Introduction

To carry out ultrasound examinations safely, mares should be suitably restrained. Ideally, one should have a set of stocks approximately 75 cm (30 inches) wide and just longer than an average mare. This is adequate for most animals and will even accommodate large draft mares. In a few cases, a twitch may be required to provide additional restraint. Foals should be restrained in front of, or to the side of the mare. Tying the tail to one side keeps it out of the way, and prevents hairs entering the rectum.

Precautions necessary for transrectal examinations also apply to ultrasound examinations and transrectal examination should always precede ultrasound examination. An initial rectal and transrectal examination ensures removal of all fecal material, facilitates rapid location of the tract during scanning and provides information on texture of structures.

The scanner should be as close to eye level as practicable and the control panel of the machine within easy reach of the operator. The scanner can be placed on either side of the mare. Where the operator’s left hand holds the transducer, the scanner is placed obliquely to the right side of the mare’s hindquarters allowing the right hand to make any notes or adjustments to the controls. To facilitate correct orientation of the transducer, a groove for the finger of the operator is usually located on the transducer, on the opposite side to the transducer face. The fingers should always be in front of the transducer as it is being introduced and later manipulated rather than pushing the transducer on ahead. For reasons of hygiene, it may be desirable to have the transducer in a plastic sleeve. Coupling gel should be used to exclude air from between the transducer and its protective cover. Using copious amounts of lubricant, which also acts as a coupling medium to ensure good contact and prevent air interference, the transducer and hand are gently inserted into the rectum. Should the mare strain, the examination should be stopped and one should wait for the rectum to relax. However, straining is usually not a significant problem.

It is best to examine the reproductive tract systematically and to scan the entire uterus and both ovaries at least twice. The transducer is usually held within the rectum in the longitudinal plane. Since the uterus of the mare is T-shaped, the uterine body appears as a rectangular image in the longitudinal plane. When scanning the uterine body, it is important to move the transducer forwards and backwards and from side to side so that no feature is missed. It is important to move the transducer slowly at all times. To image the uterine horns and ovaries the transducer should be rotated slowly to the right and then the left side. Therefore, the uterine horns appear as circular images in cross-section. If difficulties are encountered with finding a structure, the transducer can be withdrawn a short distance and the structure located by palpation. Ultrasound examination can then be resumed.

Both the ovaries and the uterus need to be examined thoroughly at every examination of an abnormal mare.

Ovarian features to note are:

• ovarian size, shape, and ultrasound appearance;
  
• follicle size, softness, and shape;
  
• echogenicity and thickness of the granulosa layer;
  
• presence of small echogenic particles within the follicular fluid.

Abnormal Follicles

In the 48 hours, but more obviously in the 24-hour period before ovulation, the wall of the follicle (granulosa layer) becomes increasingly echogenic [1]. As ovulation approaches, the follicle wall often becomes intensely hyperechoic and irregular in outline. Small echogenic particles may appear in the follicular fluid close to ovulation. These particles are thought to be the result of preovulatory hemorrhage (Figure 15.1). If the particles continue to increase in density and become widespread these follicles rarely ovulate and are termed hemorrhagic anovulatory follicles (HAFs).

Ultrasonographic collapse of the follicle may be rapid, within several seconds, or more prolonged over several minutes with eventual complete or almost complete evacuation of the follicular antrum. The length of time does not appear to impact fertility. Ovulation failure occurs in almost 10% of estrous cycles according to a study by McCue and Squires (2002) [2]. These authors report that the majority of anovulatory follicles luteinize (85.7%) but some remain as persistent follicular structures (14.3%). More recently it has been suggested that the incidence is between 5% and 20% of estrous cycles [3]. All types of anovulatory follicles are infertile since follicular collapse and oocyte release (ovulation) has not occurred. It is difficult to predict if a dominant follicle will fail to ovulate [4]. The best indicator is the “snow storm” appearance of echogenic particles in the follicular fluid and an increase in follicular size. The follicles continue to grow and may occasionally reach diameters of 125 mm. The particles are likely to be the result of hemorrhage into the follicle. In some situations, hemorrhage into the follicular antrum is minimal and the “snow storm” appearance disperses and the follicle returns to normal appearance (Figures 15.2, 15.3).

The hemorrhage in anovulatory follicles may organize to form a cobweb-like network of narrow hyperechoic fibrin strands (Figures 15.4, 15.5). Alternatively a more solid structure may form from organization of the fibrin (Figures 15.6, 15.7). These structures can be confused with a granulosa theca cell tumor.

Luteinized anovulatory follicles invariably have some echogenic material present, allowing differentiation from non-luteinized anovulatory follicles. Differentiation can be confirmed by measurement of blood progesterone values, as luteinized follicles will be associated with elevated progesterone values, unlike non-luteinized follicles. These luteinized anovulatory follicles usually respond to an injection of prostaglandin F_2α (PGF_2α) and as low a dose as possible should be used. The naturally occurring PGF_2α, dinoprost, or the synthetic prostaglandin analog, cloprostenol, are equally effective. The recommended dose for a 500 kg mare is 5 mg of dinoprost or 250 μg of cloprostenol. However, since it is reported that higher doses of PGF_2α represent an increased risk for formation of anovulatory follicles [5], a low dose of dinoprost (1.5 mg) or cloprostenol (50 μg) should be used in mares prone to formation of anovulatory follicles.

Non-luteinized anovulatory follicles are more difficult to treat, as they do not respond to prostaglandin. If the mare ovulates normally from another follicle, treatment may not be necessary, but occasionally treatment is needed either due to the physical size of the structure or its suppression of any other follicular development. Attempts to induce their disappearance with human chorionic gonadotrophin or deslorelin are usually unsuccessful. Sometimes a 12-day course of altrenogest (0.044 mg/kg PO SID) followed by an ovulation induction agent may be successful. Fortunately, they usually spontaneously regress although this can take as long as 4 weeks (Figure 15.8). Rarely, they persist beyond this period and transvaginal puncture may be useful in these rare cases.

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