Aberrations of the Equine Estrous cycle

True polyestrous

Note the year around cycles in this mare.

  • Theoretically all mares at equator will have equal light and dark year around, so they should not have an anestrous period.
  • In reality, about 20 % of mares in the Northern Hemisphere will cycle the year round. Whether this is an insensitivity to melatonin is unknown.
Silent heat
  • This is when the mare fails to show behavioral signs of estrus, even though normal physiological estrus is occurring.
  • Examination of these mares shows the progesterone to be low at estrus and rise with a normal ovulation.
  • The incidence of this syndrome seems to vary with the efficiency of the teasing program, but can be up to 15% even on a well managed farm.
  • Mares that are prone to have this problem are shy mares, maiden mares, and foaling mares (caused by over-protectiveness of the foal).
  • Poor heat detection is usually to blame, however.
  • Diagnosis
    • Determine by rectal palpation and/or ultrasound that the mare is cycling normally.
    • Evaluate the teasing program to ensure that teasing is optimized.
    • Progesterone assays may also help in determining if the mare is truly ovulating and forming a corpus luteum.
  • Treatment
    • Careful teasing may increase the chance of catching the mare in heat.
    • You may want to suggest varying the heat detection methods. Use individual instead of group teasing, change stallions to see if the mare will react to a different stallion.
    • Daily palpation or you may need to palpate every 12 hours because of the confusion between follicles and corpus hemorrhagicum.
    • Daily or every other day ultrasound exams. Ultrasound exams are a good method to tell when a mare is in estrus because of the characteristic appearance of the uterus of a mare in heat.
    • You can use prostaglandin to bring the mares into an estrus that is more predictable to time (i.e. they will watch for it more closely)
    • Artificial Insemination (AI)  or appropriate restraint for natural cover may be required. Most of these mares will break down for breeding when they are twitched. You really hold your breath though when you walk the stallion up to a kicking mare that you say is in heat based on your palpation.

Multiple ovulations
  • Up to 24-26% of ovulations are multiple. (Somewhat breed dependent, some breeds, e.g. Warmbloods may have 35% double ovulation rate). 99% are double and only 1% have more than 2 ovulations.
  • There is a 73% incidence within individual mares.
  • Generally there is about 1 day between ovulations. Progesterone rises after the second ovulation.
  • The problem with multiple ovulations is that they lead to twin pregnancy, which result in a high percentage of abortions due to placental insufficiency.
  • Diagnosis is based on palpation and/or ultrasound of two follicles at estrus forming two corpora lutea.

Note the multiple ovulations as noted by the arrows in May and June.

  • Treatment
    • You can short cycle all mares with twin ovulations with prostaglandin to bring them back into estrus, which is the way some farms handle the condition.
    • You can breed to the second ovulation, because the oocyte only has a 12- 24 hour life span so only the second oocyte should fertilize. Problems with this approach include reduced estrous behavior of the mare and decreased uterine defense mechanisms because the CL from first ovulation is producing progesterone.
    • You can take your chances and worry about twins later when you do an early pregnancy check for twins. This is probably the most common approach now. At < 16 days you can crush one  of the twins.

Diestrous ovulations
  • Ovulation can occur in the luteal phase (high progesterone) with no signs of heat.
  • A 24 % incidence reported, and these mares have been bred and become pregnant from the diestrus ovulation.
  • You must rule out silent heat by palpation of the cervix and uterus and ultrasonography, or using progesterone analysis.

Note the diestrous ovulations indicated by the arrow during high progesterone in July.

  • If the ovulation is early in the cycle there is no effect on next cycle because the corpus luteum that develops has receptors that are mature enough at the correct time to respond to the prostaglandin .
  • If the ovulation is late in the cycle the prostaglandin receptors are not mature at the time of endogenous prostaglandin release, so luteolysis of this corpus luteum does not occur and there is a prolonged diestrus. This may also explain why prostaglandin administration is not always effective in mares.

Persistent corpora lutea, also termed pseudopregnancy or spurious pregnancy.
  • This is a prolonged corpus luteum life span that may occur without uterine pathology.
  • This condition can occur with or without breeding and is the number one cause of anestrus in mares.
  • The average duration is 35 -90 days.

Note the persistent diestrus as indicated by the prolonged high progesterone.

  • Proposed mechanisms
    • Late diestrus ovulation with an immature corpus luteum at the time of prostaglandin release, as described above.
    • Failure of endometrial release of prostaglandin because of endometrial pathology.
    • Early pregnancy loss can also cause this condition. Ultrasound examination may help rule this out.
    • Follicular development and even ovulation (without estrus) may continue to occur.
  • Diagnosis
    • Diagnosis is by palpation for uterine tone and follicular activity in a non pregnant mare.
    • You can ultrasound to make sure the mare is non pregnant.
  • Treatment
    • Prostaglandin should lyse the corpus luteum and bring the mare back into heat.

