Hormonal control of Moody Mares

As we move into spring, a number of mare owners may be feeling a degree of trepidation! Moody mares can be a source of enormous frustration- one day she may be positively angelic and the next a holy terror. These sorts of behaviours can be attributable to a number of causes but are most easily divided into those resulting from normal hormonal activity, abnormal hormonal activity or factors which are not related to reproductive hormones. Here we are going to consider how to manage the first two situations where reproductive hormones play a role.


Horses are seasonal breeders, with most mares not cycling through the winter months. As the daylength increases in the spring, the reproductive hormones of oestrogen and progesterone begin to be produced and cyclic seasonal activity is observed. In the early part of spring, these cycles can be more erratic but as summer continues, most mares will settle into a regular twenty-one day cycle and often some 'mare-ishness' will subside at this point.


Typical signs of being in season (in oestrus) include:

• frequent urination

• vulval 'winking'

• raising tail

• tail swishing

• squealing

• kicking out

• change in attitude to work


Many mares will show these signs to a mild degree and it will not affect their handling or ridden work to a significant degree. In most cases when we are asked to examine a mare for moody behaviour, they may be displaying more extreme behaviours such as:


• aggressive or stallion like behaviour

• reluctance to be saddled (bucking, kicking out, biting etc)

• avoidance of being touched

• rearing


It is important to note that some of these behaviours, particularly the more extreme ones, can have a number of other causes which are not related to the reproductive hormones.


Before we would consider any treatment, it is important to ascertain that the behaviours we are trying to treat are definitely related to reproductive hormones. In many cases, this is more a process of ruling out any other possible causes. Furthermore, many of these signs are normal behaviours associated with being in season and does not necessarily mean there is something wrong with the mare, more that they respond in a particular way to different stages of their cycle. A clinical examination from your vet is also indicated and often will include a trot-up in hand and possibly under saddle along with an ultrasound of the ovaries and uterus via a rectal examination. Occasionally, ovarian tumours or uterine infections can lead to abnormal hormone activity and result in altered oestrus behaviours. Both of these can generally be ruled out with ultrasound scan and/or blood tests


Following these examinations, if it appears likely that the undesirable behaviours are related to normal seasonal hormones suppressing the oestrus cycle can be considered. There are several options for this:


1. Oral progesterone supplementation


Progesterone is one of the two reproductive hormones, along with oestrogen. In the normal cycle, as progesterone levels decline, oestrogen rises, causing the mare to come into season. If we get additional synthetic progesterone, this will prevent the natural decline and thereby prevent her coming into season. Sometimes we may use Regumate as a trial to see its use abolishes the mare-ish behaviours as it is generally very effective. As it is a liquid administered in the mare's feed once a day, it is also very easy use. However, it must be very carefully handled due to the risk of it being absorbed through human skin. There are a few further disadvantages to its use, most notably its cost and that it is not permitted under certain competition rules. Mares will generally come into season within 4-7 days of its use being discontinued and there has not been shown to be any long-term impact on fertility associated with its use. Recently, injectable formulations of synthetic progesterone have become available, requiring administration on a weekly basis which may be more convenient for some.


2. GnRH vaccine


Gonadotrophin releasing hormone (GnRH) is a hormone released from the brain which affects the release of a further two hormones, follicle stimulating hormone (FSH) and luteinising hormone (LH) which act to influence progesterone and oestrogen levels. Therefore, if GnRH levels are low, the other hormones will also be low, effectively preventing the mare from cycling. Usually mares require a primary course of two injections 21-28 days apart and then boosters every six months although in some cases boosters may be required less frequently. This vaccine is not currently licensed in the UK and requires a special importation from Australia which is costly. A pig GnRH vaccine is available in the UK and is effective in mares but has been associated with some significant and in some cases very serious side effects and as a result its use should be very carefully considered. Additionally, such vaccines should not be used in mares that have any potential for breeding in the future as the long-term fertility may be affected.


3. Uterine marble


Small glass marbles (~30-40mm diameter) have been placed in the uterus via the cervix. Theoretically, the movement of the marble in the uterus mimics the movement of an early embryo, thereby fooling the mare's body into thinking she is pregnant and not returning to being in season. That said, success rates are thought to be less than 50%. The advantages of this would be the low cost of inserting the marble and that as no drugs are used, there are no considerations for withdrawal times for competition mares. However, as well as poor success rates, there may be issues associated with removal of marbles and also the potential for infections in the uterus due to the presence of a foreign body.


4. Oil administration into uterus


A small volume of peanut or coconut oil can be placed in the uterus at a specific time within the mare's cycle and will prevent her coming back into season. Early studies into this treatment have yielded success rates of >90% and similarly to marbles, there is no drug withdrawal time to be considered. At present, there has been no further work to look at the potential effect on long-term fertility or side effects.


5. Surgical removal of ovaries


Clearly to the irreversible nature of this as a treatment, it should be considered a last resort. There are also anecdotal reports of undesirable behaviours still be displayed following this surgery, so knowing that the problem is definitely reproductive in origin is essential before considering something so invasive.


As you can see, there are a number of different options available to help to manage a moody mare, but the first consideration should always be ruling out any other cause of her symptoms.

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