Neuromuscular Diseases

In this article I will explore the interrelationships between three disease processes: Equine Polysaccharide Storage Myopathy (EPSM), Stringhalt and Shivers. These three syndromes have been seen more frequently over the past 10 years. Shivers may be a new disease for some of you, but it has been present in Draft horses for quite some time. The Drafts still have issues with this, but its presence in Warmbloods is our greater concern. The gait abnormalities associated with this disease are particularly problematic for dressage horses, but even such routine procedures as farriery and the daily picking out of hooves can be affected by it.


EPSM is a metabolic condition related to skeletal muscle dysfunction. The primary issue is an inability to properly metabolize carbohydrates from feed. Attempting to make dietary changes and maintaining a schedule of regular exercise seems to be the most effective methods to improve these horses. This is a lifelong condition and altering the carbohydrate to fat ratio will be a permanent change.

Potentially any type of horse could develop this condition, with the highest incidence involving Warmbloods, Draft-related breeds, Quarter Horses and Arabians. It is thought to be an inherited disorder, but due to variability in its onset this may not seem clear.

There are a variety of signs reported thus making it difficult to conclusively diagnose this based on history and signalment. Clients have reported: an onset of weakness, poor performance, an inability to move forward, back soreness, gait abnormalities with or without a lameness, attitude problems, poor muscling, decreased impulsion and general stiffness. From this list we could probably come up with 30 or more reasons for these signs. What also makes this difficult is that a horse can be progressing normally in a training program then some of these signs can develop subtly. Over time the gait abnormality when recognized may not be clearly linked to any particular change in the horse's diet, work or routine. An actual lameness may develop from a slight stiffness to an actual lameness that involve one or both hind limbs. From behind the gait has been described as a "goose waddle". One of the more obvious horses I saw was a 4 year old Draft gelding that walked out of the stall stiffly and would trot, but appeared stiff and lame in both hind limbs. There was not muscle loss, but they were not developing well and they seemed remarkably tight and sore for the limited work the horse was doing. The lameness and sometimes vague signs make it difficult to distinguish EPSM from other causes of lameness, including Shivers and Stringhalt plus different neurologic conditions.

Attempting to diagnose EPSM with laboratory work is a worthwhile pursuit, but it still may elude an accurate diagnosis. The primary blood test involves looking at levels of muscle enzymes before and after exercise. Muscle enzymes will increase with exercise and the higher the rise the more likely that muscle inflammation is occurring. Unfortunately not every horse will exhibit the type of elevation necessary to diagnose this from bloodwork. A second method is by doing a muscle biopsy, which allows a pathologist to make the diagnosis based on a microscopic examination of the muscle tissue. The procedure is straightforward and well tolerated by the horse. Since this is somewhat of an invasive procedure most clients opt to treat rather than to biopsy. In order to confirm the diagnosis and to get a sense of the degree that EPSM is involved in the unsoundness, biopsying is essential.

A new genetic test has become available that evaluates DNA from hair or blood to determine if the horse carries the gene for what is being called Type 1 EPSM. The Type 2 - the more common form is only diagnosed presently through a muscle biopsy.

This horse demonstrates a more extreme example of stringhalt. Typically the involuntary snapping up of the hindleg may not involve the whole body as it does in this case.
This horse demonstrates a more extreme example of stringhalt. Typically the involuntary snapping up of the hindleg may not involve the whole body as it does in this case.


This gait abnormality shares some of the signs seen with EPSM, yet there are a few things that make it unique. Potentially any breed can be affected by this condition and there is no age or sex predilection. The classic sign is described as an exaggerated flexion of the hindlimb that occurs during the walk stride. The flexion can be quite dramatic as the front of the fetlock can actually contact the horses abdomen. Once the horse continues moving and gets into trot and canter gaits the signs may dissipate altogether. It can be seen in one or both limbs. Horses with both legs involved generally have one leg more obviously affected than the other. In addition most horses with this syndrome seem to be affected more during the cold weather and stress. It may show up in the walk or when you first pick up a hindlimb and most horses improve even after a few minutes of walking. The condition is frequently noticed when picking out the feet and when the farrier works with the horse. While starting to back a horse an abnormal flexion may be shown which could be consistent with either Stringhalt or Shivers.

