By Roger Dixon MRCVS – Ashbrook Equine Hospital, Cheshire
To the frustration of many owners, some mares appear to be constantly in season. This can make ridden work very difficult and may affect the horse’s performance at competitions. The behaviour may be due to completely normal physiological events related to the mare’s oestrus cycle, or may be a sign of a more serious problem. This article will describe the more common causes of persistent oestrus and how they are diagnosed and treated.
At Ashbrook we usually treat 4-6 mares a year with ovarian tumours. There are several different kinds of ovarian tumour; the most common being the ‘Granulosa Cell Tumour’. Fortunately, these tumours in mares are usually benign and rarely spread to other organs. The behavioural changes are caused by hormones secreted by the tumour. The type of hormone determines the behaviour. For example, mares who become aggressive and ‘stallion-like’ may have excessive quantities of testosterone in their circulation. Alternatively the mare may appear to be permanently in season, or never in season.
Diagnosis is usually possible by an ultrasound scan of the ovaries; the affected ovary may be large with an abnormal ultrasonographic appearance. The other ovary is often small and inactive because the tumour secretes a hormone called ‘inhibin” which causes the normal ovary to shut down. Your vet may perform blood tests to measure hormone levels to assist with the diagnosis.
Treatment is surgical removal of the affected ovary (ovariectomy). This is usually possible by laparoscopic (keyhole) surgery performed under standing sedation. This abolishes the need for a general anaesthetic. Once the affected ovary is removed, the normal ovary is likely to regain its size and function, although this may take many months. Following confirmation of normal ovulations from the remaining ovary, successful future breeding may be possible.
Mares who have difficulty conceiving are often presented for an ultrasound scan of the reproductive tract. Chronic uterine infection (endometritis) is commonly diagnosed, usually in mares who have recently had a foal or have had repeated, unsuccessful coverings.
In addition to causing poor fertility, endometritis can cause mares to ‘short-cycle’ themselves and come into season every 5-7 days. This dramatically shortens dioestrus, the interval between successive oestrus periods and it may seem as though the mare is rarely out of season.
Diagnosis is by ultrasound exam of the reproductive tract, which may reveal free fluid in the lumen of the uterus. Laboratory tests such as endometrial swabs or uterine biopsies may be necessary to definitively diagnose the presence of infection and establish to which antibiotics the causal microorganism is sensitive. Treatment involves repeated uterine lavage and daily or every other day infusion of antibiotics into the uterus.
Transitional Oestrus (Spring and Autumn)
During the spring, it is common for mares to appear as though they are constantly in season. In winter, the ovaries usually ‘shut down’ and become inactive (winter anoestrus). In early spring, with increasing daylight hours, the ovaries gradually become active. Follicles develop on the ovaries which secrete the hormone oestrogen and the mare may appear in season for several weeks. This period is known as the Transitional Oestrus.
The follicles may become very large. Eventually, one of these follicles will ovulate and the mare will enter a regular cycle with ovulations about every 21 days. The end of the transitional oestrus may be hastened by increasing daylight hours with artificial light from mid-December and/or various hormonal treatments to induce the first ovulation of the year and therefore end persistent oestrus signs.
During the winter when the ovaries are inactive, circulating levels of the hormone progesterone are very low. This lack of progesterone may lead to the mare appearing in season throughout the winter. Many mares who are assumed to continue to cycle and ovulate through the winter actually have normal, temporarily inactive ovaries!
Behavioural issues in mares can be frustrating for owners for many different reasons. For the horse breeder, these issues may mean that the mare cannot be bred, either because she will not stand for natural covering or because of the absence of normal ovarian activity. For the horse rider, the behaviour may make meaningful ridden work impossible, particularly at competitions when lots of other horses are present.
Veterinary examination is advised in mares who persistently have behavioural problems to enable diagnosis of:
- Non-reproductive problems such as dental issues, back pain or gastric ulcers.
- Reproduction-related issues such as granulosa cell tumours, excessive oestrus behaviour and uterine infections.
Your vet will be able to advise you on the best diagnostic approach to ensure a pain-free and productive experience for both horse and rider!
If you would like to discuss any of the above issues in more detail, Roger may be contacted at Ashbrook on 01565 723030.
You may also like:
Moody Mares Part One – The Vet’s Perspective
Behavioural changes associated with the oestrus cycle.
How we diagnose problems and what are the latest treatments available