CHAPTER 2The Follicle: Practical Aspects of Follicle Control

Much time spent in stud medicine is in the practice of follicle palpation and ultrasonography with the object of predicting ovulation. Accuracy in this skill allows the veterinarian to advise the stud manager regarding key reproductive strategies:

• When the mare will need re-examination

• When to mate the mare

• When the mare may need mating again

• When the mare has ovulated (precluding mating)

Prediction of ovulation probably is the most difficult of all skills in this field, and no honest practitioner will deny that mistakes are made. An experienced stud manager will accept this uncertainty—a tradeoff between being overcautious and mating mares too early, so that a significant proportion need mating more than once per estrous cycle, and being overconfident, so that some mares have ovulated before being mated.

As a general rule, because enough sperm normally live for at least 3 days or even longer after natural mating,^1,² it is not necessary to mate mares more than once every 3 or possibly 4 days. In normal healthy mares, those not susceptible to mating-induced endometritis (MIE), mating up to 12 hours after ovulation may achieve good pregnancy rates.³ By contrast, it is contraindicated to mate susceptible mares more than once per normal-length estrus, especially close to ovulation.

Obviously, the more frequent the examination, the more accurate the prediction of ovulation should be. The use of ovulation-advancing drugs has simplified this procedure. In typical reports from the literature, when the dominant follicle reaches 35 mm in diameter, human chorionic gonadotropin (hCG) or gonadotropin-releasing hormone (GnRH) analogue is given, and ovulation is guaranteed to follow in 36 to 48 hours. What is never reported is the occurrence of ovulation in mares within 24 hours (because it was going to occur anyway), or recent ovulation from follicles smaller than 35 mm—preceding the administration of “induction agents.” Most reports suggest a response rate of less than 90% to hCG, and this can only be the result of administration too early. If all mares were treated in the 48 hours before ovulation, then the response would be 100%! Thus, even with the use of these drugs, total accuracy is not possible.

OVARIAN PALPATION

With the increasing use of ultrasonography, the art of ovarian palpation is becoming neglected. To be an effective diagnostician using this modality, the practitioner will need to perform many more examinations by palpation than are required with the use of ultrasonography. The value of palpation, however, should not be underestimated. Assessment of the texture of follicles, important in the prediction of ovulation, cannot be performed with ultrasonography. Fresh ovulations that are barely visible, and would certainly have been missed with ultrasound examination, may be detected or at least suspected on palpation. Very small anestrous or hypoplastic ovaries, which may not be found with ultrasonography alone, can be detected by palpation. Sometimes a large “follicle” is visualized with ultrasound that was not palpated in the ovary. Returning to palpation confirms the absence of any large follicle but may reveal the presence of a large soft structure adjacent to the ovary, such as a para-ovarian cyst.

The position of the whole uterus will have a major influence on the location of the ovaries. Uterine position may be quite variable. In young mares, for example, the uterus may lie predominantly within the pelvic cavity; during pregnancy and post partum, it may tip forward and downward, with the cervix lying on the pelvic brim. Although the ovaries of the mare are situated just beyond the tip of the uterine horns, their position is extremely variable. Each ovary lies on the free edge of the broad ligament and is loosely attached to the body wall by the mesovarium and to the uterine horn by the ovarian ligament. The laxity of these two attachments allows the ovary to rotate or to lie under the broad ligament. An experienced practitioner, given the cooperation of the mare, can manually rotate the ovary both along and around its axis by at least 180 degrees. In order to palpate all surfaces of the ovary with the most sensitive parts of the fingers, it is necessary to learn to manipulate the ovary.

On occasion, both the ovary and part of the kidney may be visualized together on ultrasonography. In other instances, the ovary may be situated on the midline, anterior to the uterine bifurcation. Not infrequently an ovary can be found only by running the examiner’s fingers along the anterior edge of the uterine horn to the tip; then, by pulling the tip backward, the ovary can be dragged into a more posterior position, under some tension between the tip of the horn and the body wall. In still other instances, it may be found lateral but posterior to the tip of the horn.