There are several potential causes and some type of neuropathy - a nerve dysfunction is the most common. There are times when this neuropathy develops for no apparent reason and other times a traumatic event may be linked. I know of several horses where this developed after trauma to the area above and to the side of the hock as well as in the upper cannon bone region. Some type of contusion may have resulted in a permanent change in the ability of a nerve to properly conduct impulses or for a tendon attached to a flexor muscle to function properly. In the Northwestern U.S., parts of Europe and Australia the disease has been linked to a toxic agent in a plant. If Stringhalt can be linked to plants then the goal is to remove the horse as soon as possible from that location. Unfortunately toxicity is not a cause in the New England area and our attempts to treat and manage this condition are limited. I will cover medical options at the end of this discussion.

Draft horse exhibiting the classic stance for shivers. The leg may only go about this high before it starts to shake or quiver. Often this occurs when someone reaches down to touch the leg and asks for it to be held off the ground.
Draft horse exhibiting the classic stance for shivers. The leg may only go about this high before it starts to shake or quiver. Often this occurs when someone reaches down to touch the leg and asks for it to be held off the ground.


Initially this condition was diagnosed exclusively in Draft breeds, but it has been seen with an increasing frequency in Warmbloods. Perhaps because of the similarities with respect to abnormal gaits, some of the horses previously diagnosed as being affected with Stringhalt may in fact be Shivers horses. In the Veterinary literature there have been reported cases in other breeds, however in our practice the Warmbloods have been affected the most.

There may be several signs associated with this syndrome. The classic abnormality is an exaggerated flexion of the hindlimb, where the limb may be held in the flexed position and quiver for 10-30 seconds. The shaking is different from Stringhalt which is a quicker more spastic type of movement. In addition to the quivering the leg may be held somewhat away from the body when it goes into flexion. The tail may be partially raised during one of these episodes. On occasion flickering of the eyelids and ears may occur along with a stretching or flexing motion of the neck. I frequently find they appear to be neurologically impaired yet when we do a neurologic exam with positional responses and reflexes these horses are normal. The signs may be evident in numerous situations: turning in a tight circle at a walk, starting to trot, walking backwards and when an attempt is made to pick up a hind foot. Some of these horses seem to have an inability to pick up the limb when asked and if they do "release the limb" and allow it to be held then the quivering will be seen and some of these horses may require using a wall to balance themselves on the opposing limb.

Initially the gait abnormality may consist of an unevenness in a hind limb that may progress to a "hitch". Over time this could progress to involve both hindlimbs with the flexion abnormalities as previously described. Since the horse may have functionally normal gaits beyond the walk the syndrome may be misdiagnosed as Stringhalt, EPSM or an intermittently locked stifle. A hitch in the stride could run the gamut for any cause for a hindlimb lameness.

Determining a cause for this syndrome has taken researchers in several different directions with a neuropathy being considered likely. Attempts to prove that a neurologic component is present has yet to be determined. Instead affected horses share some of the same metabolic issues that affect those with EPSM. In my opinion due to the signs occasionally seen affecting the head and neck coupled with gait abnormalities, I am still not convinced there isn't an underlying neurologic process involved.

Treatment Strategies

As previously discussed diagnosing these conditions may be difficult and treatments can be just as confusing. Generally attempts to improve affected horses involve dietary manipulation since metabolic issues play an important role in EPSM and Shivers. Only Stringhalt is amenable to a surgical procedure and success rate is generally in the 70% range. Since Stringhalt may not affect a horse's gait enough to warrant surgery most people elect to live with this condition. In the past, I have attempted using Vitamin E supplementation as a means of management, but at this point I am not convinced that this will offer many advantages.

Dietary recommendations for the two metabolic diseases (EPSM and Shivers) are designed to move the horse away from a high carbohydrate diet and to provide approximately 20-25% of total daily calories from fat. Most horses are receiving a sweet feed or pelleted ration that contains high levels of carbohydrates. Although some of the newer feeds are increasing fat content up to 10-12% there has not been a significant change in the carbohydrate content. Some horses are supplemented with vegetable oils, ( corn, cocosoya or soy ) high fat feeds and supplements such as Rice Bran. Still in order to modify the dietary content of the feed there will need to be substantial increases in overall fat content beyond what most people routinely feed. Much of the work done investigating these conditions has been done by Dr. Beth Valentine and recommendations are based on her research.

