Muscle loss in horses is a worrisome problem and one that needs to be investigated promptly. This condition is also known as muscle atrophy or muscle wasting, and is defined as a decrease in muscle mass below normal levels.
Loss of muscle tissue can be restricted to one area or generalized over the horse’s entire body. It can be caused by injury to motor nerves attached to muscles (neurogenic) or due to direct damage or atrophy of muscle fibers (myogenic). 
Muscle loss may occur gradually over time but it can also happen quickly. Sudden atrophy usually indicates a serious disease or injury, so immediate veterinary attention should be sought.
Several different underlying conditions cause muscle loss in horses, ranging from changes in the horse’s diet, management routines, vaccinations, as well as several diseases. If your horse has a normal appetite, is in full training, and continues to lose muscle mass, consulting your veterinarian is warranted. 
Top 11 Causes of Muscle Loss in Horses
The following are some of the most common underlying causes of muscle wasting in horses, some of which are more serious than others:
Once you have identified the root cause of muscle atrophy, work with your veterinarian, equine nutritionist and trainer to develop a treatment plan to support increased muscle mass.
1) Equine Motor Neuron Disease (EMND)
Equine motor neuron disease (EMND) is a neurodegenerative disorder characterized by obvious muscle weakness and irreversible muscle atrophy.  Researchers have found that EMND is associated with severe vitamin E deficiency. 
Causes of EMND
Vitamin E (alpha-tocopherol) is an antioxidant which protects cells from damage caused by free radicals. This vitamin is also associated with maintaining normal muscle function and motor nerve signals. 
Fresh green pasture is the primary source of Vitamin E for horses. Due to a lack of pasture access and an increase in stabling, many horses can experience a Vitamin E deficiency. While a short-term deficiency may not cause a problem, prolonged deficiency can result in EMND. 
Horses that are not in hard work need about 500-800 IU of Vitamin E per day. Some Vitamin E can be supplied by feeding good-quality grass hay fed at a minimum of 1.5% of body weight.  However, hay alone cannot provide all the Vitamin E a horse needs.
While fresh forage is a good source of this vitamin, as soon as grass is cut to make hay, the Vitamin E content begins to rapidly degrade. Depending on forage quality, storage conditions, drying method and time since harvesting, Vitamin E content can be between 20-90% lower in cut hay. 
Development of EMND and other neuromuscular diseases appears to depend on the age of the horse when the deficiency occurs, duration of the deficiency, the horse’s individual genetics, as well as other existing dietary deficiencies or excesses.
Signs of EMND
Diagnosis of neuromuscular disorders, such as EMND, requires testing to determine Vitamin E status, as well as clinical signs, history, and elimination of other possible diagnoses. 
A blood sample is the easiest way for a veterinarian to assess if a horse has a Vitamin E deficiency. Normal alpha-tocopherol concentrations should be between 3-10 ug /ml. 
Symptoms of EMND include:
- Acute onset of trembling
- Standing with hind limbs close together
- Prolonged periods of laying down (recumbency)
- Continual shifting of weight in the rear legs
- Muscle atrophy, most noticeable in the quadriceps, triceps, neck and gluteal areas
- Lower than normal head carriage 
Horses with EMND often have considerable difficulty standing in a confined area for more than a few minutes and may show extreme distress if forced to do so. 
2) Vitamin E Responsive Myopathy (VEM)
Some horses with a Vitamin E deficiency develop signs solely related to muscle wasting and weakness without evidence of nerve damage. This is known as Vitamin E responsive myopathy (VEM). 
The condition usually presents by decreased performance and gradual muscle loss, but it may present suddenly with muscle weakness and trembling. Muscle atrophy appears to be related to oxidative stress in muscle fibers.  Researchers aren’t sure if VEM is a separate disease or if it is the early stages of EMND. 
Signs of VEM
Clinical signs of VEM include:
- Loss of muscle mass
- Toe dragging
- Poor performance
- Muscle twitching 
Treatment of VEM
Vitamin E supplements contain alpha-tocopherol, obtained from either natural or synthetic sources. Horse owners should be aware that synthetic alpha-tocopherol, which is often added to commercial feeds, may not be as easily absorbed by the horse. 
For horses with VEM, it is recommended to feed a natural form of a-tocopherol at 10 IU/kg body weight for at least one month. 
Keep in mind that low muscle concentrations of alpha-tocopherol in horses may not necessarily be due to Vitamin E deficiency in the diet. Researchers think VEM could also result from an individual abnormality in assimilating Vitamin E into the muscle tissue. 
3) Nutritional Myodegeneration (NMD)
Another neurodegenerative disorder that can result in muscle loss is nutritional myodegeneration (NMD). This condition is commonly referred to as white muscle disease and affects skeletal or cardiac muscle of rapidly growing, active foals.
NMD is caused by a dietary deficiency in selenium that begins in utero when the dam consumes a selenium-deficient diet. 
Signs of NMD
Symptoms of NMD include:
- Laboured breathing
- Rapid, irregular heartbeat
- Trouble swallowing
- Muscle stiffness
- Firm muscles
- Difficulty getting up
- Excess of myoglobin in the urine
- Sudden death if the myocardium is involved [6 ]
Aspiration pneumonia is also a frequent complication associated with NMD. 
