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


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