Enlarged ovaries are frequently observed in mares, especially those used for breeding. This enlargement often stems from an ovarian cyst. In most instances, these cysts are benign and resolve spontaneously, typically not affecting the mare’s fertility.

However, it’s crucial for mare owners and breeding veterinarians to be vigilant and consider other potential causes, such as hematomas or tumors, when encountering a mare with an enlarged ovary.

In the case of non-breeding mares, enlarged ovaries are often detected only when there is a noticeable change in behavior, such as exhibiting stallion-like traits.

A common culprit behind such behavioral alterations is a granulosa theca cell tumor, which is the most prevalent type of ovarian tumor in horses. These tumours often require surgical removal of the ovary to resolve the behavioural issue and prevent future complications.

This article discusses common causes of enlarged ovaries in mares, starting with ovarian tumours.

Enlarged Ovaries in Mares

The ovaries are a pair of reproductive organs in mares located in the abdominal cavity, near the kidneys. The ovaries are integral to both reproductive health and the endocrine system, performing several critical functions, including:

  • Production of Eggs: Ovaries are responsible for producing and releasing eggs (ova). In mares, this occurs in a cyclical pattern, known as the estrous cycle.
  • Hormone Production: Ovaries secrete sex hormones, primarily estrogen and progesterone. These hormones regulate various reproductive functions, including the estrous cycle, pregnancy, and parturition.

Enlarged ovaries in mares refer to a condition where one or both ovaries are abnormally large. This enlargement can be due to several reasons, ranging from normal physiological changes to pathological conditions.

Granulosa Theca Cell Tumours

Granulosa theca cell tumours (GTCTs) are by far the most common ovarian tumour in horses. In fact, GTCTs make up around 2.5% of all reported equine tumours. [1]

Most mares first show symptoms of GTCTs around 10 years of age. Usually only one ovary develops a mass. [1]

These tumours arise from sex-cord stromal tissues that support the ovaries. The tumours can consist of one or both of these cell types: [1]

  • Granulosa cells which are cells that produce the hormones follicle-stimulating hormone (FSH) and estrogen
  • Theca cells which are are cells that produce the hormones luteinizing hormone (LH) and progesterone

Symptoms

The main symptoms of granulosa theca cell tumours reported in mares are: [1][2]

  • Failure to cycle normally
  • Continuous estrus (“in heat”) behaviour
  • Stallion-like behaviour

Some mares exhibiting stallion-like behaviour may show physical changes such as increased muscling, a cresty neck, and an enlarged clitoris. [2]

Behavioral Signs

Recent studies indicate that the primary reason most owners bring their mares to a veterinarian is due to poor performance under saddle or other atypical behaviors. The behaviors reported include: [3]

  • Poor performance, such as unwillingness to engage the hindquarters or poor contact with the bit
  • Bucking or rearing when ridden
  • Increased sensitivity over the flanks
  • Biting other horses
  • Kicking at humans or other horses
  • Aggressive behaviour when approached from behind

Non-Specific Symptoms

Around 10% of mares with GTCTs do not show behavioural or reproductive symptoms. [4] Other reported symptoms associated with GTCTs include: [4]

Signs of GTCT Rupture

In rare cases, the GTCT may rupture, causing massive blood loss into the abdomen. [4] Symptoms may include:

  • Pale gums
  • Rapid breathing
  • Severe colic symptoms
  • Collapse
  • Sudden death
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Diagnosis

Mares with GTCTs are typically examined by their veterinarian due to noticeable behavioral changes. In some cases, owners interested in breeding may notice abnormal cycling in their mare, prompting them to see a veterinarian. [1]

It is important to note that behavioral issues in horses can arise from numerous causes, ranging from diseases to feeding and management to training and riding techniques. [5][6] The veterinarian must perform a thorough investigation of the mare’s overall health and management. [5]

Reproductive Exam

As part of this investigation, the veterinarian will often perform an ultrasound exam of the reproductive tract. GTCTs can be readily identified on ultrasound, appearing as an enlarged ovary resembling a honeycomb or cluster of grapes. [1]

Some small GTCTs may resemble normal ovarian structures, which prevents diagnosis if further testing is not conducted. [6]

Hormone Levels

Based on the ultrasound findings and clinical signs, the veterinarian may investigate further by measuring hormones in the mare’s blood, to confirm the tumour is a GTCT. [1]

Since the granulosa theca cell tumor arises from ovarian tissue, it can secrete large amounts of reproductive hormones, altering the mare’s hormonal balance.