A change in a horses diet to one that contains these high fat levels is safe and may be used as a "test" to evaluate if a horse is a candidate for these type of syndromes. It may take 1-2 weeks to make the necessary changes some of this depends on the horses current diet, body condition and acceptance of the addition of oil. For an average 1000 pound horse one simple diet is to replace grain with an equal quantity of alfalfa pellets and gradually add vegetable oil until the horse is eating 2 cups of oil per day. The alfalfa was suggested as an adequate source of protein, calcium and Vitamin E with good palatability. Alfalfa hay can also be added to the mixture. For horses that are not accepting of this diet feeding a high fat feed instead of the alfalfa pellets is an option: Purina Athlete, Purina Strategy, Blue Seal Demand or Blue Seal Hunter. If the oil is a factor the palatability can be improved by using Cocosoya oil, which is a blend of coconut and soy oil. The two oils that should not be used are linseed and flax seed. Rice Bran which is 20% fat can be used to supplement or to replace some of the oil. The conversion of the rice bran to an equivalent amount of oil is as follows: 5 pounds of rice bran provides the same amount of fat as 2 cups or 1 pound of oil. Feeding with rice bran as the primary fat source will substantially increase costs and not reduce the carbohydrate or starch content as much as desired. Dried fat sources are also available and provide an alternative to the oil. These products weigh half of their volume so twice the volume will be required. You would start with at least 4 cups of dried fat per 1000 pounds of horse's weight. Combining dried fats, oils, rice bran and/or high fat feeds can be used to achieve similar results. The difficulty lies with the feeds that are reportedly high fat. The actual % of fat may be 10-12%, but what they do not report is the carbohydrate %. As more feeds are used in the feeding regime the potential for throwing off the carbohydrate to fat ratio exists. For example, a feed with 10% fat would require feeding up to 10 pounds in order to achieve 1 pound of fat from the feed. Obviously this will be too much grain and by increasing this in order to help manage fat content you will undoubtedly significantly increase carbohydrate levels. Therefore the use of oils as a 100% fat source is the safest and most efficient means to get the desired results. The diets discussed here are designed for horses weighing approximately 1000 pounds which is about the weight of the average Thoroughbred or Quarter Horse. A Warmblood may weigh closer to 1200-1300 pounds and the feed should be increased accordingly.

The minimum caloric requirements for a 900-1000 pound horse is 10,000-12,000 calories per day. As the horse's work regime increases so will its demand for calories. The high-fat oil-based diet provides 16,000 calories per day. One cup of oil contains 2,000 calories, and in this diet the 2 cups will provide 4,000 calories, which is 25% of the total calories. If a horse gains too much weight or is already heavy then it may be necessary to decrease grain and/or access to grass, plus increase the work schedule.

Feeding extra sources of vitamins and minerals is generally not a bad idea if a general supplement is needed. However I feel particular attention should be given to Vitamin E which should be fed at higher levels. Generally we suggest giving the horse at least 1 unit of Vitamin E for each pond of the horse's weight. In my opinion these horse do better with at least 2-5 times that amount. I recommend using E-5000 sold through Vita Flex that contains 5000 units per measure. If you are already feeding a Vitamin E-Selenium supplement we don't want to use this to increase Vitamin E as there is a risk of toxicity if Selenium is increased beyond the 1 mg per 1000 pounds of weight.

If this type of oil-based diet is chosen it may be necessary to increase oil every few days by 1/4 increments to not throw off palatability. Results may take up to 4 months in order to realize full fat adaptation. Some horses have exhibited improvement in as little as 1 month with an improvement in gaits, attitude and energy. Over time improvement in the horse's musculature is to be expected. Potential set backs have occurred and may be associated with variables such as access to grass or hay. There can be a significant increase in grass carbohydrate content especially in the spring and after the initial onset of cool weather in the fall as grasses will concentrate sugars. Paying particular attention to the pastures and hay, such as first versus second cut will help. see article on Feed Information


Valentine, BA Mechanical Lameness in the Hindlimb. In Lameness in the Horse Ross, MW and Dyson, SJ St. Louis, Mo. 2003

Valentine, BA Diagnosis and Treatment of Equine Polysaccharide Storage Myopathy Journal of Equine Veterinary Science 2005; 25:52-61.