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.

Understanding and treating depression in horses

Introduction

While depression is widely recognised in humans, it is a condition which is often overlooked in animals. The symptoms and clinical signs of depression vary from species to species but recent studies have shown that humans and horses share some characteristic warning signs which are important to recognise.  

Horses are prey animals with a well-developed fight-or-flight response and their behaviours are controlled by the nervous and endocrine system.  Like all animals, behaviour is influenced by many external factors that include genetic predisposition, environment, physiology, experience and learning. Foals are neurologically mature at birth and soon after birth can identify and react to dangerous stimuli, but like humans, the horse’s brain function does decrease with age resulting in increase in anxieties and fears. 

The brain and its neurotransmitters play an instrumental role in the temperament and behaviour of the horse; therefore, abnormal levels of various hormones can lead to a change. Normal survival instincts for the horse fall into two broad categories that includes ‘something to fear and must flee’ and ‘something not to fear and should be explored or ignored’. New research has also demonstrated that there is not a linear dominance hierarchy but rather the herd communicates with positive reinforcement and less from punishment.

In order to achieve the best results when training, it is important to consider these natural behavioural instincts. Maintaining a positive mental attitude in both training and management regimes for the horse will have considerable benefits to performance and reduce negative behaviours. 

What is depression? 

Depressive disorder (also known as depression) is a mental disorder that can occur in horses.  It involves a depressed mood or loss of pleasure or interest in activities for long periods of time.  Depression affects how the horse thinks and behaves and may lead to a variety of potential physical problems.  

While depression is widely recognised in humans, it is an illness which is often overlooked in animals.  The symptoms and clinical signs of depression vary from species to species but recent studies show that humans and horses share some characteristics. With racehorse welfare being a key topic at present, understanding and recognising small behaviour changes can allow for small management adaptations to be made thus enhancing the horses wellbeing. 

The research 

Recent research from France alongside the growing body of research, suggests that horses may develop something similar to depression in a response to physical or social discomfort.   

Researchers have observed horses that become withdrawn because of undergoing a cognitive shift.  It has been found that horses tune out to their surroundings due to physical or psychological stress.  Horses have the cognitive ability to be attentive; however, with the presence of chronic stressors there is a delay in a horse's response as they have “switched off” from their environment and demonstrated sensory inattention. 

Because we do not truly know how the horse is feeling, the hypothesis currently being considered is that the horse develops an “inward-oriented attention” when subjected to chronic disorders.  It must be stressed that this long delay in attentiveness does not indicate a state of calm for the horse but a withdrawal from its surroundings. 

Dr Georgia Mason from The Ontario Veterinary College has suggested that a horse may respond similarly to humans with depression.  It is thought that the horse’s withdrawal can be a result of a ‘learned helplessness’ in that responding to negative stimuli does not make any difference.  Thus depressed people are prone, for example, to judge ambiguous stimuli as being unlikely to be positive and to recall unpleasant memories more readily than pleasant ones.

In addition, a number of studies have assessed cortisol levels in those horses that are withdrawn. Cortisol acts to assist in relieving stress by increasing glucose metabolism to provide energy, which then enables the horse to escape from the stress. In the short-term, cortisol release is beneficial to the horse to help it cope with a stressor. This study found that abnormally low levels of cortisol (hence a physiological depression) were found in withdrawn horses that are in a pathological and depressed state.  These depressed horses also expressed anhedonia, which is the loss of pleasure to feed on an appetent substance.  

Further studies and research are needed to better understand the pathology of depression in horses.  It has been suggested that the life conditions, such as food, space, social conditions and health problems should be questioned and observed in order to identify potential acute or chronic stressors that may lead to depression. 

However, there are reviews that question the current animal models of depression and suggest that ethological models of mood disorders based on animals living under natural conditions need to be improved and developed. 

Symptoms of a depressed horse 

It is important to become acquainted with potential signs of depression in the horse so that you can identify them quickly.  By identifying the symptoms of depression, one can rapidly begin the task of identifying the underlying cause such as an injury, illness, social or environmental stressor.  With these tools, one can reduce the risk of the short-term and long-term negative effects that could ultimately hinder the horse’s health and performance.  

Lack of response

In humans and animals, being attentive is one aspect of subject cognitive abilities and capacities. The delay in responding to stimulation shows that withdrawn horses ‘switch off’ from their environment and show sensory inattention. Such lapses of attention are likely to be associated with the chronic effect of stressors, which might be expected to induce a lowered state of arousal.

A French study from the Universite de Rennes, demonstrated that depressed horses that were subjected to a series of five new, unique and unusual sounds were significantly less likely to pay attention to the noises compared to normal horses.  Their findings demonstrated that the withdrawn horses had undergone a cognitive shift that they were so physically or psychologically stressed that they developed sensory inattention. 

Heightened anxiety 

While the depressed horse will often be disinterested in everyday activities, people and companions, they are likely to demonstrate heightened emotions towards challenging situations and new stimuli as they try to express their discomfort. Stimuli that are usually unremarkable to the horse may become uncomfortable for the horse and make them “spooky”, anxious or fearful when they weren’t before. 

Lack of interest in food 

A change in mental well-being in your horse can lead to a loss of appetite.  The potential complications are the loss of weight, condition, muscle development, fitness, energy levels as well as the increased susceptibility to infection and prone to other health conditions such as colic or gastric ulcers. 

There are other illnesses and ailments that can lead to inappetence; hence, it is worth seeking veterinary advice in order to rule out any other physical problems.  