Hormones that your veterinarian may measure include: [1][2]

  • Inhibin: a hormone that prevents normal estrous cycling
  • Testosterone: the main male sex hormone
  • Progesterone: the main female sex hormone during ovulation and pregnancy

The altered hormonal balance is responsible for the unusual behaviours observed in affected mares. Usually, inhibin and testosterone levels are high in affected mares, and progesterone levels are low. [1][2][7]

Around 90% of mares show increased inhibin levels, with around 50-60% of mares having increased testosterone levels. [8] Mares that exhibit stallion-like behaviour are more likely to have elevated testosterone. [1]

Biopsy

Your veterinarian may decide to also examine the health of the uterus by taking a sample of the endometrium – the inner lining of the uterus. [1]

In some cases cells of the endometrium may also be abnormal in structure, appearance or organization. [1]

Treatment

The main treatment available for GTCTs is surgical removal of the affected ovary. [1] There are several surgical approaches to remove the tumour, depending on its size, experience of the surgeon, and equipment available. [1][9]

The primary surgical approaches are: [1][9]

  • Colpotomy, where the ovary is removed through an incision in the vaginal wall
  • Flank incision, where an incision is made in the mare’s flank near the ovary
  • Ventral median celiotomy, where the abdomen is opened along the midline
  • Laparoscopic removal, where a camera and surgical instruments are introduced into the abdomen through small incisions in the abdominal wall

Risks of Surgery

Abdominal surgery carries several potential risks, regardless of the surgical approach used. Major complications include: [9]

  • Hemorrhage
  • Colic during the postoperative period
  • Abdominal infections
  • Scar tissue forming between abdominal organs
  • Damage to nerves resulting in paralysis of an affected area
  • Infection of the incision
  • Failure of the sutures to hold an incision closed
  • Sudden death

Reported rates for complications from GTCT removal vary depending on the type of surgery, but can be as high as 63%. Laparoscopic surgery has a lower complication rate than other methods of surgical removal. [4]

GnRH Vaccines

A recent study investigated the use of vaccines against gonadotropin-releasing hormone, the main hormone that stimulates ovarian activity, as a means of treating the behavioural issues associated with GTCTs. [10]

This study showed that repeated vaccination reduced behavioural issues and slowed the rate of tumour growth in affected mares. [10] These vaccines show promise as a potential management strategy for GTCTs, but require further study.

Prognosis

The majority of mares that undergo surgical removal of their GTCT typically resume normal estrous cycling within a year. [1] Generally, the first ovulation post-surgery occurs within 6-8 months. [8] These mares are often found to have a favorable prognosis for long-term fertility.

Various studies have documented the resolution of behavioral issues following treatment, with success rates ranging from 64 – 86%. [3][5][11]

However, some mares treated for GTCTs may enter a permanent or prolonged state of anestrus, meaning they do not exhibit regular estrous cycles. [1]

Interestingly, these mares can still be responsive to hormonal treatments designed to manipulate estrous cycles. As such, they may still be viable candidates for breeding, despite not cycling naturally. [1]

Other Tumours

Other types of tumors can also develop in the ovaries and cause enlargement, but they are less common compared to GTCTs. The approach to diagnosing and treating these other tumor types is generally similar to that of GTCTs.

In the case of benign tumors, treatment is often not required.

Teratomas

Teratomas arise from pluripotent germ cells, which are primitive cells that can develop into any type of tissue. Tumours formed by these cells contain multiple tissue types, including fat, skin, teeth, muscle, nerves, etc. [1][12]

Teratomas are the second most common ovarian tumour in horses after GTCTs, however they are rare overall. [12] Most of these tumours are benign, meaning they do not metastasize to other locations. [1] Some large ovarian teratomas may cause abdominal discomfort in affected mares. [1][2]

The tumours do not produce any hormones, so affected mares do not usually show the behavioural changes associated with GTCT. [2][12] Additionally, the opposing ovary usually maintains its functionality, so most affected mares cycle normally and are able to conceive. [2][12]

Cystadenomas

Cystadenomas arise from the lining of the follicles, and therefore often resemble hemorrhagic anovulatory follicles on ultrasound examination. [1] These tumours are benign and typically do not affect estrous cycling of the mare. [2]

Some mares may have elevated testosterone levels due to these tumors. [2][8]

Dysgerminomas

Dysgerminomas are very rare tumours that arise from the same pluripotent germ cells as teratomas.[1][2]

Dysgerminomas are aggressive and can metastasize widely throughout the abdomen and thoracic cavity. Mares may show weight loss associated with dysgerminomas. [1][8]

Although dysgerminomas do not interfere with normal estrous cycling, removal of the tumour is highly recommended due to their aggressive nature. [8]

Adenocarcinomas

Adenocarcinomas are very rare aggressive tumors that arise from the surface lining of the ovary. [1] Unlike other tumours that spread through the bloodstream, these tumours tend to spread when the mass ruptures, releasing cancerous cells into the abdomen. [1]

Metastatic Tumours

In rare cases, tumours arising in other areas of the body can metastasize or spread to the ovaries through the bloodstream or lymphatic system. The most common metastatic tumours in the ovary are melanoma and lymphoma. [1][2]

Other Causes

Tumours are not the only potential cause of an enlarged ovary. Ovaries are also prone to developing cysts, or fluid-filled sacs. Careful palpation and ultrasound examination by a veterinarian can usually distinguish between a cyst and a tumour.

