Suppressing unwanted hormonal behaviours in training
/Article by Kate Dugher
The desire to suppress unwanted behaviour in the horse can present for many different reasons. The behaviours that we are talking about can be anything from poor performance to hyper-excitability, distraction, discomfort on girthing up, not responding to the jockey, bucking, rearing, squealing, kicking or aggression.
Often it is assumed that overt behaviours are hormonally driven; however, it can be easy to discount many other possible causes of these behaviours, especially those that are related to pain. A full clinical examination by a veterinarian is always warranted when considering unwanted behaviour in the horse in order to appropriately identify the cause and consider the most appropriate treatment options.
Common causes of abnormal/unwanted behaviour can include:
Musculoskeletal pain (lameness)
Gastric ulceration
Dental disease
Poorly fitting tack
Stress
Hormonal influence
Learnt behaviour
There are also many reasons for normal and abnormal behaviours that can be associated with the reproductive system. Some of these could be identified as undesirable behaviours when associated with performance.
The equine reproductive cycle
Horses are seasonal long day breeders and are influenced by daylight length. This means that the majority of mares have inactive ovaries in the winter and do not exhibit oestrus behaviour during this time. In comparison, in the summer months, they exhibit a reproductive cycle that lasts an average of 21 days. They spend, on average, 5-7 days in oestrus, ‘in season’, and 14 days in diestrus, ‘not in season’.
In the spring and autumn months the mare undergoes a transitional period. During this time, oestrogen concentrations are variable, and oestrus behaviour can be seen irregularly. Whilst stallions are also affected by seasonality, they still exhibit reproductive behaviour all year round. The mare’s reproductive cycle can also be influenced by artificial light and therefore, it is worth considering that performance horses who are exposed to stable lights beyond the normal daylight hours in spring, autumn or winter may cycle for a longer period of the year or even throughout winter.
Puberty
Timing of puberty in the horse is varied and affected by both genetic and environmental factors. Not only by age but also by time of year in which they were born, body condition and social cues. Puberty in fillies is usually at around 12-19 months compared to colts at around 10-24 months, however, there are wide variations from these reference ranges.
Normal reproductive behaviour in the mare
Normal oestrus behaviour occurs under high oestrogen and low progesterone influence. Commonly associated behaviours include receptivity to stallions/geldings, vocalisation, increased frequency of urination and presentation of hindquarters in a wide based stance.
Normal diestrus behaviour under a dominant progesterone state includes repulsion to the stallion and can occasionally be associated with aggressive behaviour to other horses. During pregnancy, the mare will also be under a dominant progesterone influence and is unlikely to exhibit oestrus behaviour particularly in the first trimester. Later in gestation a peak in testosterone and oestrogen levels may be associated with changes in behaviour.
Abnormal reproductive behaviour in the mare
Ovarian pain
Many mares will show an obvious reaction upon rectal palpation of the ovary when close to ovulation, suggesting that the dominant follicle/ovary can sometimes be tender at this time. Comparatively, humans often describe some ovarian pain around the time of ovulation. Therefore, it can be assumed that some mares could also experience discomfort around the time of ovulation.
Other possible causes of ovarian pain that can occasionally occur in normal cyclicity include ovarian haematomas and haemorrhagic anovulatory follicles. It is also a consideration that external pressure placed onto the lumbar region close to the ovary around the time of ovulation could rarely elicit a painful response in some individuals.
Vaginal pain
Vaginal pain has occasionally been associated with conditions such as vaginitis and pneumovagina. These conditions describe inflammation and/or air in the vagina. These are most commonly associated with poor perineal conformation and can be evident in some performance mares.
If vaginal pain is suspected due to poor perineal conformation, then placement of a caslicks vulvoplasty may prove to be beneficial. If concurrent infection or urine pooling is suspected, then further intervention may be required.
Reproductive tumours
Reproductive cancer affecting the ovaries is one of the most common causes of cancer in the mare, the most common being the granulosa theca cell tumour (GTCT). These are generally locally invasive and are unlikely to cause any further health problems if the affected ovary is removed. They are often identified with a change in behaviour. On rectal examination a common finding would be to identify one enlarged and one small ovary.
Depending on which reproductive hormones the tumour secretes is likely to influence the associated behaviour. This can include stallion-like behaviour, aggression, persistent oestrus behaviour or complete absence of reproductive behaviour. The severity of this often depends on the stage at which this condition is identified. Other types of ovarian tumours are less common but depending on if/which hormones are secreted will dictate which hormonal behaviours are associated. It is suspected that occasionally there could be ovarian pain associated with some of these cases particularly when the ovary is very large in size.
Reproduction related treatment options
Mares
To have the most successful outcome in controlling reproductive hormonal behaviour in the mare, it is important to understand whether the unwanted behaviour is being exhibited all year round or just in the summer months and whether it is related to a particular stage of the oestrus cycle.
Whilst it is commonly assumed that most behaviour problems are associated with the mare being in season, occasionally some mares can show unwanted aggressive behaviour under the influence of progesterone – when they are not in season.
Furthermore, it can be tricky to interpret this when trying to link hormonal behaviours to performance based unwanted behaviours and these signs can often be very individual. Keeping records of behaviour versus stage of the reproductive cycle can help to try and decipher whether reproductive hormones are likely to be playing a part in the unwanted behaviour. However, this does require careful monitoring and, most likely, multiple reproductive ultrasound examinations.
