The mare under the influence of estrogen will have an increase of edema of the reproductive tract. This includes mild hyperemia
of the vulvar lips and vagina with a concomitant relaxation of the vaginal vault and cervix. The mare has an average of 5-10
endometrial folds that when under the influence of estrogen become markedly edematous. A mare in standing heat will display
a characteristic pattern of edema that could be described as a cart wheel pattern. As the endometrial folds become hyopechogenic
in the center and hyperechogenic in the borders. Just prior to ovulation and during the diestrus period the reproductive tract
will starts a progesterone dominance and the endometrial edema is no longer present under normal circumstances. The appearance
and disappearance of endometrial edema are related with the onset of estrus and ovulation in mares and is a progressive phenomenon
that has a typical pattern during the estrus period. The present paper describes a scoring system for endometrial edema (SEE)
detected by transrectal ultrasonography, and the use of the pattern of endometrial edema as a tool to help veterinarians:
a) improve the prediction of the time of ovulation in normal mares, b) detect mares that need post-breeding therapies such
as uterine lavage, oxytocin or caslicks operation.
A subjective scoring system (0-5) for the degree of endometrial edema (score of endometrial edema SEE) was established. A
grade of zero (0) was given to mares that were in diestrus and had no uterine edema and a grade of five (5) was assigned to
mares with maximal uterine edema. In order to use this scoring system the mares were examined on a regular basis throughout
their estrus cycle and the pattern of endometrial edema determined. Teasing was done on a daily basis in the thoroughbred
population and behavioral signs correlated with appearance of uterine edema. In addition SEE, follicle size and time of ovulation
were recorded after injection of 2500 IU of HCG or Ovuplant treatment.
In the normal mare, there is a typical pattern of appearance and disappearance of the uterine edema that can be followed ultrasonographically.
Mares will start displaying signs of behavioral heat when the ultrasound score reaches around two. (X= 1.83 range 1-4) From
there on, the score will increase gradually until it reaches its maximal degree (X= 4.3 range 3-5). It is at this point when
mares will respond most consistently to hCG with follicular size very often larger than 35 mm. Of the mares that I have treated
with hCG 98% have ovulated within 48 hrs. Follicle size in these mares has been an average of 39.8 mm ranging between 33 and
49 mm. From the time that mares reach their peak SEE or are treated with HCG, endometrial edema starts to gradually decrease
as the mare approaches ovulation. The normal mare will ovulate within a few hours after the SEE has decreased to an average
of 1.3 range between 1-4. In most mares impending ovulation will coincide with a history of behavioral estrus for four or
five days and a preovulatory size follicle 40-55 mm in size. The ultrasonographic appearance of the follicle that is approaching
ovulation is quite distinct with irregular borders, and hyperechogenic walls. This follicle in general is painful and will
be soft at palpation depending on its location within the ovary.
Frequently veterinarians working at breeding farms do not have the opportunity to follow a mare during the entire estrous
cycle and often the mares are already in standing heat when they are presented.
Mares in estrus
Frequently a mare is presented for breeding when she is in standing heat. Veterinarians often are puzzled on when to breed
these mares particularly when they should be bred to a heavily booked stallion. The degree of uterine edema combined with
follicular size is an important marker to decide when these mares should be bred. Low SUE (1-2) and the presence of a large
follicle often > than 40 mm, is a good indication of imminent ovulation. On the other hand low SEE with follicles less than
38 mm would be suggestive of a mare in early estrus. Furthermore the presence of uterine edema is the most reliable indicator
of heat in the normal mare even when they do not respond to teasing or a teaser is not available.
Mares early in the breeding season might display behavioral signs of heat. These heats in general can be quite erratic and
unpredictable and very often are anovulatory heats. The presence of uterine edema in these mares unlike the cycling mare does
not follow the pattern described above. However, the detection of edema is indicative of estrogenic competence of the follicles.
Mares with distinct uterine edema and follicles less than 35 mm could be started on progesterone or progestagen therapy. Mares
with uterine edema and a follicle grater than 35 mm, can be treated with an ovulation inducing agent.