Anhedonia

Dr Carole Fureix looked at anhedonia which is the loss of feeling pleasure from experiences that used to be pleasurable.  This symptom is prominent in the depression of humans. Her study demonstrated that horses who are at times withdrawn are more anhedonic than those that are not withdrawn.  She concluded that this is a key symptom of clinical depression and suggests that withdrawn horses are indeed in a depression-like state.  

The horse may show unwillingness to work or no longer take pleasure in work they used to enjoy. This is especially important when considering rider and horse safety too as a withdrawn horse may not react appropriately to potentially dangerous situations.

Posture

Standing facing the stall wall for periods of time while demonstrating a withdrawn posture showing a fixed gaze, neck stretched out level with back, eyes open but lack of eye and ear movement. This differs from a content horse that is resting and displaying relaxed muscles, laterally rotated ears, the drooping of its eyelids and lips and holding its neck such that it slopes lower and rounder.  

Physical changes are perhaps somewhat easier to spot with a withdrawn posture being one of the typical signs of depression.  A healthy and content horse will stand with their head up and ears moving inquisitively.

Causes of depression 

The domesticated horse is often managed in such a way that they cannot express their natural behavioural instincts, thus exposing them to stressors which can lead to depression.

Lack of adequate positive social interaction with other horses 

A study from Michael Steger and Todd Kashdan explained that dysfunctional social behaviour has been implicated in the experience of depression. 

In addition, Timothy Elmer and Christoph Stadtfeld described individuals with depressive symptoms are more likely to be isolated from their social group(s), which can further increase their symptoms.  

Similarly, lack of social interaction without conscious choice for extended periods of time can lead to depression in humans.

Lack of adequate exercise and/or turnout 

It is well documented in human medicine that exercise and physical activity can improve many health conditions. Research on depression, anxiety and exercise demonstrates that the mental health and physical benefits can help improve mood and lessen anxiety.  The link between depression, anxiety and exercise are not entirely clear.  It has also been shown that exercise and physical activity may help keep depression and anxiety from coming back. 

Companionship reduces stress in the horse, hence naturally  living in a herd.  Horses are able to form companionships with their own species and other animals including humans.  In fact, horses kept in isolation, such as in closed stables, can become anxious, flighty and difficult to manage. 

Exercise releases endorphins which are feel-good chemicals in the body and also allows for more social interaction.  A recent University of London study demonstrated that people with low aerobic and muscular fitness are nearly twice as likely to experience depression.  

Illness or injury that causes pain or discomfort 

In humans studies have found that anywhere between 30-85% of patients suffering from chronic pain are also clinically depressed. Other illnesses in humans such as nutritional deficiencies, cushing’s disease, lyme disease, chronic pain, and insulin resistance can cause depression.

Stress during training, competing or in the daily routine 

If there is chronic stress in which the horse’s physiological stress response is taxed beyond what it is designed to do it can begin to impair the horse possibly leading to depression 

Inflammation and depression 

There is evidence that there is a link between inflammation and depression. Inflammations that lead to serious depression in humans can be caused by infections such as those caused by bacteria, viruses or even parasites.  There is increasing evidence that inflammation can cause depression because of the increase of cytokines setting the horse’s brain into “sick mode”.   

Treating depression

Simple changes in management can help treat the underlying cause of depression and can substantially improve the horses well-being thus having a positive impact on performance.

Light therapy

Modern management methods mean that horses can spend a large amount of time stabled which impacts the horse’s circadian rhythm (body clock). 

A study was performed at Nottingham Trent University where trial horses were put under an hour of high intensity broad spectrum light every day for six weeks. Their behaviours regarding feeding, sleep patterns and attitude to being handled and ridden were compared to a group of horses that had not received the therapy. The untreated horses were noted to be sleeping longer, lazier at ridden exercise and grumpier.

Light therapies have many positive benefits that can reduce signs of depression and improve overall well being; better daytime alertness and improved rest and sleep pattern, extending summer coat and body condition and encouraging the body to convert feed to muscle mass, noticeably fewer stable vices such as box walking, cribbing, weaving and reduced bacterial pathogens and fungal load leading to improved respiratory function.

Management

Racing is already making moves to ensure the best welfare conditions for horses, with German trainers now having to comply with regulations regarding size of stable, the amount of light in the stable, the provision of windows enabling horses to socialise and turn out capacity for a minimum two hours per day.

Increasing turnout time is reported to have positive effects on their horses' mental wellbeing, reduces stiffness and lowers the rate of stable vices. It also allows the horse to make social connections whilst allowing them to show their natural instinctual behaviours. 

With horses in varying routines in large yards, this can sometimes become stressful to some individuals. By providing company of other horses when stabled, this can help with social interactions and reduce stress. Stable enrichment and adlib forage can also replicate positive behaviours in the stable. 

It is known that exercise releases ‘feel good’ endorphins, thus, keeping the horse in a regular training regime complimented with the correct nutrition will be beneficial. 

Veterinary advice

It is important to seek veterinary advice should the horse be exhibiting signs of depression to help identify the underlying cause. Diagnostics such as blood tests can ensure there is no infection present and work-ups will highlight any pain or discomfort.

Conclusion 

While depression is widely recognised in humans, it is an illness which is often overlooked in animals. Depression in horses can be significant and harmful regardless of the underlying cause and can be temporary or long term. It is vital to ascertain the underlying cause of depression in a horse in order to treat and remedy the illness.