Some left-handed practitioners find the left ovary difficult to palpate, whereas others find the right difficult, and vice versa. No apparent advantage is associated with the use of either hand. Whichever feels more comfortable generally is the best. Some authorities argue for a bilateral ability, but then each hand will acquire only half the experience, and changing hands is both awkward and time-consuming. To be ambidextrous, however, is advantageous when the examination area is suitable for only the left-handed (or right-handed) or when a hand becomes injured. On balance, it may be better to give one hand all of the available experience.

The ovarian examination should begin with a total evacuation of the rectum, which may necessitate several entries. The presence of even small pieces of feces, when encountered by the hand and fingers, makes it difficult for the brain to “focus” on the structures being palpated. For the less experienced, it may be best to first identify the uterine bifurcation and then extend the examination up to the tip of the horn and “look around” from this point in every direction. The ovary normally is found anterolaterally, but not always.

The examiner may not know what size structure to feel for. The deep anestrous ovary of a young mare may be less than 4 cm long, whereas at times the ovary with a number of large follicles can be greater than 10 cm. If the ovary cannot be found, then maneuvers to pull it into a more posterior position, as already described, may be necessary. Even if palpation of the ovary is not the object of the examination, ascertaining its position manually may save time looking for it with ultrasonography. When the ovary lies underneath the broad ligament, the examiner must “flip” it back up with the middle finger in order to palpate it effectively. Occasionally, gaseous intestines seem to prevent access to the ovary, and on rare occasions, even the experienced practitioner cannot find an ovary. Absence of an ovary usually is the result of a previous ovariectomy, but one case of agenesis has been reported.⁴

ESTIMATION OF FOLLICULAR MATURITY AND PREDICTION OF OVULATION

Cyclic mares may be presented for examination either on arrival at the stud farm, at the end of diestrus (for pregnancy diagnosis), at a predetermined time after luteal regression (e.g., day 16), at the first signs of estrus, or already several days into estrus. The veterinarian is expected to be able to estimate the stage of the cycle and, by examining the dominant follicle, predict when ovulation may occur.

Palpation

Before the use of ultrasonography, palpation of the follicle was the most reliable method of predicting ovulation. Not only can its diameter be estimated with some accuracy, but also its turgidity (hardness/softening) and its prominence within the ovary can be assessed. The value of diameter already has been discussed; however, some practitioners agree to a phenomenon whereby the diameter can be assessed with more accuracy nearer to ovulation. When followed serially with palpation and ultrasonography, the follicle appears to grow more rapidly with palpation. A large follicle apparent on ultrasonography but located deep within the stroma may palpate as smaller and much less prominent. Ultrasonography may suggest ovulation within 2 days, yet palpation suggests 3 to 4 days and frequently is more accurate. This may be explained by an increase in the area of palpable follicle surface. Although the follicle eventually ovulates from the ovulation fossa at the hilus of the ovary, the follicle bulges from surfaces away from the fossa and communicates with the fossa only immediately before and during ovulation. The experienced practitioner may well prefer to rely on palpation than on ultrasonography.

Occasionally but not invariably, the follicle appears to be painful when palpated. This symptom when present is a good predictor of imminent ovulation. It must not, however, be confused with the tenderness of a fresh ovulation. When this area is particularly tender, maneuvers to distinguish between the two may be unrewarding, difficult, and even dangerous.

Turgidity

A great deal of emphasis is placed on the turgidity of the follicle. Smaller follicles are relatively hard and exhibit less preovulatory softening than that found with larger follicles. With examination every 48 hours, no detectable softening of smaller follicles may be found. Larger follicles appear to be more turgid than hard and may not exhibit any softening. Distinct and reliable softening normally only occurs in the immediate preovulatory period, which may be as much as 24 hours before ovulation but more frequently is only a few hours before ovulation.

Larger follicles may undergo some degree of softening several days before ovulation, but such changes also may be detected in those follicles that are undergoing regression. This distinction presents a challenge during the transitional phase; therefore, other features such as development and regression of endometrial edema and the growth pattern of the follicle should be taken into account in deciding whether to mate a mare found to have follicular softening.

Ginther (1986) concluded that softening was not a reliable feature.⁵ Unless mares are examined at least every 12 hours, this phase may well be missed. It is therefore inadvisable to wait for softening before mating.