Ovarian Tumors

Whether you operate a breeding operation or have a mare as a riding or competition mount, there are certain problems that you might face just because your horse is female. One of those problems is the ovarian tumor. Tumors can cause estrous


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Whether you operate a breeding operation or have a mare as a riding or competition mount, there are certain problems that you might face just because your horse is female. One of those problems is the ovarian tumor. Tumors can cause estrous problems in competition mares so severe as to lead to early retirement, and they also can cause severe behavioral changes. Mares used for breeding purposes can suffer damage to their reproductive organs, thus limiting their careers as producers.

There might be little or no out-ward signs that something is grow-ing in your mare, and it might take a skilled practitioner to diagnose when an ovarian tumor is involved. However, any owner with a mare should take heed that ovarian tumors do occur with some frequency, and they can disrupt the mare’s ability to function either as a mount or as a breeding prospect.

The word tumor is a frightening one for most people, but it often is misunderstood. When most people hear the word tumor, they immediately think the diagnosis is cancer. The word tumor actually means a swelling or morbid enlargement and is used to des-cribe a growth of tissue in which the cells of that tissue are growing uncontrollably. This doesn’t mean that the tumor is cancerous.

Cancerous tumors are those which not only grow quickly, but spread to other areas of the body. This spread of cancerous cells is called metastasis. Tumors can either be benign, meaning they do not metastasize, or malignant, meaning that the tumor is likely to metastasize and can eventally kill the individual. This distinction is very important for determining the appropriate course of treatment and prognosis. Fortunately, malignant ovarian tumors are very rare in horses.

Ovarian tumors are not unusual occurrences in mares, and as was said before, there might not be obvious outward signs that a tumor is present. There are four common types of ovarian tumors that will be discussed in this article; these tumors are the granulosa-theca cell tumor, teratoma, dysgerminoma, and the serious cystadenoma.

Regardless of the type of tumor, an enlarged ovary can only be determined during a rectal examination. These types of exams are common in broodmares, but not so much in performance mares. Outward signs of an enlarged ovary might include intermittent abdominal pain or erratic estrus (heat) cycles, which often warrant a reproductive examination.

After an enlarged ovary is discovered during a rectal examination, the next step is to determine what effect, if any, it has had on the opposite ovary. Is the opposite ovary small and non-functional, or is it active, with the mare having normal and regular heat cycles? These facts will help your veterinarian determine the type of tumor that is causing the ovary to be enlarged. The next step is to perform a transrectal ultrasound examination on the abnormal ovary, as certain tumors have very characteristic ultrasound findings. Other diagnostic tests might evaluate blood hormone levels. Finally, depending on the probable diagnosis, an ovariectomy might be necessary to remove the enlarged ovary.

Granulosa-Theca Cell Tumors

These tumors are named for the particular ovarian cells that are involved–granulosa and theca cells. This type of ovarian tumor is by far the most common in the horse. Luckily, these tumors are almost always benign and usually are unilateral (involve only one ovary), but a few cases have been reported where both ovaries were involved. Granulosa-theca cell tumors have been reported in all breeds and ages of mares, including newborns, but most commonly these tumors occur in mares between five and nine years of age.

The unique aspect of these tumors is that they can produce several different hormones in the mare, leading to several different behavioral abnormalities. The hormones that are produced by these tumors include estrogens, progesterone, and testosterone. Depending on the amount of each hormone produced by the tumor, mares with granulosa-theca cell tumors might display different types of behavior, even though they all have the same type of tumor. Abnormal behaviors can range from stallion-like aggressiveness (mounting other mares and lunging at other horses) to complete lack of normal estrus or heat cycles, intermittent cycles, or constant cycling (nymphomania).

A rectal examination will reveal an enlarged ovary on one side, and because of the hormones produced from the tumor, the opposite ovary usually is very small and inactive. These tumors vary in size, but can become very large–the largest reported was 59 kilograms (130 pounds). Furthermore, an ultrasound examination might help in the diagnosis, as the tumors usually have multiple small cysts within the tumor that create a honeycomb-looking appearance on ultrasound. Although this ultrasound appearance is not diagnostic, it is very suggestive of a granulosa-theca cell tumor.

