THE MERCK VETERINARY MANUAL
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Behavioral Problems of Horses

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Many behavioral problems are associated with confinement. Under free-ranging circumstances, horses wander and spend >60% of their day foraging. The remainder of their time is spent resting (standing or lying down), grooming, or engaging in another activity. This same pattern is seen under barn conditions; even with free choice of grain, horses will eat many small meals a day. Horses are highly social animals that require contact with others for normal daily maintenance and well-being. Isolating horses can lead to development of problems. The main goal of managing behavior problems in horses is to identify the deviation from normal equine behavior and correct it.

Aggression

Aggression is a common problem in horses and includes chasing, neck wrestling, kicks and bites, and other threats. Signs of aggression include ears flattened backward, retracted lips, rapid tail movements, snaking, pawing, head bowing, fecal pile display, snoring, squealing, levade (rearing with deeply flexed hindquarters), and threats to kick. Submissive horses respond by avoiding, lowering the neck and head, clamping the tail, and turning away from the aggressor.

This behavior is seen mostly in stalls in which the horse feels confined in a small space that is also easily defended. The varieties of aggression toward people include fear, pain induced, sexual (hormonal), learned, and dominance related. Some horses, especially young ones, play with each other while showing signs of aggression such as kicking and biting. Although benign to other horses, this can be dangerous to people.

The first step in managing equine aggression is identifying the cause, and if possible, removing it. Training and positive reinforcement to establish control over the horse are also used, along with desensitization and counterconditioning. Dominance-related aggression in horses is different from canine status-related aggression (also known as dominance aggression) in that it is not context-dependent. Environmental management is important as well; good management should include sufficient resources such as space, food, and water. Some horses are considered to have pathologic dominance aggression; they will attack other horses and people that are near them. These horses should be separated completely from other people or horses and have a poor prognosis.

Aggression toward other horses is mostly associated with sexual competition, fear, dominance, or territory (protecting the group and resources). As with aggression toward people, some horses may be pathologically aggressive toward other horses. The first step is separation of aggressive horses from other horses, and keeping subordinate away from dominant horses. Separation is achieved by solid walls or two fences to avoid kicks through the fence. Horses should have sufficient resources, and desensitization and counterconditioning is the best treatment approach. In cases of sexually related aggression, castration and progestins (eg, medroxyprogesterone 70–80 mg/300 kg/day) can help. Adverse effects of such treatment should be weighed carefully, and the horse should be monitored closely. Adding tryptophan to the daily ration or administering selective serotonin reuptake inhibitors may be helpful in some cases. Punishment should be avoided.

Aggression by mares toward people is normal during the first few days after parturition. This behavior is hormonally driven and usually wanes with time. Mares should be familiarized with their caretakers before delivery and have minimal contact with other people after delivery. No treatment is required in most cases.

Stallions that are aggressive when used for breeding are often overused or used out of season. Stallions can develop preferences for mating and may not be compatible with the chosen mare; changing the mare may help. If stallions were stabled with mares when they were colts, they may have some social inhibition for mating, and forced mating can result in aggression. The goal of treatment is to treat the main cause of aggression; changing the mare (because of preferences) or artificial breeding can also be attempted. Physical restraint (eg, hobbles) and desensitization can help as well. Clicker training has been used successfully to desensitize stallions with this problem.

Stereotypic Behaviors

Compulsive behaviors in horses can be divided into movement-related behaviors and oral behaviors. They can be called stereotypic because they are repetitive, occupy a large part of the daily activity, and serve no function. Confinement and poor management practices are the primary contributing factors. In addition, bedding, feed, and social contact influence stereotypic behaviors. Horses that have more social contacts, are fed more roughage and more than one type, are fed two or more times daily, and are bedded on straw are less prone to these behaviors. Cribbing and wood chewing are examples of oral behaviors, whereas weaving, stall walking, and pawing are examples of locomotor stereotypies. Horses with one stereotypic behavior are likely to exhibit another. In Thoroughbreds, these behaviors are commonly seen in mares and 2-yr-old foals.