The other consideration is that unwanted behaviours are related to reproductive pain or abnormal hormone production due to pathological conditions of the reproductive tract as previously described.
Ways to mimic the diestrus state and suppress oestrogen related behaviour
Progesterone/Progestins
Progesterone is the dominant hormone produced by mares in diestrus. There are a multitude of systemic progestin (progesterone-like medications) available for use in horses in injectable and oral formulations.
Altrenogest is a synthetic progestin commonly used to suppress oestrus behaviour by acting as a progesterone agonist. This means that the horse is likely to exhibit normal diestrus behaviour for that individual whilst it is being administered. Altrenogest is molecularly very similar to the anabolic steroids trendione and trenbolone. Occasionally the product may contain trace levels of these anabolic steroids. Therefore, its use for horses in training is to be taken with extreme caution and withdrawal times adhered to. It is banned for use in racing thoroughbreds in some countries.
There is also evidence to show that altrenogest can exhibit a reduced stress response and sedative-like effects in some horses, particularly mares. This effect may be beneficial in anxious individuals in training circumstances. However, arguably, dependent on the individual, a reduced stress response could have either a positive or negative effect on performance.
Injectable progesterone applications have been used in racing thoroughbreds with appropriate clearance times before racing. These are often available in oil-based preparations which are commonly associated with injection site reactions and therefore, many trainers would avoid administering these within 3 days of racing.
Upon cessation of progesterone supplementation, many mares will present with oestrus signs 2-7 days after treatment, as this mimics normal luteolysis at the end of the diestrus phase. Therefore, the timing of administration and cessation of progesterone/progestin treatments is a crucial consideration when being used for the prevention of oestrus behaviour.
Intra-uterine devices (IUDs)
IUDs have been historically utilised to mimic early pregnancy in the mare with varying success. These require an ovulation to act upon to extend the life of the corpus luteum by blocking the hormonal release that normally brings them back into season. Therefore, they are only useful once the mare is already cycling.
Glass marbles have been the most used IUD historically; however, these are no longer recommended due to multiple evidenced side effects including risk of glass fragmentation in the uterus. The use of PMMA spheres or magnetic devices such as the iUPOD would be a preferable and safer alternative if an IUD was going to be used.
Interestingly, in the author’s experience speaking with clinicians who have administered these devices, there is surprisingly positive client satisfaction despite the inconsistent and variable evidence of the success of these devices in the literature.
Oxytocin
Administration of the hormone, oxytocin, at specific time points when the mare is in diestrus can extend diestrus by up to 60-90 days. This technique is evidenced by multiple studies. For optimal success, reproductive ultrasound would be used to identify ovulation and carefully plan the timing of injectable administration.
However, some studies have also evidenced successful extension of the diestrus phase without known timing of ovulation. The major downside of this technique is the need for administration of multiple injections/multiple reproductive examinations to time ovulation.
Immunological approach
Gonadotrophin releasing hormone (GnRH) is a hormone produced by the brain that is responsible for stimulating follicle growth in the ovaries and activation of a hormonal cascade to bring the mare into oestrus.
GnRH vaccinations generate an immune response against GnRH, suppressing the hormonal cascade and ovarian activity and therefore, oestrus behaviour. An equine licensed product has previously been available in Australia. However, this is no longer in production. We have the option of a swine formulation, Improvac®, which has commonly been used in equids off licence.
Major drawbacks for the use of this are common adverse injection site reactions, risk of anaphylaxis and concern over extended length of ovarian suppression. Therefore, this option would not be recommended in mares with a future breeding potential.
Surgical approach
Ovariectomy is a treatment option for hormonal behaviour in mares. The ovary is the only supply of progesterone in the horse but is not the only supply of oestrogen.
Ovariectomy has been associated with good client satisfaction in many cases to resolve unwanted hormonal behaviour. However, in some mares, whilst removal of the ovaries would prevent cyclicity, it can occasionally result in persistent oestrus behaviour in the absence of progesterone produced by the ovaries. This is also a permanent option that will remove breeding potential.
The techniques discussed so far are not exhaustive and there are many other methods that have been used to affect cyclicity or hormonal behaviour including pregnancy, induction of diestrus ovulation, GnRH analogue medication and infusion of intrauterine medical grade plant oils.
Colts/stallions
There are a few medicated options for hormonal manipulation in males. Progestagen administration e.g. oral altrenogest administration can quieten stallion like behaviour in males but is banned for use in racing and training.
Immunisation with off licence GnRH vaccines such as Improvac®, suppresses pituitary-gonadal hormone production aiming to cause a ‘chemical castration.’ However, results can be variable, particularly in mature stallions. As mentioned previously with mares, the downside of these vaccines are injection site reactions, risk of anaphylaxis and risk of prolonged sterility in future breeding animals.
Occasionally nutritional supplements have been used with effect in stallions such as L-tryptophan, a precursor of the neurotransmitter serotonin. This has induced calm and fatigue-like behaviour in a number of species.
Synthetic preparations of calming pheromones based on an equine appeasing pheromone produced in perimammary gland secretions of lactating females have also been used with such success. Of course, the use of these to calm behaviour vs the desire to generate an athletic performance animal is a consideration and results are likely to have wide individual variation.