Germinal Inclusion Cysts

Germinal inclusion cysts develop when a small piece of connective tissue in the abdomen enters a recently ovulated follicle. A follicle is the structure that contains the oocytes (eggs) prior to ovulation. These cysts are more common in older mares. [1]

When a follicle ovulates, the surface of the follicle ruptures, which can allow fragments of surrounding tissue to enter the structure. [1]

The ruptured follicle eventually heals over, trapping the abdominal tissue fragment within the ovary. Often, the trapped tissue secretes a small amount of fluid, which over time produces a large cyst. [1]

Typically, germinal inclusion cysts do not cause issues for the mare. In rare cases, they may become so large that they put pressure on the ovarian tissue surrounding the cyst, which may prevent normal functioning of the ovary. [1]

Hemorrhagic Anovulatory Follicles

Hemorrhagic anovulatory follicles (HAFs) are follicles that reach a mature size, but never ovulate (rupture) to release an oocyte. [1] They are very common, occurring in about 25% of heat cycles, particularly during the spring and fall transition periods. [1][13] Older mares may be more likely to develop HAFs. [1]

HAFs can interfere with breeding plans, as they cause abnormal estrous behaviour and increase the length of time between ovulations. [8] HAFs usually regress on their own, however it can take several weeks before they start reducing in size. [1][8]

Most veterinarians will ultrasound affected mares repeatedly a few weeks apart to ensure that the HAF is regressing. [1] In some cases, administering human chorionic gonadotropin (hCG) or gonadotropin-releasing hormone can induce ovulation and resolve the issue. [8]

If the HAF does not regress, other diagnoses such as GTCTs and hematomas should be considered. [1]

Hematomas

Hematomas are large, blood-filled sacs that are a common cause of ovary enlargement in mares. [8]

When a follicle ruptures during ovulation, the surrounding tissue produces a small amount of blood which enters the space the follicle left behind. This blood rapidly clots, forming a corpora hemorrhagica.

The mare’s behaviour and hormone levels are usually normal because hematomas typically only affect one ovary. [8]

Hematomas can arise if there is a large amount of bleeding after ovulation. [1] The hematoma forms a large blood clot, which is difficult for the body to remove, and it may take weeks or months before the ovary returns to a normal size. [1][8]

Typically, hematomas do not affect the mare’s fertility or ability to conceive. [1] Very large hematomas may be painful during rectal palpation, so veterinarians must be cautious when performing reproductive work on a mare with a hematoma. [1]

Summary

  • There are many possible causes of an enlarged ovary, ranging from cysts to tumours
  • Some conditions may cause behavioural changes or interfere with the mare’s fertility
  • Granulosa theca cell tumours are the most common ovarian tumour, and must be surgically removed

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References

  1. McKinnon, A. O. et al., Equine Reproduction. Wiley-Blackwell, 2010.
  2. McCue, P. M. et al., Granulosa Cell Tumors of the Equine Ovary. Veterinary Clinics of North America: Equine Practice. 2006. View Summary
  3. Straticò, P. et al., Behavioral Disorders in Mares with Ovarian Disorders, Outcome after Laparoscopic Ovariectomy: A Case Series. Veterinary Sciences. 2023.View Summary
  4. Sherlock, C. E. et al., Granulosa Cell Tumours in the Mare: A Review of 52 Cases. Equine Veterinary Education. 2016.
  5. Nout-Lomas, Y. S. and Beacom, C. L., Granulosa Cell Tumours: Examining the ‘Moody’ Mare. Equine Veterinary Education. 2015.
  6. Renaudin, C. D. et al., Equine Granulosa Cell Tumours among Other Ovarian Conditions: Diagnostic Challenges. Equine Veterinary Journal. 2021.View Summary
  7. Ball, B. A. et al., A Retrospective Analysis of 2,253 Cases Submitted for Endocrine Diagnosis of Possible Granulosa Cell Tumors in Mares. Journal of Equine Veterinary Science. 2014.
  8. McCue, P. M., Review of Ovarian Abnormalities in the Mare. Proceedings of the Annual Convention of the AAEP. 1998.
  9. De Bont, M. P. et al., Standing Laparoscopic Ovariectomy Technique with Intraabdominal Dissection for Removal of Large Pathologic Ovaries in Mares. Veterinary Surgery. 2010. View Summary
  10. Behrendt, D. et al., Active Immunisation against GnRH as Treatment for Unilateral Granulosa Theca Cell Tumour in Mares. Equine Veterinary Journal. 2021. View Summary
  11. Melgaard, D. T. et al., Moody Mares—Is Ovariectomy a Solution?. Animals. 2020.View Summary
  12. Duarte, L. et al., Equine Ovarian Teratomas: Diagnostic Challenges Illustrated by Case Reports. Equine Veterinary Education. 2022.
  13. Ginther, O. J. et al., Incidence, Endocrinology, Vascularity, and Morphology of Hemorrhagic Anovulatory Follicles in Mares. Journal of Equine Veterinary Science. 2007.