Ovariectomy is a procedure that generally is performed to remove an abnormal ovary. Occasionally, an ovariectomy is performed in fillies or mares which have particularly bad behavior during their heat cycle in an effort to make them more manageable. Ovarian problems that might indicate an ovariectomy include tumors, hematomas, and, rarely, abscesses.

Ovarian tumors account for approximately 6% of neoplastic (abnormal growth) conditions in the horse. The most common ovarian problem in the horse by and large is a granulosa thecal cell tumor. These tumors are benign in the sense that they do not metastasize (invade and spread to other parts of the body), but they can exert some severe physiological effects via hormones that they secrete (see below).

Another benign ovarian tumor that occurs in the mare is called a teratoma. Other malignant or invasive tumors do occur (carcinomas), but are rare when compared to granulosa thecal cell tumors. Hematomas are generally a result of excessive hemorrhage from an ovulated follicle and usually resolve spontaneously, but may take up to 3-5 months to do so. Ovarian abscesses are very rare.

As we discuss tumors, cancer, and the various "--omas," I think it is again important to define the terminology as this is another area where words are often used inappropriately and confuse the true meaning of what we are trying to say. The following are defined by Dorland's Medical Dictionary.

  • Tumor: Latin for "swelling"--note that an abscess as well as a pimple could be called a tumor and does not necessarily mean cancer.
  • Cancer: Any malignant cellular tumor.
  • The suffix --oma: Latin for "a tumor or morbid growth."
  • Carcinoma: A malignant tumor (cellular) tending to invade local tissues and to give rise to metastases.
  • Malignant: Having the properties of invasiveness and metastasis (malign is Latin for "bad or evil").
  • Metastasis: Transfer of disease from one organ or part of the body to another not directly connected to it (the movement of abnormal cells with respect to malignant tumors).
  • Benign: Not malignant (Latin for "good or kind").

Granulosa thecal cell tumors are large, benign, steroid-producing tumors that often are associated with behavioral changes and/or poor reproductive performance. These tumors have been reported in mares of all ages, but have the highest frequency in mares between five and nine years of age. The abnormal cell growth occurs in cells that naturally produce sex hormones, i.e., testosterone, estrogen, etc., and therefore can cause excessive production of those hormones.

A common finding in mares with a granulosa thecal cell tumor which are exhibiting stallion-like behavior is a high level of the male sex hormone testosterone. The level of such hormones in the blood's serum can be measured, but, as can be imagined, in very small amounts. A serum level of 100 picograms/milliliter (that's 0.000000000001 grams) is generally associated with stallion-like behavior in a mare.

Other clinical signs are intermittent or continuous estrus and nymphomania, also related to excessive production of various sex hormone steroids by the abnormal ovary. If the mare is breeding, poor reproductive performance also is related to excessive hormone production. There is negative feedback on the normal ovary that essentially makes it small and inactive.

The second benign tumor is called a teratoma. Tera is Greek for "a wonder or a monster"; thus the monster swelling definition alludes to the character of this strange tumor. Teratomas are benign and do not secrete steroids that arise from "germ" cells within the ovary. Germ cells are the cells genetically programmed to become eggs and therefore contain the genetic material for all the components of the body. It is that fact that leads to the bizarre nature of these tumors; teratomas might contain cartilage, skin, bone, hair, nerves, and even teeth--something right out of the new TV series "Dark Skies." Because these tumors do not secrete sex hormones, there is no negative feedback on the other ovary and reproductive performance is generally not affected. These tumors are often discovered as incidental findings on reproductive examination.

The diagnosis of an ovarian abnormality is generally made by rectal palpation. Both ovaries can be evaluated in very good detail via rectal palpation. Complete examination is important. As mentioned above, in cases of ovarian hematoma and teratoma, the other ovary usually is functioning normally. In the case of granulosa thecal cell tumors, the other ovary is usually small and inactive.