When cribbing, the horse usually grasps an object in the stall (such as the water bucket) with its incisors, flexes its neck, and sucks air into the pharynx. Some horses will aspirate or swallow the air. In some cases, horses will suck air without grasping any object. Feeding highly palatable food (eg, grains, molasses) is associated with cribbing. Lack of exercise is also associated with cribbing; endurance horses are less likely to do it than race or dressage horses. Thoroughbreds are more prone to cribbing than other horses. The rate of cribbing is higher in confined horses; however, even if the horse is turned to pasture once the behavior is established, it will persist. It is possible that GI discomfort can lead to cribbing. One of the major complications of cribbing is damage to the incisors. Other problems include gastroduodenal ulcers and epiploic foramen entrapment. In most cases, cribbing is a benign behavior that does not affect the horse's welfare and does not require treatment. Close to 10% of foals 20 wk of age will start cribbing when weaned and placed in stalls. Those kept on pasture will not start. It has been speculated that horses can learn cribbing by watching other horses; however, no clear evidence exists.

Cribbing can be diagnosed by finding U-shaped pieces missing from fences and horizontal surfaces in the stall, and worn incisors and enlarged neck muscles in horses that crib. In some cases, the caretaker may directly observe the behavior. Management should include more roughage, exercise, and social contact. Turning confined horses to pasture may help, and providing toys and stimulation is also advocated. Placing a strap around the horse's neck behind the poll will apply pressure each time the horse tries to flex its neck. This essentially punishes the horse for cribbing, with the punishment associated with the behavior and not the caretaker. Alternatively, an open-end muzzle can be applied. This will allow the horse to eat and drink but prevent it from grasping objects to crib on. Some horses find a way to crib with the muzzle (eg, grasping a linear object, such as a stick), and most horses seem to tolerate the strap better than the muzzle. Keeping stalls free of horizontal surfaces and objects that the horse can grasp can help minimize cribbing. A variety of surgeries have been suggested to manage cribbing; however, the varying success rates and negative impact on animal welfare are significant disadvantages.

Like a horse that cribs, a wood-chewing horse will grasp pieces of wood with its incisors, but unlike in cribbing, it will swallow the pieces. The definitive cause of wood chewing is lack of roughage in the diet. Confinement, high-concentrate diets, and lack of exercise and stimulation increase incidence of wood chewing. Horses on pasture normally spend 8–12 hr/day grazing, while confined horses spend <3–4 hr/day feeding. Wood chewing increases in cold, wet weather. Providing more roughage, exercise, stimulation, toys, or social contact can reduce incidence of this behavior. Eliminating exposed wood and covering fence edges with wires and taste repellents can also help minimize wood chewing.

Most horses will ingest sand or dirt. Soil that is ingested is richer in iron and copper than other soils, and this may attract the horse. Ingestion of stones can become a serious problem, because it can lead to intestinal obstructions. Management should include increasing roughage and exercise and providing salt blocks and toys.

Polydipsia in horses is similar to behavior seen in dogs (see Eating Disorders). The most common presenting complaint is a wet stall due to frequent urination. It is important to exclude medical causes (eg, diabetes insipidus).

These behaviors are seen in confined horses, serve no purpose, are hard to interrupt, and are usually slower than other types of movement. Horses that stall walk usually walk in circles in the stall, and when released to a larger space (eg, pasture or barn) continue to circle in a small area. Tying the horse to prevent walking will only transform the behavior into weaving, ie, lifting the legs and shifting weight and head position from side to side in the same spot. Possible causes of stall walking include lack of exercise and social contact and claustrophobia. Stress and anxiety appear to aggravate the problem. Treatment should focus on increasing exercise and stimulation, providing social contact, and turning the horse to pasture. Providing thick bedding and feeding more than twice daily can help as well. In extreme cases, selective serotonin reuptake inhibitors might be necessary to control the problem. Providing a large mirror in the stall in front of the horse can help decrease weaving.

Horses may kick the walls of the stall because of boredom, aggression, or frustration. The horse may kick in anticipation when food is being prepared but is out of reach. When the horse is then fed, the behavior is reinforced. The horse may also be frustrated when it cannot achieve its goals (eg, exercise, mating, or social contact). It is possible that this behavior is a form of self-mutilation. Many horses that kick and make holes in the walls of the stall also eat wood from these holes. Treatment should be directed toward eliminating the underlying cause. Treatment for aggressive kicking is discussed above (see Aggression to People), and for most other causes the treatment is similar to that for stall walking (see Stall Walking and Weaving). Owners should never reinforce kicking by providing food when the horse kicks. Providing more social contact, exercise, and stimulation can also help.

