogens in clinical endometritis (with significant growth) include Proteus sp., Corynebacterium sp., Staphylococcus, Shigella.
  • In cases of fungal endometritis, Candida albicans, Aspergillus sp., Mucor sp. Are the most commonly isolated species.
  • Repeated or prolonged antibiotic treatment, especially with a variety of antibiotics, is many times associated with the establishment of a chronic endometritis.
  • The first step in treatment should be to correct any anatomical defects. 
    • Place a Caslick's to close the vulva; 
    • Perform a urethral extension to correct urine pooling; 
    • Repair R-V tears; etc.
  • Treatment timing
    • Prebreeding
    • Postbreeding
    • Postovulation
  • Antibiotics are often used to treat a bacterial infection before breeding or as routine post breeding treatments. 
    • Their use as routine post breeding treatments is controversial but may depend on the timing of the treatment.
    •  Ideally, antibiotic use is based on sensitivity. 
    • Superinfections should be avoided by using antibiotics with some discretion.
  • Fungal infections are difficult to treat, at best. Some suggested medications are: 
    • Clotrimazole - 500 mg daily for 7 - 12 d; 
    • Amphotericin B - 200-250 mg daily for 7 d; 
    • Nystatin - 5X106 U for 7 - 10 d; 
    • dilute (1 part stock solution containing 1% available iodine in 10-20 parts final solution) betadine. Avoid strong iodine and chlorhexidine diacetate solutions. Chlorhexidine is very irritating to the equine endometrium. Some mares are sensitive to even dilute iodine, therefore


Persistent mating induced endometritis

  • Normal uterine defense mechanisms are comprised of a number of factors
    • Local inflammatory response 
      • In response to an insult, either bacteria or semen, the uterus mounts an inflammatory response. 
      • Neutrophils, antibodies, complement, and other factors enter the uterus. 
      • Both complement and antibody are required for killing of bacteria which are then phagocytosed by neutrophils.
    • Mechanical evacuation of the uterus. 
      • Physical removal of bacteria and inflammatory byproducts then follows.
  • All mares have a transient post breeding endometritis which is considered physiologically normal and most mares are able to clear the endometritis in a timely fashion. 
    • After AI with fresh semen, the inflammatory reaction peaks 6-12 hr post AI and the uterus eliminates excess sperm. 
    • Clearance of the inflammatory processes must occur by 4-5 d post ovulation in order for the embryo to survive. 
    • The normal mare is able to rapidly eliminate contaminants.
  • Endometritis becomes established or persists when the natural defense mechanisms fail. Treatments should be performed only during estrus and the mare evaluated before subsequent treatments. Mares should be observed closely for signs of irritation.