History also plays an important role in diagnosis. Mares with granulosa thecal cell tumors often have behavioral changes as mentioned above and/or poor reproductive performance if they are used for breeding.

Ultrasonography can be extremely valuable in assessing an ovarian mass. Hematomas have a characteristic ultrasonographic "texture," as do granulosa thecal cell tumors and teratomas. Hematomas are usually a single mass with a very homogeneous texture, whereas granulosa thecal cell tumors generally are multiloculated, fluid-filled structures, but they also can be one large fluid-filled structure. When imaging a teratoma, there are often characteristic shadows caused by more dense structures such as teeth or bone.

Hormone assays can also be performed on the blood to gain laboratory support for the evidence of a granulosa thecal cell tumor. The levels of testosterone, estrogen, and progesterone can be quantitated, but not all granulosa thecal cell tumors produce enough of these to show a significant elevation.

It is a case where a positive test means something, but a negative test means nothing.

A newer test that measures the blood level of the hormone inhibin has been reported to be more accurate. The diagnosis of an abnormal ovary that requires removal usually can be arrived at given the history and findings of rectal palpation and ultrasonography.

Removing the Ovary

The decision to remove the normal ovaries from a filly or mare in an effort to eliminate behavioral problems during estrus must be carefully thought over for obvious reasons--once they're gone, they're gone! Certainty that the behavioral problem outweighs the breeding potential is paramount. My personal opinion is that all other potential causes for the behavioral problem should be ruled out and hormone therapy, such as Regumate or progesterone, should be proved ineffective.

The treatment of choice for granulosa thecal cell tumors is surgical removal. The surgical approach depends on the size of the abnormal ovary and preference of the surgeon. A common approach for tumors that are 15 centimeters or less in diameter is through the flank. The incision is made in the center of the para-lumbar fossa (flank) between the last rib and the point of the hip. This procedure is usually performed with the horse under general anesthesia, but occasionally is done with the horse standing while under heavy sedation and local anesthesia. One of the main benefits to the flank approach is an overall decreased convalescent period. The other approaches that are performed if the tumor is large or out of surgeon preference via the ventral mid-line or low on the flank.

Once the ovary is exposed, regardless of the approach, the pedicle (connective stalk of the ovary) must be ligated in order to tie off the ovarian artery. The ovarian pedicle is composed of the ovarian artery and vein, nerves, and connective tissues. It is extremely important that the ligation be secure as fatal hemorrhage has been reported to occur via the ovarian artery. The pedicle can be ligated by hand, using suture material or more commonly using surgical stapling instruments. The use of surgical stapling instruments for aid in removal of ovarian tumors was reported in 1988 by Rick Duran, DVM, Diplomate ACVS, in the Equine Veterinary Journal. A 1993 article by Scott Palmer, VMD, Diplomate ACVS, in the Journal of the American Veterinary Medical Association reported on a standing laparoscopic laser technique for ovariectomy.

Another technique that is commonly used for the elective removal of normal ovaries for behavior modification is called a colpotomy. Colp is Greek for "vagina" and indicates the approach for this procedure. The horse is standing and heavily sedated and an epidural anesthetic is performed. An incision made deep inside the vagina above the cervix is the opening into the abdominal cavity for ovary removal. The ovarian pedicle is ligated by crushing and excised at the same time using an instrument called an ecraseur. Advantages of the method include being relatively inexpensive, not requiring general anesthesia, and not leaving an external scar.

Regardless of the method used for ovariectomy, this procedure is generally a painful one and the use of peri-operative analgesics is important. The horses often are hospitalized for 3-7 days and very carefully monitored in the immediate post-operative period for any signs of hemorrhage, which is a serious complication that can occur.

For mares which were showing behavioral changes related to a granulosa thecal cell tumor, their behavior rapidly (sometimes within a week) returns to normal. In addition, there is generally an improvement in reproductive performance in 5-6 months, although they might not begin cycling until the following spring, depending on the time of year the tumor is removed.

About the Author

Michael Ball, DVM

Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.

Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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