Lactational anestrus

  • The ' 9 day' or 'foal heat' is usually readily observable, but subsequent heats may not be readily observed. This is usually a result of subestrus because of the mare being so concerned about the foal.
  • However, this may also be a persistent corpus luteum.
  • May be due to seasonal anestrus if the mare foaled very early in the year
  • May be due to other unknown causes
  • Diagnosis
    • Progesterone analysis and/palpation will usually be able to determine if the mare is cycling normally.
Granulosa-theca cell tumor

  • This is the most common ovarian tumor of mares. 
  • It is referred to as a sex cord stromal tumor and includes granulosa cells, theca cells with Leydig and Sertoli-like cells occasionally noted. 
  • These tumors are endocrinologically active with variable production of gonadal steroids. 
  • There is limited tendency for metastasis in mares. 
  • There is no age distribution. They have been reported in mares as young as 2 yr old. 
  • This is the only tumor that causes estrus behavioral changes. Mares may exhibit anestrus, continuous or intermittent estrus or stallion-like behavior. 
  • Diagonsis
    • On palpation per rectum, an enlarged firm ovary is found with the opposite ovary small and inactive due to lack of gonadotropin stimulation resulting from inhibin production by the tumor. The affected ovary typically lacks a palpable ovulation fossa (although with very large ovaries, this can be hard to discern). 
    • On ultrasound exam, the typical appearance is multiloculated, although there is much variation.
    • Probably the most definitive diagnostic tool is the inhibin assay (presently done at UCD). A granulosa cell profile is done in which inhibin is assayed along with progesterone and testosterone.
    • anti-Müllerian hormone (AMH) strongly expressed by granulosa cells in equine GCT and in normal antral follicles. - may be a useful biomarker for detection of GCT in the horse (Ball UCD).
  • It is important to differentiate this condition from an ovarian hematoma. 
    • With a hematoma, the mare continues to cycle, an ovulation fossa is present (but may be hard to palpate if the hematoma is very large). 
    • The opposite ovary is normal in size and activity. 
    • The hematoma will regress with time and may respond to PGF with a decrease in size. 
    • With a hematoma, inhibin and testosterone are low.
  • Treatment of a granulosa cell tumor is surgical removal. 
    • Removal leads to resumption of normal activity in the remaining ovary in most mares within 2-16 mos. post-op (ave, 6 mos). 
    • The time depends on how long the tumor has been present. The prognosis for future fertility is as good as before the tumor developed.

Other ovarian tumors found in mares include:
  • Dysgerminoma: This is a malignant ovarian tumor of germ cell origin. There is rapid metastasis to the abdominal and thoracic cavities. Typical history is of chronic weight loss and abdominal discomfort. On exam, an enlarged multilobulated ovary or abdominal mass is found.
  • Teratoma: These are benign tumors of germ cell origin. They may (and often do) contain teeth, hair and bone. They are usually an incidental finding at slaughter or necropsy. They are usually small and nonpalpable and do not affect fertility.

  • Serous cystadenoma: These are benign primary epithelial ovarian tumors. They typically have a gradual onset and the affected ovary contains palpable cystic areas on its surface. The other ovary is unaffected.


Anovulatory Hemorrhagic Follices
  • Hemorrhage into follicular lumen before ovulation
  • May be about 8% incidence
  • Formation of luteal tissue minimal 
  • Regress over time
  • Often observed in autumn
  • Ultrasound appearance
    • Thick walled
    • Free floating spots in antrum as follicle develops
    • Once growth stops, contents become “organized” – echogenic with fibrin strands
    • May luteinize to some degree 
    • Older mares
  • Treat - wait and/or prostaglandin
  • Article - click here for more information.

Constant estrus

  • Constant estrus may just be an over interpretation by the owner of the mares actions. Urinating and winking do not always mean estrus.
  • Vaginal irritation may make a mare wink and appear to be estrus.
  • True nymphomaniacs have normal cycles, have abnormal CNS signs at estrus.
  • They may be mild and be completely normal except during estrus. These mares may be too crazy to breed.
  • Severe nymphomaniacs have constant aggression and are not in heat. They are not cystic and the ovaries are small and hard. Ovariectomy results in no change in their behavior.
  • This may be the result of adrenal production of steroids. If so, 10-15 mg dexamethasone may suppress the signs. Progesterone may also suppress the signs, but do not count on it. Recently it was reported that these mare may be hypothyroid and supplementation with thyroxine restored normal behavior.

Racing mares
  • Many mares on the track are given anabolic steroids, such as Equipoise.
  • Equipoise is definitely detrimental to stallions, however the effects on mares are much less.
  • In a study (JAVMA; 186:583, 1985) when 46 mares were given equipoise for 53 weeks, or 19 injections, the last of which was in December, the first ovulation was at the same time, but the conception at the first heat was lower, resulting in an overall longer time to get the mares pregnant.

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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