Similar to kicking but less dangerous, pawing or digging may be a result of frustration and anticipation. This could also be a displacement behavior. Changing the floors to concrete may stop pawing; however, it will not change the motivation to do so, and some horses (especially stallions) may rear up instead of pawing. Treatment is similar to that for kicking (see Stall Kicking).

A head-shaking horse shakes or jerks its head uncontrollably, without any apparent stimulus. Some horses will also snort, rub their head on objects, and display an anxious expression. Most commonly, horses shake up and down. There are five grades to this problem: 1) intermittent signs, mainly facial twitching; 2) moderate signs with noticeable shaking such that can interfere with riding; 3) advanced stage, and difficult to control; 4) uncontrollable and unrideable horse; and 5) dangerous behavior with bizarre patterns. In most cases, the horse looks as if it has nasal mites or is being attacked by biting flies. Many medical conditions can cause head shaking (eg, seizures, respiratory tract diseases and parasites, ear and eye disease, GI disorders, pain, trauma, nasal foreign bodies), and these must be excluded. Behavioral causes of head shaking include an improper bit, an incompetent rider, fear and anxiety, dressage leading to extreme cervical flexion, and compulsive disorders. Geldings seem to be affected more frequently than stallions or mares. Management should include treating any underlying medical problem, desensitization and counterconditioning, and potentially use of selective serotonin reuptake inhibitors.

Some horses hurt themselves by biting or kicking the abdomen with their hindlegs. Some of these horses also vocalize. Underlying causes include displacement behavior, self-reinforced behavior, and redirected behavior. Skin diseases and pain can also lead to self-mutilation and must be excluded. This problem seems more common in young males (<2 yr old) and may possibly be triggered by environmental stressors. Management should include sufficient stimulation and exercise and increased social contact.

Fear and Phobia

Like dogs, horses can have fears and phobias. The two main presentations are noise and location or environment phobias. Horses have an innate fear of new things (neophobia) that explains some behavior issues such as trailer-related problems (see Trailer-Related Problems). The management is similar to that in dogs and cats (see Treatment of Fears, Phobias, Anxiety, and Aggression).

There are two main presentations of trailer-related problems: loading into the trailer and travelling. Horses may be afraid to load into a trailer because of innate factors (eg, neophobia, a dark interior, instability of the trailer, noise) and/or learned factors (eg, previous accident, motion sickness, previous punishment while loading). A horse may load into a trailer readily but misbehave while inside. This could be because the horse finds it difficult to keep its balance while the trailer is moving, anticipates a stressful event such as a race after the trailer ride, or has motion sickness. In a small number of cases, horses are reluctant to leave the trailer. Heart rate and salivary cortisol levels have been shown to increase during and after trailering.

The best approach to managing trailering problems is slow desensitization and counterconditioning using food and treats. This may take a long time and may not be suitable for an acute problem. Desensitization should be done long before the expected trailer ride. Punishment should be avoided, because it may aggravate the situation and be dangerous for both the horse and the caretaker. Some horses prefer to see their environment and may load better in a rear-facing trailer. Horses are herd animals and learn from each other; having a horse with a trailer-related problem view another horse loading can facilitate learning. Foals should be loaded with their mothers at an early age. Sedatives such as xylazine can be used in acute situations; however, the horse may not learn to load or ride better while drugged and may be less able to balance itself for the ride and any performance after the ride.

Sexual Behavior Problems

Behavioral anestrus is a common problem in young mares, especially during the first cycle. Palpation of the ovaries reveals normal ovaries, and ovulation occurs normally. However, the mare will not accept the stallion. Causes include environmental stressors and mating preference. Presenting several stallions may help, and if the mare is still nursing a foal, weaning may help.

