ABSTRACT: Ovariectomy is a treatment employed in the mare for removal of granulosa theca cell tumours (GCTI, and for behaviour modification in mares that become aggressive during oestrus. The most common technique is laparoscopic ovariectomy under standing sedation, using sequential electrocoagulation and transection of the mesovarium. In the treatment of GCT the prognosis for resolution of behaviour and return to normal fertility is excellent. When treating mares exhibiting aggressive behaviour during oestrus, approximately 75 per cent of mares will show a significant improvement in their behaviour, over a course of up to 18 months.


Ovariectomy (removal of either one or both ovaries) in the mare can be performed for a variety of reasons: unilateral ovariectomy in the case of pathologically enlarged ovaries, such as a granulosa-thecal cell tumour (GCT); or a bilateral ovariectomy where no ovarian pathology can be identified but a mare becomes aggressive or difficult to handle during oestrus.1'2


Removal of a GCT is the most likely reason for performing an ovariectomy in a mare. A GCT is the most common tumour of the equine female reproductive tract, and is a benign tumour that can occur in any age of horse. Behavioural changes are the most common presenting sign in a mare with a GCT, ranging from stallion-like behaviour and aggression to persistent oestrus (seasons’) – with stallion-like behaviour being the most frequently reported behavioural change, possibly because it contrasts so readily with the previous behaviour exhibited by the mare (Figure 1).

Figure 1: A granulosa cell tumour weighing 3kg. (Image courtesy of Bonny Millar]

Granulosa-theca cell tumours may be diagnosed following palpation per rectum, alongside trans-rectal ultrasonography, where the affected ovary will be enlarged, with a cystic, ‘honey-combed’ appearance. Hormone levels can also be measured to confirm the diagnosis of a GCT.

Increased blood testosterone levels were considered to be indicative of a GCT in a mare that exhibited stallion-like behaviour; however, further investigation has shown that up to 50 per cent of mares with a GCT will not have elevated testosterone levels. In fact, the hormone inhibin is a more reliable marker of the presence of a GCT.

If a mare becomes difficult to handle or aggressive during oestrus, there are a number of treatments that can be tried, including the synthetic progestogen, altrenogest (Regumate, MSD Animal Health) and implanting a sterile marble into the mare’s uterus.

If it has been shown that a mare’s undesirable behaviour is a consequence of her seasons – with the changes in hormone levels that occur as a result – another possible treatment is a bilateral ovariectomy. However, this is a treatment that should not be undertaken lightly, as the mare will then never have a future as a brood mare.

It is used as a treatment in mares that have initially responded well to other therapies, the effects of which seem to be wearing off with time; or in mares that are deemed to be so dangerous that this is the only treatment option prior to euthanasia.

A bilateral ovariectomy may also be performed in mares used as ‘jump mares’, for semen collection from stallions to be used for artificial insemination (AI).


A number of different techniques of ovariectomy in the mare have been described over the years, including colpotomy, flank laparotomy, and ventral midline and paramedian oblique laparotomy (Figure 2).3'4&5 Limited exposure to the ovaries, lack of visualization and haemorrhage from the severed ovarian pedicle are some of the major problems associated with this surgery.4

Figure 2: A mare being draped for laparoscopic surgery

Laparoscopic ovariectomy in the standing mare is now considered to be the technique of choice, offering several advantages including the advantages of minimally invasive surgery and the ability to visualise the ovarian pedicle, whilst avoiding the risks of general anaesthesia.1

Several different methods of achieving haemostasis have been utilised as the mesosalpinx and mesovarium are dissected, including ligatures, clips, endoscopic staplers and electrosurgery. The most commonly employed technique would be to utilise an electrosurgical instrument that combined electrocautery with a guillotine action, although this is dependent on surgeon preference.8&9

In preparation for laparoscopic surgery, the mare is starved for 36 hours pre- operatively, in order to reduce the volume of ingesta within the viscera and improve the visualisation of the surgical field. The mare is then restrained in stocks, sedated and administered pre-operative broad spectrum antimicrobials and non steroidal anti-inflammatory drugs.

The three laparoscopic portals are in the ipsilateral paralumbar fossa for the ovary being removed, with the initial portal being for introduction of the camera and insufflation of the abdomen with carbon dioxide (C02). All subsequent portals are then made under direct visualisation using the laparoscope.

The mesovarium is infiltrated with 10ml of 2% mepivicaine hydrochloride, via a laparoscopic injection needle, prior to the ovary being grasped with laparoscopic Babcock forceps via an instrument portal, to allow for manipulation during the procedure. An electrosurgical instrument that combines electrocautery with a guillotine action (Ligasure) can then be employed to sequentially cauterise and transect the mesovarium (Figure 3).

Figure 3: Grasping the ovary through the laparoscopy portals

In enlarged, pathologic ovaries, the ovary may require to be decompressed prior to removal from the abdomen, in order to minimise the length of the final incision required in the paralumbar fossa.


Postoperative incisional complication rates, including incisional drainage and partial dehiscence, up to 50 per cent have been reported following laparoscopic ovariectomy (Figure 4).8'10 In addition, mild colic signs have been reported in mares in the immediate post-operative period, attributed to the irritant effects of C02 on the serosal surfaces of the abdomen.

Figure 4: Incision ready for closure

When considering long-term outcome, if a mare is diagnosed with a GCT, then only the affected ovary need be removed, and the prognosis for return to normal behaviour and future reproductive function is good. Aggressive behaviour in mares with granulosa theca cell tumours is frequently associated with an elevated serum testosterone concentration, which will remain in the circulation longer than the post-operative hospitalisation period.11*112 It is important, therefore, that owners are aware of the likely timescale over which the undesired behaviour will subside.

One study showed that 75 per cent of mares undergoing bilateral ovariectomy for the purpose of behavioural modification showed a significant improvement in behaviour.10 However, it is important to note that this change in behaviour occurred over a protracted period of time, ranging from three to 18 months.

An owner must have realistic postoperative expectations for any horse undergoing a bilateral ovariectomy, as any longstanding cases are likely to have a component of learned behaviour in their aggression, which is unlikely to be resolved immediately by surgery. 


Luisa Smith BVMS PhD CertES (Soft Tissue) DipECVS MRCVS

Luisa graduated from Glasgow University Veterinary School in 2000, and since then has worked in specialised equine practice, both in the USA and in the UK. She completed an equine surgical residency, with her time split between the Royal Veterinary College, London and Bell Equine Veterinary Clinic, attaining her Diploma from the European College of Veterinary Surgeons, before becoming a partner in House and Jackson – a mixed practice in Essex.

To cite this article use either

DOI: 10.1111/j.2045-0668.2011.00115.x or Veterinary Nursing Journal Vol 27 pp 109-111


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• VOL 27 • March 2012 • Veterinary Nursing Journal