Ovarian Tumors in Horses: Granulosa Cell Tumors and Other Masses

Quick Answer
  • Granulosa-theca cell tumors are the most common ovarian tumors in mares and often affect only one ovary.
  • Common clues include stallion-like behavior, persistent heat-like behavior, aggression, infertility, or failure to cycle normally.
  • Diagnosis usually involves rectal palpation, transrectal ultrasound, and hormone testing such as anti-Müllerian hormone (AMH), often with inhibin and testosterone.
  • Many mares do well after surgical removal of the affected ovary, and behavior and fertility may improve over the following months if the other ovary is healthy.
  • See your vet promptly if your mare has sudden abdominal pain, a rapidly enlarging abdomen, marked behavior change, or breeding problems that are not resolving.
Estimated cost: $400–$900

What Is Ovarian Tumors in Horses?

Ovarian tumors in horses are abnormal growths that develop in one or both ovaries. In mares, the most common ovarian tumor is the granulosa-theca cell tumor (GTCT), often also called a granulosa cell tumor. These tumors arise from the hormone-producing cells of the ovary, so they can change both reproductive cycling and behavior. Many are unilateral, meaning one ovary becomes enlarged while the other becomes small and inactive.

Because these tumors can produce hormones, the signs are not always a visible mass. Some mares act like they are in heat for long periods, while others stop cycling. Some show stallion-like behavior such as aggression, mounting, flehmen, or urine marking. Less common ovarian masses in horses include teratomas, cystadenomas, adenocarcinomas, hematomas, and non-neoplastic enlargements that can mimic a tumor on exam.

Not every enlarged ovary is cancerous, and not every ovarian mass spreads. In fact, many granulosa-theca cell tumors are slow-growing and are managed successfully with surgery. The key is getting a clear diagnosis from your vet so the plan fits your mare's age, use, breeding goals, comfort, and budget.

Symptoms of Ovarian Tumors in Horses

  • Stallion-like behavior such as aggression, mounting, flehmen, or urine marking
  • Persistent heat-like behavior, frequent urination, tail raising, or vulvar 'winking'
  • Irregular cycles, failure to come into heat, or apparent infertility
  • One enlarged ovary and one small inactive ovary found on reproductive exam
  • Poor performance or discomfort related to behavior changes during training or breeding
  • Abdominal enlargement or a palpable mass in larger tumors
  • Colic signs or internal bleeding from a large ovarian mass, which is uncommon but urgent

Many mares with ovarian tumors are first noticed because of behavior or breeding changes, not because they look sick. A mare may become unusually aggressive, act persistently receptive to other horses, or stop cycling altogether. These signs can overlap with other reproductive problems, so they are a reason to schedule an exam rather than assume the cause.

See your vet immediately if your mare has colic, weakness, pale gums, collapse, or a suddenly enlarged abdomen. Those signs are not typical for every ovarian tumor, but they can happen with large masses, bleeding, or other serious abdominal disease.

What Causes Ovarian Tumors in Horses?

The exact cause of most ovarian tumors in mares is not well defined. Granulosa-theca cell tumors develop from the sex cord-stromal tissues of the ovary, which are the cells involved in follicle development and hormone production. They are considered the most common ovarian neoplasm in mares, but that does not mean every mare is at high risk.

These tumors are usually sporadic, meaning they are not linked to anything a pet parent did or did not do. There is no well-proven feed, supplement, housing, or routine management factor known to prevent them. Some masses found on the ovary are not true tumors at all. Hematomas, anovulatory follicles, abscesses, and pregnancy-related ovarian changes can sometimes mimic a tumor until your vet completes imaging and hormone testing.

Because hormone secretion varies from tumor to tumor, one mare may show stallion-like behavior from testosterone production, while another may show prolonged estrus or infertility. That is why the cause of the signs is really the combination of the mass itself and the hormones it may release.

How Is Ovarian Tumors in Horses Diagnosed?

Diagnosis usually starts with a detailed history and reproductive exam. Your vet may ask about cycle changes, breeding history, aggression, mounting, poor performance, or heat-like behavior. A rectal palpation and transrectal ultrasound are often the first major steps. Granulosa-theca cell tumors often appear as an enlarged ovary with a multicystic or honeycomb-like pattern, while the opposite ovary may be small and inactive.

Hormone testing helps confirm the picture. Anti-Müllerian hormone (AMH) is widely used because it is often elevated in mares with granulosa cell tumors. Your vet may also recommend inhibin and testosterone, especially when behavior changes are prominent or the ultrasound findings are not classic. Basic bloodwork may be added before surgery or to check for other causes of illness.