Blood hormone levels, including testosterone (and according to a recent report, possibly inhibin), also can be studied to help determine if a granulosa-cell tumor is likely. If these hormone levels are elevated as compared to normal horses, then a granulosa-theca cell tumor is very likely. Unfortunately, a definitive diagnosis cannot be made without a biopsy, but the decision to remove the ovary usually is based on the history, rectal examination findings, ultrasound examination findings, and blood hormone levels. Often the sheer size of the tumor will necessitate removal.

Surgical removal of the affected ovary will almost always result in the resumption of normal heat cycles and the ability to carry a normal pregnancy. The mare usually will begin cycling normally within a few months to one year, depending on the time of year when the tumor is removed. For example, if the tumor is removed dur-ing the winter months, the mare might begin cycling within a few months–in the early spring. However, if the tumor is removed during summer months, the remaining ovary might not begin to function normally until the following spring after the normal winter anestrus (lack of estrus cycles).


This type of tumor is the second most common type of ovarian tumor in mares. It is almost always benign, and usually occurs on only one ovary. The tumor does not produce any outward signs of its presence, and unlike granulosa-theca cell tumors, the opposite ovary remains active. Affected mares will continue to cycle normally and can carry a pregnancy despite the tumor’s presence.

Teratoma tumors usually are only discovered after a routine reproductive examination. They are unique in that they arise from very primitive cells called germ cells, which are capable of differentiating into several different types of tissues or organs. As a result, on the inside of these tumors, many different tissue types are found. It is not unusual to find teeth, hair, bone, cartilage, nerve, and adipose (fat) tissue all within one tumor. These tumors can become large and might cause discomfort or pain (manifested as colic) due to their size. Even though the tumors are benign, surgery is recommended to remove them since they can progress in size. Removal of these tumors is curative.


These tumors are similar to teratomas in that they also originate from germ cells, but the similarities stop there. Dysgerminomas are composed of only one type of germ cell and are one of the few equine ovarian tumors that are malignant. These tumors spread rapidly from the ovary to the rest of the abdominal cavity, then can spread to the chest cavity. Luckily, this type of ovarian tumor rarely is found in horses.

Dysgerminoma tum-ors often are silent in nature (cause no outward signs) until they spread, resulting in weight loss and/or colic. These tumors can be found on a rectal examination as a single enlarged, abnormally shaped ovary. If this type of tumor is suspected, then a sample of abdominal fluid (abdominocentesis) might reveal that there are tumor cells in the abdomen, meaning the tumor has spread (metastasized).

Radiographs of the horse’s chest should be taken to determine if there is visible metastasis to the chest cavity. It is very important to determine whether or not the tumor has metastasized, as this greatly affects the prognosis for survival with this malignant tumor. Horses without metastasis might have a fair chance of survival if the abnormal ovary is removed quickly. In general, horses with this tumor have a very poor prognosis, as anti-cancer therapies for this disease have not been successful.


This type of tumor originates from the ovarian capsule or epithelium that covers the ovary, and produces multiple cyst-like structures within the ovary. Cystadenoma tumors are considered locally invasive, meaning they invade the ovary itself and destroy most of the normal ovarian tissue. However, these tumors are benign and will not spread throughout the horse.

This tumor is found only rarely and usually only affects one ovary, so affected mares often have normal estrus cycles. However, all horses which have been reported to have this tumor were presented for infertility. Although many of the reported cases exhibited elevated testosterone levels, none of the horses had any behavioral alterations. Because of this tumor’s benign nature, your veterinarian might stumble upon this tumor during a routine reproductive examination. Surgical removal of this tumor is curative.

Ovarian Hematoma

Although an ovarian hematoma is not technically a tumor, because it causes ovarian enlargement and must be differentiated from other ovarian tumors, it is worth mentioning here. An ovarian hematoma occurs when a previous ovarian follicle becomes engorged with blood following ovulation. Mares with an ovarian hematoma will continue to cycle normally, but it can take a few weeks to several months for the affected ovary to return to normal size, depending on the original size of the hematoma.

