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.

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