ANYONE involved in keeping horses will appreciate the array of medical and surgical conditions which they can develop. One such condition is a type of ovarian tumour known as a Granulosa Cell Tumour (GCT).

These tumours are slow-growing and benign, which means they do not spread around the body. They can, however, produce a number of hormones which can result in an array of behavioural or reproductive issues in affected mares. Occasionally, tumours up to 40cm in size can be found in mares displaying no outward symptoms at all!

What are the signs?

One of the most interesting aspects of this condition is that it can present in a variety of ways. The tumours can develop in mares of any age with the average age at time of identification being around 11 years. In many cases, there may be no outward symptoms at all and they may go undetected for long periods of time, or picked up during routine reproductive examinations. In other cases, however, these tumours can be more problematic.

If there is a significant testosterone-producing element to the tumour, mares may begin to display behavioural signs such as aggression, or even stallion-like behaviour. This can sometimes be so severe that the mares become dangerous. Even in the absence of these signs, circulating testosterone can be present in the bloodstream which in some rare circumstances can result in an unexpected positive drugs test in a racing or competition mare.

In broodmares, these tumours are sometimes picked up when a very enlarged ovary is noted during routine examination. The ovary will usually appear abnormal with a multi-cystic or “honeycomb” appearance although this can vary between cases.

A GCT is problematic from a reproductive viewpoint as the hormones released by the tumour will eventually suppress normal function of the other ovary and mares will fail to cycle normally. Some of these mares may be noted to either be anoestrus or fail to come into season at all, or even show nymphomania where they appear to be in season all of the time.

In the early stages, the mare may still come into season and ovulate but in most cases, as long as the tumour is present, the mare is unlikely to have normal fertility. In rare cases, other problems such as colic or internal bleeding can occur due to the presence of a GCT.

Diagnosis

If a granulosa cell tumour is suspected in a mare with unusual behaviour or with an absence of normal cycles, you should talk to your veterinary surgeon. It is important to remember that not every moody mare or enlarged ovary will be due to a GCT.

In most cases, a work-up will start with a rectal exam and ultrasound examination of the reproductive tract. In most mares this is performed simply and easily, often with the help of some sedation or restraint in the stocks to safely facilitate examination. It is important to first rule out that the mare is not pregnant as this can cause some of the behavioural signs. If an abnormal or enlarged ovary is observed, this must be distinguished from other causes such as anovulatory follicles, an ovarian haematoma or other types of tumour.

There are a number of blood tests that can help distinguish a GCT from other causes of ovarian enlargement. Probably the most useful is a test for anti-mullerian hormone which, if increased, is very accurate at diagnosing the condition. Other tests for hormones such as inhibin, testosterone or progesterone may be selected by your veterinary surgeon to gain more information about your mare’s condition.

It is very important that an accurate diagnosis of GCT is obtained by your vet before embarking on treatment.

Treatment

If your mare is diagnosed with a GCT which is having adverse effects on her behaviour or reproductive function, the best treatment is removal of the affected ovary, termed ovariectomy.

This surgery is generally performed by a specialist equine surgeon in a hospital environment. In many cases, the ovary can be removed by standing laparoscopy or essentially key-hole surgery without the need for a general anaesthetic. Small incisions are made in the mare’s flank so that a camera and instruments can be passed into the abdomen, the tumour is located and carefully removed.

This surgery is generally very successful and mares recover well from it. In some cases, particularly with very large tumours, surgery under full general anaesthesia might be considered.

Complications after surgery are uncommon and usually mild such as swelling or infection around the surgical site or mild post-operative colic.

Outcome after surgery

Any behavioural signs associated with the tumour will disappear rapidly after removal and mares which have experienced reproductive suppression will generally start to come into season and cycle normally within six to eight months, depending on the time of year in which it is removed.

Mares should be able to breed normally and carry healthy pregnancies with one functioning ovary and we see a number of mares on studs every year who successfully produce foals after ovariectomy.

The exception to this would be the very unlucky minority of mares who develop bilateral GCTs, meaning the tumours affect both ovaries.

Biography

Emma Chedgey MVB MRCVS is a graduate of Veterinary Medicine from UCD and works in equine practice in Lisadell Equine Hospital, Navan, Co. Meath. The practice offers a full range of first opinion and referral services including extensive diagnostic, treatment and surgical facilities. Emma’s professional interests include internal medicine, reproduction and foal care and she is in the process of completing certificates in Advance Veterinary Practice in Stud Medicine and Internal Medicine through the University of Liverpool. Her time is split between calls to a range of clients and studs in the Meath area and the hospital where she performs medical investigations and manages hospitalised patients and mares for artificial insemination.