Changes in a mare’s ovaries can lead to a variety of physical and behavioral issues. While some changes are a normal part of the reproductive cycle, abnormal growths like cysts can cause problems ranging from poor performance to significant behavioral shifts.
What Are Ovarian Cysts?
Ovarian cysts are fluid-filled sacs that can develop on or within a mare’s ovary. While many cysts are benign and may resolve on their own without affecting fertility, larger or persistent cysts can cause discomfort and hormonal disruptions. These are distinct from normal follicles that develop and release an egg during the mare’s cycle. Sometimes, a follicle fails to ovulate and becomes a large, persistent structure known as an anovulatory follicle, which can be mistaken for a cyst.
Clinical Signs and Behavioral Changes
The signs of a problematic ovarian cyst can be subtle or dramatic. Owners may notice:
- Behavioral Issues: Irritability, grumpiness, and unpredictable mood swings are common. This can manifest as kicking, biting, or general unwillingness to be handled or ridden.
- Performance Problems: A mare might show reluctance to move forward, bucking, rearing, or sensitivity over her flanks and loins when being tacked up or ridden.
- Irregular Cycles: Signs can include the mare being in season constantly (nymphomania), having irregular cycles, or not showing signs of being in season at all.
- Pain: In some cases, large cysts can stretch the ovarian ligaments, causing signs of low-grade or recurring colic.
- Seasonal Worsening: These behaviors may become more pronounced during the transitional periods of spring and autumn.
Ovarian Cyst Diagnosis
A thorough veterinary examination is the first step to diagnosing an ovarian issue.
- Rectal Palpation and Ultrasound: The primary method for diagnosis is a transrectal ultrasound. This allows the veterinarian to visualize the ovaries, identify any abnormal structures, and measure their size. Cysts can appear as single, large fluid-filled structures or multiple smaller cysts.
- Differentiating from Tumors: It is crucial to distinguish benign cysts from more serious conditions like a Granulosa Cell Tumor (GCT), the most common type of ovarian tumor in horses. GCTs are hormonally active and can cause more extreme behavioral changes, such as aggressive, stallion-like behavior (e.g., mounting other mares, developing a cresty neck). While ultrasound can provide clues—a GCT often has a classic “honeycomb” appearance—the findings can be variable.
- Blood Tests: If a tumor is suspected, a blood test can be performed to measure hormone levels. The Anti-Müllerian Hormone (AMH) test is considered the most sensitive marker for diagnosing a GCT. Levels of other hormones like testosterone and inhibin may also be checked.
Treatment Options for Ovarian Cysts
Treatment depends on the severity of the signs and the definitive diagnosis.
- Hormonal Therapy: For mares with behavioral issues related to their cycle, a synthetic progestin called Altrenogest (marketed as Regu-Mate or Altren) can be prescribed. This medication is given orally and works to suppress the estrous cycle, which can alleviate the problematic behaviors. It is often used as a first-line treatment to see if suppressing the cycle resolves the issue.
- Surgical Removal (Ovariectomy): In severe cases, or if a tumor is confirmed, surgical removal of the affected ovary (an ovariectomy) is the treatment of choice. This procedure is often performed laparoscopically (“keyhole surgery”) on the standing, sedated mare, which is less invasive and allows for a quicker recovery compared to traditional open surgery. If one ovary is removed, the mare can often return to normal cycles with the remaining ovary and can still be used for breeding.
If you notice any significant behavioral changes or signs of discomfort in your mare, a veterinary consultation is recommended to determine the underlying cause and the most appropriate course of action.