Ovarian Cancer (Mares) in Horses

Cancer of the ovaries in your mare may present with symptoms or may not. Aggression and changes in the estrus cycle are two signs that your mare may be affected by this condition.  Ovarian cancer may mimic other conditions of the reproductive organs in mares; therefore, it will be important to identify the cause of your horse’s symptoms. Ovarian cancer only accounts for 2.5% of all cancers diagnosed in mares, so while somewhat common, there is a relatively low risk your mare will be diagnosed. Surgery is the typical treatment; early diagnosis is best as is the case with any type of cancer.

Ovarian cancer in your mare is due to one of any number of tumors of the ovaries. These are somewhat common amongst mares compared to other domesticated animals.

• Change in cycles – Extended periods of heat or estrus
• Biting – Your mare may begin to bite and act aggressively towards humans 
• Kicking – Mares may kick when being led or performing and where they were not aggressive previously, may become so 
• Aggression – A typically easy tempered mare may begin to present with aggression towards humans or other horses 
• Discomfort – May present when your mare is moving or working due to their ligaments on the affected ovary being pulled
• Irritability – They may no longer be tolerable of typical irritants and become easily frustrated 
• High levels of testosterone – Upon blood work being drawn, it may be found that your mare has high levels of testosteron
• Colic-like symptoms may be seen 

Types

Granulosa-theca Cell Tumor

•  Slow growing and benign
• Impacts one ovary only
• Most common in mares from ages of 5-9 years
• Does not move to other areas of the body
• The affected ovary is usually enlarged 

Cystadenoma

• Only impacts one ovary versus both ovaries
• Multiple cyst type structures can be found in the ovary impacted
• These cysts do not spread beyond the ovary it is found in 

Dysgerminoma

• Very rare tumors
• Can move from one ovary to the abdomen and chest cavities
• Produces colic type symptoms

Teratoma

• These are tumors which have other material in them typically found in other areas of the body
• These tumors can contain bone, cartilage, teeth, and hair possibly
• Do not typically impact reproductive performance

There are no known causes of ovarian cancer in mares. The incident rate appears to be the same across the board for all breeds. The only possible indicator is age when it comes to the most common type of ovarian cancer which is granulosa-theca.

If you suspect that your mare is suffering from ovarian cancer, it will be important to seek out medical care as soon as possible. Your veterinarian will want to know the types of changes you have noticed in your mare. They will also want to know when you first noticed the symptoms and whether they have progressed over time or not.

Your veterinarian will want to perform a physical exam including a rectal palpation which may or may not provide an indication of a problem. An ultrasound may be requested as well, this will be used to identify the tumorous area. This area in granulosa-theca tumors appears “honeycombed.” In other cases, the tumor may look like a large cyst or solid mass. Often, the affected ovary will be enlarged.

Blood work may be requested to look for inhibin levels and testosterone levels as well, both of which most likely will be elevated. Additionally, the progesterone level may be low.

Treatment options are typically surgical in nature and involve the removal of the cancerous ovary. There are many ways to perform this procedure, however the standard procedure is a celiotomy while the mare is under general anesthesia. A paramedian approach can be used for larger tumors (the size of grapefruits or larger.) Laparoscopic techniques can be used as well. The only risk for surgery are the typical ones for any animal put under anesthesia. Laparoscopic surgical procedures reduce the dangers when the mare is standing during the procedure.

Follow up will be needed as directed by your veterinarian and will be most likely necessary for surgical checkups. Once the tumor is removed, the odds are rare that she will develop more tumors in the other ovary or in the same ovary again. It may take 4 to 8 months before your mare’s follicular activity returns to normal in the remaining ovary. You can expect your mare to return to her normal cycle by the following spring/summer following the removal of the tumor/ovary.