Moody mares: Hormonal and medical conditions

By Gayle Hallowell

23 February 2010 11:57

In addition to the normal variation in behaviour due to the reproductive cycle, there are also a few conditions that affect the ovaries and hormones, which can lead to extremes of behaviour.

Some of these include: 
Permanent oestrus ornnymphomania this can be seen for a variety of reasons, which include ovarian tumours or other tumours that secrete hormones.

Behavioural problems that don’t relate to hormones at all (such as social dominance in a group).

Infections of the bladder leading to the mare squirting urine all the time (causing people to think they are constantly in season).

Pain in the back or hindlimbs making it difficult to posture to urinate (so they squirt little bits of urine often). If this relates to the ovary, removal of the ovary will abolish the behaviour.

However, other causes should be ruled out first by your vet, with blood
tests and/or examination of the urine and hormone measurements in the blood.

Ovarian tumours can also cause problems, particularly granulosa cell tumours, which are benign ovarian growths that don’t spread. They produce a lot of testosterone, which is the main hormone in the stallion.
This causes the mare to show stallion-like behaviour, which includes mounting other mares, aggressive behaviour, excessive squealing and striking out.

Usually these mares are constantly in season or have irregular seasons – about one quarter of mares have no seasons at all.

Diagnosis by your vet involves palpation of the ovaries with or without examination using an ultrasound machine and blood tests to look at hormone concentrations. The ovary usually needs to be surgically removed and normal behaviour is seen once the hormone concentrations fall in the blood.

Some mares can bleed from their ovary following egg release and this can cause them to suffer colic. It’s hard to tell the difference between this and other forms of colic, so ask your vet to make a diagnosis.