We have a mare that we purchased a few years back as a young colt that behaved similar to what you describe,mounting another mare at times and trying to be dominant with the herd only to have her but kicked time and time again. Finally had to remove her from the two more aggresive members of the herd to get any peace! We're still not sure what caused this behaviour but a good while back I read an article somewhere,(possibly in Equus magazine), that described a similar behaviour caused by ovarion cysts creating a hormone imbalance and stallion like behaviour in mares. I did a search and could not find the article that I had read but did find this very limited information on anther site. If you find anything similar, please share it since I meant to have this information as a reference.

4 April 2007

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Cancer of the Ovary in Mares

CANCER OF THE OVARY

Goulburn Valley Equine Hospital, Victoria, Australia

WHAT ARE THEY?
Granulosa theca cell tumours (GTCT) are one of the more common cancers in the horse accounting for ~ 2.5% of all horse cancers. Although other ovarian cancers occur they are rare. GTCT’s are benign (i.e. they do not spread elsewhere around the body) and once present generally continue to grow in a relentless way. Unlike most cancers, they are more likely to occur in younger to middle aged animals (highest frequency 5-9 years of age). The neoplastic (cancerous) cells are actually made from the cells of the follicles that grow within the ovary (either granulosa or theca cells).

HOW DO THEY AFFECT MARES?
They are a problem for mares because commonly the mare stops cycling and also the cancers can become quite big (20 Kg) (See figure 1).
The follicles that grow in a normal ovary have different cells which produce a number of reproductive hormones. Because the tumour has more of these cells than a normal follicle, more of these hormones may be produced. Depending on which hormones predominate, the mare can show any one of a number of behavioural changes;

Failure to show oestrus- this is the most common presentation, when you will notice that your mare stops showing her regular signs of heat (oestrous). This is because the tumour is producing large amounts of the hormone inhibin that stops all ovarian activity.

Continually showing oestrus- these mares are sometimes described as nymphomaniacs, as they appear to be continuously in heat.

Stallion-like behaviour- this is seen less commonly, and occurs if the tumour has lots of cells producing androgens (like testosterone).

Not uncommonly mares may have all the signs above with different levels of behavioral expression on different days or weeks. We have seen mares teasing one day and mounting other mares the next.

It is also possible for a mare with a GTCT to cycle normally early in the clinical course of the disease. This was thought to be quite uncommon but with newer diagnostic equipment such as ultrasonography, we have been able to demonstrate that GTCT’s occur in cycling mares more than originally thought.

Occasionally if the tumour is very big, the mare might show signs of colic.

HOW DO I FIND OUT IF THIS IS WHAT MY MARE HAS?
Your vet will examine her, and will be able to feel an abnormally big ovary, often with the other one being small and inactive. They have a very typical appearance on an ultrasonographic examination (figure 2), however if we need further confirmatory information a blood test can be submitted. This measures the concentrations of different hormones, and some, especially inhibin can be fairly specific for the condition.

WHAT NEEDS TO BE DONE TO TREAT IT?
If your mare is a broodmare then the cancer must be removed (usually the earlier the better).
If your mare is not a broodmare, the tumour is small and there are only mild behavioural changes, you may not have to treat her for the time being. However, if she is difficult to manage, shows occasional colicky signs, or you would like to breed from her in the future, then the affected ovary should be removed.
Surgical removal can be performed through either the flank or ventral midline under general anaesthesia and they are by far the most common methods. Occasionally smaller ovaries can be removed in the standing tranquilised mare through the flank or even the vagina. All the procedures carry a risk to the mare but experienced anaesthetists and surgeons have little problems.

WILL THE MARE BREED AGAIN?
Yes. The remaining ovary takes some time (2-16 months) to recover from the hormonal suppression of ovarian activity, but your mare should be able to conceive and carry foals normally with the one healthy remaining ovary.

IS SHE LIKELY TO GET THE PROBLEM AGAIN?
No. Bilateral GTCT (both ovaries affected) have been reported exceedingly rarely. We have only seen one mare with a bilateral GTCT.

Goulbourn Valley Equine Hospital Media Release

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