Ovarian hematomas only occur during the time of the year the mare is cycling, unlike tumors that can be found at any time during the year.

An accurate diagnosis of a hematoma usually can be made via an ultrasound examination and palpation of the ovary. Ovarian tumors will destroy the normal architecture of the ovary, but with an ovarian hematoma, the ovulation fossa (depression) remains and can be palpated in almost all cases. This is an important point, as a veterinarian wants to avoid removing an otherwise normal ovary.

Surgical Removal Of Ovaries

Once an abnormal or enlarged ovary has been detected, your veterinarian needs to determine if the tumor (and possibly the ovary) must be removed. This requires him/her to perform a series of procedures, including tests for blood hormone levels, rectal palpations, ultrasound examinations, and possibly a biopsy. Because of the risk of infection and subsequent peritonitis (inflammation of the smooth, transparent, serous membraneth that lines the abdominal cavity) following an ovarian biopsy, this procedure is not often recommended or performed.

The horse might be sent to a referral center for a second evaluation and potential surgery. Once a decision has been made that the ovary has a tumor and it must be removed, then a surgical plan is made. There are several techniques that can be used for ovariectomies (surgical removal of an ovary).

One approach used to remove normal ovaries is ovariectomy via colpotomy (surgical incision of the vagina). This approach, otherwise know as spaying, is used most often to remove both normal ovaries in order to eliminate hormone-related behavior problems in performance mares not intended for breeding. A colpotomy is performed with the mare standing. However, other approaches provide better options for removing larger tumors, and most importantly for controlling blood loss during tumor removal.

Most of the time, the size of the ovary and tumor will dictate the approach and/or technique used to remove the ovary. If the ovary is less than 15 centimeters in diameter, then the ovary can be removed via the flank, and it is the surgeon’s preference whether this is performed with the horse standing or under general anesthesia.

Because of the risk of post-operative hemorrhage with ovarian tumors, many veterinarians prefer to perform ovariectomies with the horse under general anesthesia. This allows the surgeon to completely ligate (tie off) the blood supply to the tumor since ovarian tumors often have a much larger than normal blood supply. If the ovary is much larger then 15 cm, then it might have to be removed via a ventral midline incision (straight up the center of the abdomen).

Many veterinary clinics also remove normal ovaries and moderate-sized ovarian tumors via laparoscopy. This surgical technique utilizes a fiber-optic camera and long-handled instruments and has many advantages, including the need for only small (2-3 cm in length) incisions to insert the instruments. These long-handled instruments then can be used to ligate the blood supply to the ovary and remove it. Sometimes one of the incisions is lengthened slightly to accommodate removal of the enlarged ovary. These small incisions allow for a quicker return to normal exercise, decreased chance of problems at the surgical site, and decreased chance for peritonitis after surgery.

The most serious potential complication of ovariectomy is blood loss occurring within the first 24 hours following surgery (because of the vascular nature of these tumors). Therefore, mares often are hospitalized for several days post-operatively. Following surgery, the mares will require a variable period of rest–depending on the technique used–ranging anywhere from two or three weeks to two months.

Malignant ovarian disease is quite rare in the horse, and mares with benign ovarian tumors have a very good chance of returning to a normal, healthy life with no long-term complications following ovariectomy. Although at first the word ovarian tumor might be very alarming, many times the final outcome can be successful.  


Daels, P.H.; Hughes, J.P. The Abnormal Estrous Cycle. Equine Reproduction. Eds. McKinnon and Voss. Philadelphia: Lea & Fibiger, 1993.

Myers, P. Ovary and Oviduct in The Horse. Diseases & Clinical Management. Eds. Kobluck, Ames, and Geor. Philadelphia: W.B. Saunders Co., 1995.

Trotter, G.W.; Embertson, R.M. The Uterus and Ovaries. Equine Surgery 2nd ed. Eds. Auer and Stick. Philadelphia: W.B. Saunders Co. 1999

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Written by:

Christina S. Cable, DVM, Dipl. ACVS, owns Early Winter Equine in Lansing, New York. The practice focuses on primary care of mares and foals and performance horse problems.

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