Mares behaving badly: Would removing their ovaries help?

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Mares demonstrating unwanted behaviours not obviously related to their reproductive cycle may benefit from having their ovaries removed, it has been reported.

A Danish study explored whether the removal of the ovaries provided a solution for troublesome mares showing unexplained unwanted behaviours.

Changes in their behaviour and/or rideability following the ovariectomies were assessed from the perspective of the owners.

Some of the mares involved in the study had ovarian cancer. The others had all undergone a full diagnostic work-up in a bid to identify the reason for their unwanted behaviour.

“Some mares are still without a diagnosis that can explain their behaviour,” Daniel Taasti Melgaard and his colleagues wrote in the open-access journal Animals.

“The mares participating in the study were a subset of mares that had already been screened for painful conditions stemming from outside the reproductive tract, but included mares with ovarian tumours.”

Removal of their ovaries, they said, was used as a last-ditch attempt to solve their problems. In all cases, the use of drugs to stop their reproductive cycle had failed to alter their behaviour, and no problems outside their reproductive tract had been identified.

The study team said unwanted behaviour in mares is a common problem dealt with by veterinarians. Behaviours may range from the mare being uncooperative or aggressive when handled on the ground; kicking, bucking or rearing when ridden; or being aggressive towards other horses.

In some cases, ovarian cancers cause the behaviour change, but in other cases there is no apparent reason.

The retrospective study was based on surgical removal of the ovaries in 28 mares, all of whom had undergone their operations six to 24 months earlier. The study team examined the case records for each mare, asked owners to complete a questionnaire, and conducted a telephone interview.

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Fourteen of the mares had ovarian cancer in either one or both ovaries, 10 had normal ovaries, and the ovaries of the remaining four mares had not been examined to identify if cancer was present.

Afterward, rideability improved in 80% (8 out of 10) of the mares with normal ovaries and in 57% (8 out of 14) of the mares with ovarian cancer.

A behavioural improvement was observed by owners in 40% of the mares with normal ovaries, and in 43% of the mares with cancer,  which amounted to no statistical difference.

The results suggest that mares with and without ovarian cancer can equally benefit from ovariectomy to improve behaviour and rideability, the researchers said.

The authors noted that previous papers reporting on the effects of removing the ovaries described an improvement in unwanted behaviour in relation to oestrus.

“In the present study, the unwanted behaviour was not obviously related to the oestrus cycle,” they said.

© Jan Milne

Researchers in a 2007 study removed the ovaries of mares with only behavioural problems, apparently not related to the reproductive cycle. They found an improvement in behaviour in 83% (19 of the 23) of mares.

In the present study, 80% of mares with normal ovaries and rideability problems, and 40% of mares with behavioural problems on the ground, improved after removal of their ovaries.

“There is no obvious explanation as to why the mares with normal ovaries benefited from an ovariectomy. Whether there was an undetected pathological problem present in the ovaries cannot be ruled out.

“Another hypothesis is that some mares may benefit from an ovariectomy in the same manner as stallions to reduce hormone-driven unfavourable behaviour and rideability following orchidectomy (testicle removal).

“To our surprise,” they continued, “no observable statistical difference in rideability or behaviour following ovariectomy could be identified between mares with or without ovarian neoplasia (cancer).”

It is generally accepted that mares with ovarian cancer will benefit from an ovariectomy, but in the present study, the mares with normal ovaries equally improved in behaviour.

The authors acknowledged that owners’ perceptions of a change in behaviour and rideability could be a placebo effect due to the assumption or hope that the operation would work.

Two mares underwent ovariectomies despite an inapt clinical presentation. “The ovariectomies were conducted as a last resort before euthanasia, but neither case benefited from the ovariectomy. One of the mares was euthanized and a brain tumour was diagnosed at the post mortem examination.”

The authors say that, before carrying out an ovariectomy in mares with normal ovaries and behavioural problems, it is important to do a thorough diagnostic workup to rule out other pathology.

“Despite the significant improvement observed in the present study, further research is necessary to confirm whether mares with unwanted behaviour not obviously related to the oestrus cycle and to painful conditions may benefit from ovariectomy to alter their behaviour and rideability,” they concluded.

The study team comprised Daniel Taasti Melgaard and Martin Soendergaard Thoefner, both with Hoersholm Equine Clinic in Kongevejen; Trine Stokbro Korsgaard, with DyrDoktor in Sjaellandsgade; and Morten Roenn Petersen and Hanne Gervi Pedersen, both with the University of Copenhagen.

Melgaard, D.T.; Korsgaard, T.S.; Thoefner, M.S.; Petersen, M.R.; Pedersen, H.G. Moody Mares—Is Ovariectomy a Solution? Animals 2020, 10, 1210.

The study, published under a Creative Commons license, can be read here

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