In some cases, referral imaging or repeat exams are needed because not every ovarian enlargement is a tumor. Definitive diagnosis is often made after ovariectomy when the removed tissue is examined by a pathologist. That step matters because treatment and long-term expectations can differ between a granulosa-theca cell tumor, a hematoma, a teratoma, or a more aggressive ovarian cancer.

Treatment Options for Ovarian Tumors in Horses

Spectrum of Care means you have options. Here are treatment tiers at different price points.

Budget-Conscious Care

$400–$1,200
Best for: Mares that are stable, not in pain, and need confirmation of the diagnosis before committing to surgery. Also reasonable when breeding is not an immediate goal or finances require a stepwise plan.
  • Farm call or clinic reproductive exam
  • Rectal palpation and transrectal ultrasound
  • Baseline bloodwork as needed
  • Targeted hormone testing, often AMH with or without testosterone/inhibin
  • Short-term monitoring with repeat exam if the diagnosis is uncertain or surgery must be delayed
Expected outcome: Fair for clarifying the problem and planning next steps, but the mass itself usually does not resolve if it is a true granulosa-theca cell tumor.
Consider: This approach may control spending up front, but it does not remove the tumor. Behavior, infertility, or enlargement may continue. Delaying surgery can make management harder if the ovary becomes very large.
Most Common

Recommended Standard Treatment

$2,000–$4,500
Best for: Most mares with a confirmed unilateral ovarian tumor that are otherwise stable and good candidates for standing surgery.
  • Referral reproductive or surgical consultation
  • Pre-op bloodwork and ultrasound confirmation
  • Unilateral ovariectomy, often by standing laparoscopy for suitable cases
  • Pain control, hospitalization, and pathology on removed tissue
  • Follow-up exam and guidance on return to work or breeding
Expected outcome: Good to excellent in many cases after removal of a unilateral granulosa-theca cell tumor. Behavior often improves over weeks to months, and some mares return to breeding if the remaining ovary is healthy.
Consider: Requires referral-level care and sedation or surgery. Recovery time, transport, and hospital costs still matter. Very large tumors may not be removable through a smaller standing approach.

Advanced / Critical Care

$4,500–$6,500
Best for: Large tumors, complicated cases, mares with acute pain or bleeding, pregnant mares needing specialty planning, or pet parents who want every available diagnostic and surgical option.
  • Advanced referral hospital workup
  • General anesthesia or laparoscopic-assisted/open abdominal surgery for very large masses or difficult anatomy
  • Management of complications such as hemorrhage, colic, or pregnancy-related surgical planning
  • Extended hospitalization, intensive monitoring, and full histopathology
  • Additional reproductive planning for valuable broodmares or complex bilateral disease
Expected outcome: Variable but often still favorable when the mass can be removed safely. Outcome depends on tumor type, size, surgical complexity, and whether there are complications.
Consider: Higher cost range, more intensive hospitalization, and greater anesthesia-related risk. This level is not necessary for every mare, but it can be the safest path in complex cases.

Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.

Questions to Ask Your Vet About Ovarian Tumors in Horses

Bring these questions to your vet appointment to get the most out of your visit.

  1. Does my mare's exam look most consistent with a granulosa-theca cell tumor or another type of ovarian mass?
  2. Which hormone tests do you recommend in her case, and how would AMH, inhibin, or testosterone change the plan?
  3. Is the other ovary active and likely to return to normal after treatment?
  4. Would you recommend monitoring first, standing laparoscopic ovariectomy, or referral for more advanced surgery?
  5. What cost range should I expect for diagnosis, surgery, hospitalization, and pathology in my area?
  6. How long should I expect before behavior improves after the affected ovary is removed?
  7. If I want to breed her later, what is the outlook for fertility with the remaining ovary?
  8. What warning signs at home would mean she needs urgent recheck before the planned surgery?

How to Prevent Ovarian Tumors in Horses

There is no proven way to prevent most ovarian tumors in mares. These masses are not known to be caused by routine riding, breeding, turnout, or standard feeds. Because the cause is usually not something controllable, prevention focuses more on early detection than on avoiding a specific trigger.

Regular reproductive exams are especially helpful for broodmares, mares with unexplained infertility, and mares that develop sudden behavior changes. If your mare starts acting persistently in heat, becomes unusually aggressive, or stops cycling normally, an early workup can identify an ovarian problem before the mass becomes very large.

Keeping good records can help your vet, too. Track heat behavior, breeding dates, teasing responses, performance changes, and any episodes of colic or abdominal enlargement. That information can make diagnosis faster and can help your vet decide whether monitoring, surgery, or referral is the best fit for your mare.

Find a Vet Near You