Use of Ultrasonography for Evaluation of Mare Reproductive Abnormalities

Use of Ultrasonography for Evaluation of Mare Reproductive Abnormalities

Ryan A. Ferris1 and and Kristina G. Lu2

1 Summit Equine, Gervais, OR, USA
2 Hagyard Equine Medical Institute, Lexington, KY, USA


Ultrasound examination is a commonly utilized diagnostic tool used by veterinarians to evaluate the mare’s reproductive tract, while significant advances in ultrasound technology now provide better-quality images and machines with battery power. The crux of the examination is the person performing the examination, no matter the capabilities of the ultrasound machine. In the author’s experience, the ability to palpate the reproductive tract with good accuracy allows for an accurate and detailed ultrasound examination. The ultrasound machine will not replace good palpation skills.

While performing the examination, the probe is held in the hand below the middle finger and palm. This allows your other fingers to hold the ovary or trace the uterus as you scan the mare. A clinical tip in performing ultrasound examinations is to hold the probe in a 45–60-degree downward angle almost imaging the uterus from the front to the back, as opposed to holding the probe in a horizontal fashion. By holding the probe in this downward fashion, it is in the author’s opinion that this enhances imaging of the uterine bifurcation, resulting in a more thorough ultrasound examination.

For consistency, every ultrasound examination is performed in the same manner. This can be adjusted but should include the following steps. Start the examination with the left ovary imaging from pole to pole (off the ovary all the way through the ovary and off the ovary) followed by measurement of any significant follicles (>30 mm). Scan down the left uterine horn up the right uterine horn to the right ovary. When imaging the uterine horns, the probe should be held perpendicular to the uterus keeping it circular in appearance. The right ovary is imaged similar to the left. Come back down the right uterine horn to the uterine body. The uterine body is imaged in longitudinal fashion by sweeping the hand/probe to the left and right as you move caudally. Continue the examination through the cervix and cranial vaginal vault. Careful assessment of the cervix and vaginal vault via ultrasound can detect abnormalities such as cervical tears and fluid (urine) in the vaginal vault.

Many ultrasound machines used in the equine reproductive field have a 5 MHz probe. However, with advancements in technology, many of the machines available today have 7–15 MHz probes that allow for greater detail of ovarian and uterine structures. Commonly, the authors of this chapter utilize ultrasound machines with a 7.5–10 MHz range. A common depth for routine scanning is 6–8 cm, depending on the length of the rectal probe. If the probe is relatively short, the depth may have to be increased (8 cm) to completely fit a dominant (40–45+ mm) follicle fully on the image. Setting up your ultrasound machine is key for consistent examinations and the image quality can be adjusted to fit your needs by altering gain, dynamic range, and other settings on your machine. Once you have identified the settings you prefer, these can often be saved as presets for future examinations.

Imaging the equine reproductive tract is unique in that it can vary greatly depending on the stage of the cycle, time of year (day length), age, parity, nutrition, and pathology. In many of these circumstances, one examination may not be enough to make a diagnosis and serial examinations may be required. Additionally, other diagnostics may be required to confirm the suspected pathology seen on ultrasound.

The goal of this chapter is to provide guidance in diagnosing common reproductive abnormalities. Comments will be brief regarding pathophysiology and treatments.


Anovulatory Follicles

Approximately 5–10% of follicles in a cycle will end with an anovulatory follicle. There are no signs to predict this event from occurring with normal follicular growth rates and normal rise of uterine edema. Mares that develop an anovulatory follicle during the season are at a higher risk of developing an anovulatory follicle in subsequent cycles. It is suspected that mares with metabolic disease (pars pituitary intermedia dysfunction and/or insulin resistance) are at an increased risk for developing anovulatory follicles [1]. Appropriate screening for metabolic disease should be considered in mares with anovulatory follicles (Figure 15.1). It should be noted that anovulatory follicles are relatively common during the spring and fall transition periods, and are considered normal events during these times of year.

Figure 15.1 Images from mare presenting for failure to develop a dominant follicle and ovulate. Mare was diagnosed with insulin resistance and treated with metformin. Normal follicular development, ovulation, and pregnancy was established after treatment.

These structures will often luteinize and eventually secrete progesterone. Thereafter, the majority will respond to prostaglandins but the time period from formation to response to prostaglandins is prolonged from a normal ovulation and will be 7–10 days from noting the debris in the follicular lumen, as noted in Figures 15.2A and 15.3. Following administration of prostaglandins, these structures will shrink in size and no longer be hormonally active. However, the remnants of these structures can be seen with ultrasound for several weeks to months following development of an anovulatory follicle.