Medical conditions that can cause excessive sexual behavior in mares include granulosa cell tumor and persistent ovarian follicles. These conditions should be differentiated because a persistent follicle may resolve without treatment (although it may require treatment with gonadotropins or luteinizing hormone or increasing daylight to ≥16 hr), whereas granulosa cell tumors require surgical resection. Excessive estrous behavior is manifested as squatting and urinating frequently, receptiveness to males, and exposing the clitoris (“winking”). Some granulosa cell tumors produce testosterone, leading to stallion-like behavior (eg, aggression, mounting, flehmen, and urine marking). Management should be directed toward the underlying problem.

Mares with psychic estrus show estrous behavior without any of the physiologic correlates of heat. Treatment is as for nymphomania (see Nymphomania).

Overused stallions, submissive stallions presented to aggressive mares, and stallions with previous negative experiences may be unwilling to mate. Masturbation is a normal behavior in horses; colts will start mounting within the first few weeks of their lives. Ejaculation is rare, and fertility rates are unaffected. Some owners use various devices to stop masturbation; these essentially punish the horse for masturbating and cause pain. They can cause fear while trying to breed and predispose the stallion to poor libido. Management should be directed toward eliminating the underlying problems. The stallion should be presented with teaser mares, should be well rested and fed, and have increased social contact with mares on pasture. Using an artificial vagina to desensitize the stallion can help, and treatment with anxiolytics such as diazepam may address underlying anxiety. Other cases may require semen collection and artificial insemination.

Approximately 50% of geldings have some stallion-like behavior, including courting and mounting females, flehmen, fighting, and attacking foals. The brain of the male horse is masculinized before birth; therefore, some of these behaviors do not require androgens to be expressed. Some geldings may achieve erection and intromission while mounting the mare. In advanced cases, the horse should be checked for pituitary adenoma. Normally, the testosterone level should be <0.2 ng/mL. Treatment includes separation, progestins, and selective serotonin reuptake inhibitors.

Eating Disorders

Eating feces is mainly seen in foals during the first 8 wk of life. Foals usually consume fresh feces of their mothers, and it is believed that maternal pheromones play a role in this behavior. Deoxycholic acid is found in feces and may help protect against infantile enteritis and aid in deposition of myelin. The behavior may also provide vitamin B and introduce normal intestinal flora. In adults, the behavior is mainly associated with a low-roughage diet.

Decreased exercise and overfeeding a palatable and highly concentrated diet can lead to obesity. On pasture, horses spend 8–12 hr grazing while moving from place to place; confined horses spend <3–4 hr. Increasing exercise, social contact, and roughage can help maintain appropriate body weight.

Horses are herd animals, and any changes in social relationships or the environment can increase stress and lead to anorexia. Appropriate weaning of foals is also important to prevent anorexia. Submissive horses may not eat near aggressive horses if they have previously been attacked. Management should be directed toward the underlying problem. Increasing social contact and separating affected animals from aggressive horses can help.

Foal Rejection

There are three types of foal rejection: 1) avoidance, seen mainly in primiparous mares that appear fearful of the foal—the mare will not attack the foal but will not allow suckling; 2) intolerance of suckling, which is seen in primiparous mares or mares with a painful udder; and 3) aggression toward the foal, in which mares exhibit stallion-like behavior and may kick and bite the foal. There may be a genetic factor for this type of aggression in Arabians and Morgans. Some mares paw at the foal to stimulate them to stand, and this should be differentiated from aggression.

The most important aspect of treatment is to protect the foal. In extreme cases, the foal should be supplied with colostrum within the first 12 hr and then bottle fed or cross-nursed to another mare. Restraining a primiparous mare and letting the foal suckle may teach the mare that nursing is pleasurable and encourage her to let the foal nurse without restraint. Avoiding any disturbances while the mare is nursing the foal is paramount for successful nursing. Any evidence of mastitis or retained placenta should be addressed. For an aggressive mare, appropriate restraints such as a barrier or tying should be considered. Feeding the mare treats while the foal nurses can help desensitize her. In some cases, stimulating maternal behavior by separating the mare from the foal or faking a threat to the foal (eg, other horses, dogs) can help. Medications such as acepromazine, xylazine, diazepam, and progestins may help; however, these drugs can enter the milk and affect the foal.

Last full review/revision May 2014 by Gary M. Landsberg, BSc, DVM, MRCVS, DACVB, DECAWBM; Sagi Denenberg, DVM, DACVB, Dip. ECAWBM (Behaviour), MACVSc (Behaviour)

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