  • Much effort has gone towards determining why some mares develop a persistent endometritis after breeding while others are resistant. 
    • It appears now that the primary defect is a failure of mechanical clearance. 
    • Why susceptible mares have reduced clearance is not totally clear. It is probably due to a number of factors including reduced lymphatic function, reduced myometrial response, conformation, etc. 
    • This is probably a gradual process resulting in a loss of efficiency in bacterial clearance, although it can occur abruptly following serious trauma.
    • Studies examining uterine clearance found that after infusion of Strep. zoo. into the uterus, resistant mares had cleared the bacteria by 24 hr, yet susceptible mares had not cleared it by 96 hr. 
    • Studies have also shown that the progesterone dominated (e.g. post ovulation) uterus has reduced physical clearance. 
      • The clearance of non-antigenic charcoal began within 2 hr in estrogen treated mares and was completely cleared within 3 d 
      • Clearance was significantly delayed in progesterone treated mares. 
    • Lymphatic drainage is also impaired in susceptible mares. 
      • After infusion of India ink in normal mares, ink was found in the lymphatic vessels and lymph nodes. 
      • Decreased lymphatic clearance (both amount and speed) was found in susceptible mares compared to resistant mares. The ink remained in the lumen of susceptible mares.
  • Mechanical clearance can be effectively enhanced using oxytocin or cloprostenol. 
    • Uterine lavage is also very effective in removing accumulated debris and improving the intrauterine environment. 
      • Generally, when performing uterine lavage, volumes of 1L are allowed to enter the uterus and then retrieved. 
      • This is repeated until the reflux is clear. 
      • Lavage may be performed as early as 4 - 6 h after breeding without reducing pregnancy rates.
      • In fact, there is evidence that early lavage may be beneficial. 
    • Oxytocin, 20 IU, i.v. stimulates rapid clearance within 30 min. 
      • Long acting oxytocin is available in Europe (
    • Cloprostenol has a longer period of action but does not stimulate as rapid or thorough clearance as does oxytocin and may have detrimental effects on the developing CL. 
    • Evidence indicates that sexual stimulation may enhance uterine clearance by causing endogenous oxytocin release. 
  • Various protocols exist such as uterine lavage followed by 20 IU oxytocin IV, 12 hr post insemination (or 6-24 hr after each breeding). 
    • Basically, the idea is to prevent a persistent inflammatory condition from developing.
    • Therefore, the sooner treatment is initiated, probably the better because you are helping to prevent a rapid increase in bacterial numbers and accumulation of excessive fluid and inflammatory byproducts.
  • Mucus or biofilm disruption
    • Mucus or biofilm may protect bacteria
    • DMSO - 30% solution infusion post-breeding improved conception rates
    • Kerosene - 50 mL infusion - breed on next cycle
    • N-acetylcysteine (NAC) -
      • disrupts disulfide bonds in mucin polymers
      • 3.3% solution infused not detrimental, however fertily data is not available
  • Chlelating agents
    • Chelates Ca or Mg to help disrupt cell wall - speculated
    • Tricide - no fertilitly data yet
  • Steroids
    • Dexamethasone - 50 mg IV 1 hr  post mating - improved conception rates (given along with other routine treatments)
      • 6-12 hrs post mating (10 or 20 mg) did not improve conception (n=783 cycles)
    • Prednisolone - 0.1 mg/kg  q12h 4 days starting 48 hours before breeding
Diagnosis of endometritis
  • Physical signs such as an exudate or evidence of inflammation on a vaginal speculum exam are clear indications of endometritis. 
  • Repeated failure to conceive after being bred to fertile stallion is also cause for suspicion of endometritis. 
  • Probably one of the best ways to diagnose persistent post mating induced endometritis is to observe an accumulation of fluid by ultrasound post-insemination. 
    • While normal mares may still have some fluid 6 h after breeding, the fluid is gone by 12 h. 
    • If on examination at 12 -24 h post mating fluid is observed in the uterus, a diagnosis of persistent post mating endometritis can be made.
  • Other means of diagnosing endometritis include evidence of chronic inflammation on endometrial biopsy and a positive Cytology/Culture. 
    • Cytology is more important than culture in the diagnosis of endometritis. 
    • Neutrophils are only present in the lumen when an active inflammation is present (need not be septic). 
    • Quantitative and qualitative culture results must be correlated with evidence of inflammation.
    • Culture is used to suggest the etiology of an already confirmed inflammatory process.
Management to prevent recurrence is as important as treatment afterwards.
  • First, reduce contamination.
  • Minimize the number of breedings per estrus by predicting when ovulation will occur. This will decrease the number of insults to the uterus. Avoid repeated breedings during the transitional period.
  • Keep in mind that fresh sperm cells normally survive in the mare 's oviducts for days after deposition, often up to a week.
  • The ovum, however, is fertile for only a few h after ovulation. (Therefore when breeding with frozen semen, mares are checked every 6 hrs and bred as soon as ovulation is detected.)
  • Sperm cells are transported into the oviduct very rapidly, therefore treatments can begin within 4 hr after breeding.
  • Use hCG or Ovuplant to induce ovulation. They are effective in inducing ovulation of a >35 mm follicle within 24-48 h. These management techniques also reduces overuse of the stallion.
  • When possible, use AI with a semen extender containing an antibiotic. In breeds requiring natural service, the Minimum Contamination Technique may be used, where semen extender containing an antibiotic is infused into the uterus before breeding.
  • In mares with endometritis before breeding, pre-breeding treatments are indicated, whereas in mares which do not have endometritis before breeding but are prone to developing persistent post mating endometritis, post-breeding treatments are more effective.
  • Treatments can also be used after ovulation because the embryo does not enter the uterus for 5 days after ovulation and the CL is not responsive to PGF until approximately 5 d after ovulation.
  • Therefore, examinations after breeding should be performed not only to check for intrauterine fluid but also to detect ovulation.
  • There is some evidence that the effectiveness of oxytocin diminishes somewhat as progesterone rises so there may be reason to try and accomplish uterine clearance after breeding but before ovulation.


  • There are several possible etiologies for pyometra in the mare. 
    • Cervical adhesions or malfunction with failure to drain contents are often implicated. 
    • Reduced endometrial resistance or other unknown factors may also be involved. A variety of organisms can be involved and in some cases a negative culture may be obtained.
  • There is usually no sign of systemic illness.
  • In some cases there may be a discharge.
  • On rectal exam, a fluid filled uterus is found which must be differentiated from pregnancy. The majority of affected mares cycle normally.
  • In some cases, with severe endometrial damage, PGF release may not occur so that luteal function is prolonged. In other cases, the mare may cycle at shortened intervals due to endometritis and PGF release.
  • In all cases, however, the prognosis for future fertility is poor. 
    • The damage to the uterus is most severe in long standing cases or those with a closed cervix.
    • In these cases the endoetrium may be replaced by granulation tissue. 
    • In less severe cases endometritis with atrophy and fibrosis of the endometrium is found. If the pyometra is cleared up, it will likely reoccur.

Cervical trauma
  • These are often the result of a laceration following a foaling injury. Adhesions may follow a dystocia with subsequent cervicitis/vaginitis or following inappropriate therapy. Correction requires surgery but is difficult and often unsuccessful.

Salpingitis/oviductal blockage
  • Salpingitis has been noted in abattoir studies as not uncommon, however oviductal but blockage is rare.

Endometrial atrophy may be found on biopsy if the affected area was biopsied. Alternatively, it may be suspected on the basis of placental inspection after parturition.

Breeding accidents
  • Whenever bleeding is observed after natural mating, it is important to determine the origin of the hemorrhage. It may be due to vaginal perforation. If so, the location and extent of the tear must be determined. Vaginal tears may be peritoneal vs. retroperitoneal depending on their location. Treatment will depend on their location (retroperitoneal -antibiotics; peritoneal -antibiotics, surgery or cross tie the mare).
  • Prevention should be practiced if any problems are anticipated (large stallion with small mare, stallion with previous history). AI would certainly avoid the problem but if natural service is required, the use of a stallion roll will improve safety. Varicose veins in the vagina may rupture during breeding but are not of any serious consequence.
  • Rectal breeding can cause a rectal tear. If a stallion penetrates the rectum during breeding, do a rectal exam to check for tears. If present, treat as previously discussed.

contributed by Bruce E Eilts and modified on 8 September 2009

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contributed by Bruce E Eilts on 25 September 2012


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