Figure 15.2 Development of an anovulatory follicle that failed to respond to an ovulation induction agent. The top figure is often first observed with an excessively thickened follicular wall and echogenic material free floating in the follicle. The hemorrhage in the follicle continues until a hematoma and eventual luteinization of the structure occurs. (A) 48 hours after GnRH agonist administration. Note flecks and debris in follicular fluid. (B) 72 hours after GnRH agonist administration. Note free blood in the follicular lumen. (C) 120 hours after GnRH agonist administration. The structure had fibrin strands and appears similar to a blood clot. (D) 144 hours after GnRH agonist administration. The structure has stabilized and formed a large mass on the ovary. The mare’s progesterone value was 2.3 ng/mL.

Figure 15.3 Anovulatory follicle 4 days apart. Hyperechoic material within the follicle is an indicator of the follicle being anovulatory.

Unfortunately, often by the time it is recognized that an anovulatory follicle has developed, the expense of breeding (live cover or artificial insemination) has already occurred. As the mare never ovulates, the oocyte is not released and is unable to be fertilized.

Granulosa Cell Tumor

A granulosa cell tumor (GCT) is the most common tumor of the ovary. The classical presentation is that of an enlarged ovary with the contralateral ovary being small and inactive (Figure 15.4). The contralateral ovary is inactive due to inhibin production by the GCT down regulating FSH release. Mare behavior is ambiguous with anestrus, nymphomania, or stallion-like behavior noted. During early development of a GCT, an abnormal area on the ovary may be noted but the mare may continue to cycle on the contralateral ovary and can become pregnant [2]

Figure 15.4 Image of a mare with a granulosa cell tumor. (A) Left ovary that is small and inactive due to production of inhibin by the tumor. (B) The ovary with the granulosa cell tumor where little normal ovarian tissue could be recognized.

If presented with a mare with abnormal behavior and an abnormal area on the ovary, additional diagnostics to confirm or rule out a GCT are warranted. Currently, it is recommended that blood be submitted for inhibin, testosterone, progesterone, and Anti-Müllerian Hormone (AMH) to confirm suspicion of a GCT. Both authors have noted abnormal structures on the ovary that have taken 2–3 years to mature to the point of being able to be confirmed on a blood panel for a GCT. Careful evaluation of the ovary should be performed in these early detected tumors in which the contralateral ovary is still active to ensure that the abnormal ovary is removed. Surgical removal of the ovary will result in a drop in inhibin and resumption of cyclicality in 4–12 months. The broad range on return to cyclicity depends on when the ovary was removed in reference to the physiological breeding season.

Parovarian Cysts

Parovarian (or periovarian) cysts are remnants of embryonic development and cystic accessory structures. These structures do not appear to interfere with oocyte capture by the infundibulum or oocyte/early embryo migration within the oviduct. The greatest consequence of these structures is that they can appear to be within the ovarian parenchyma with cursory ultrasound examination. Occasionally these structures can be similar in appearance to a dominant follicle (Figure 15.5). This may result in mismanagement of the mare due to the inadvertent diagnosis of a large dominant follicle, not a parovarian cyst.

Figure 15.5 Image of ovary with adjacent parovarian cyst with ovary circled in blue, cyst highlighted by red arrow, and areas of tissue delineating ovary and cyst in orange.

These structures are often easily identified with manual palpation of the ovary and oviduct region. If the tip of the uterine hold is held in the hand as you move toward the ovary, you will often feel the parovarian cyst followed by the ovary. A suggestion is to carefully palpate the ovaries and uterus of every mare at least for the first examination of the season to note any parovarian cysts that could cause diagnostic issues in the future.

Epithelial Inclusion Cysts

Epithelial inclusion cysts arise from the surface epithelium that lines the ovulation fossa and is taken up into the ovarian cortex during ovulation (Figure 15.6). These cystic structures are common in older mares. The effects on fertility are unknown but it is speculated that a large cyst or numerous cysts can obstruct the ovulation fossa, preventing release of the oocyte during ovulation.

Figure 15.6 Image of epithelial inclusion cysts, indicated by blue arrow.

Ovarian Hematoma

Ovarian hematomas can be similar in appearance to an anovulatory follicle (Figure 15.7

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Nov 6, 2022 | Posted by in EQUINE MEDICINE | Comments Off on Use of Ultrasonography for Evaluation of Mare Reproductive Abnormalities
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