Treatment of NMD
Treatment for NMD includes selenium and Vitamin E therapy, given intramuscularly, intra-venously, or through oral supplements. Supportive treatments may include antimicrobials, feeding through a nasogastric tube, and maintaining fluid and electrolyte balance. 
4) Immune-mediated myositis (IMM)
Immune-mediated myositis (IMM) affects Quarter Horses and related breeds, usually between the ages of 8-17. This condition leads to atrophy in the lumbar (back) and gluteal muscles, commonly referred to as the horse’s topline. The muscle atrophy is usually accompanied by stiffness and general malaise. 
With IMM, atrophy occurs rapidly and can involve a loss of 40% of gluteal and back muscle mass within a 48-hour time period. The atrophy can last for months, and severely affected horses may also have weakness and frequent periods of recumbency. 
Recently, a genetic mutation was discovered in horses with IMM. Genetic testing will soon be available to diagnose this condition in breeds that may be at risk. Early identification is important for stopping muscle loss with this disease. 
Treatment for IMM is dexamethasone or prednisolone – steroid medications which can stop muscle atrophy. In some cases, steroid treatment is combined with antibiotics if an infection is present. For some horses, muscle mass recovers without treatment within 2-3 months. 
Unfortunately, recurrence of muscle atrophy is seen in at least 40% of horses with IMM. If this happens, the horse will need additional corticosteroid treatment. 
5) Type 2 Polysaccaride Storage Myopathy (PSSM)
Type 2 Polysaccharide Storage Myopathy (PSSM) is another muscle condition that affects certain horse breeds such as Quarter Horses and Warmbloods. It is common in high-performance Quarter Horses as well as pleasure and halter horses. 
Both type 1 and type 2 PSSM results in abnormal glycogen storage in muscle. However, unlike type 1, researchers are not aware of a genetic mutation that causes type 2. 
In Quarter Horses and light breeds, chronic tying up is the most common sign of type 2 PSSM. Horses with this condition may show signs of sore muscles, undiagnosed lameness, poor performance, reluctance to go forward under saddle, reluctance to collect, and sometimes, slow onset of topline atrophy, especially if taken out of work.
For horses with PSSM, it’s important to feed a forage-based, low-sugar and starch diet and meet additional energy requirements with dietary fat. Amino acid supplements are often recommended for horses with topline muscle atrophy.
6) Equine Myofibrillar Myopathy (MFM)
Myofibrillar myopathy (MFM) is a newly discovered disease that affects Arabian and Warmblood horses more than other breeds.
Previously, these horses may have been mistakenly diagnosed with type 2 PSSM. However, MFM differs from PSSM in that it is marked by physical changes in muscle cells that involve contractile proteins called myofibrils. 
In Arabians, especially those competing in endurance, the most common sign of MFM is intermittent tying up. The severity of these episodes are often much milder than those that occur with exertional rhabdomyolysis (rapid dissolution of muscle). 
In Warmbloods with MFM, there may be a decline in performance as they reach 8-10 years of age. Tying up is very intermittent or nonexistent in these horses.
More common is an unwillingness to move forward, vague hindlimb lameness, sore muscles, and decreased energy. These horses may also be reluctant to collect and engage their hindquarters and have a slow onset of muscle wasting, especially when out of work. 
It should be noted that MFM is much less common than other types of immune-mediated muscle disorders in horses. 
7) Equine Protozoal Myeloencephalitis (EPM)
Equine Protozoal Myeloencephalitis (EPM) is a disease of the central nervous system. It is caused by the microbe, Sarcocystis neurona, commonly found in opossums.
EPM can affect different parts of the horse’s brain and spinal cord, and the severity can range dramatically between horses. 
Signs of EPM
There are three main symptoms of EPM:
- Asymmetry: symptoms that are worse on one side of the body than the other.
- Ataxia: incoordination which results in the horse having trouble moving its legs and body normally.
- Atrophy caused by damage to the nerves that control the muscles. However, atrophy isn’t seen as commonly with EPM as asymmetry and ataxia are. 
It is also common to see muscle weakness in horses with EPM. Some may have an abnormal gait, lameness, or possibly loss of sensation along the face, neck or body. Paralysis of muscles around the eyes, face, or mouth can also occur, often appearing as droopy eyes, ears or lips. 
Horses with EPM may also have a head tilt or difficulty swallowing or vocalizing. Rarely, seizures or collapse can occur. EPM progresses differently in every horse, but most horses with EPM will continue to have a bright, alert temperament. 
Treatment of EPM
EPM is diagnosed through blood tests or a spinal tap. There are two standard treatments for this disease:
- A six-month course of antibiotic (trimethoprimsulfonamide) and anti-protozoal (pyrimethamine)
- A 28-day course of anti-protozoal (ponazuril). However, some horses may need a second round of this treatment. 
Horses with EPM also need general supportive care.
8) Traumatic Nervous System Injury
Occasionally, horses can experience injury to their peripheral nervous system. This happens through blunt trauma or penetrating injuries.
Symptoms of this type of injury in horses include an abnormal stance or gait, reduced sensation along the skin, and muscle atrophy. 
Horses with traumatic nervous system injury may develop rapid muscle atrophy, continuing until the nerve heals, which may take weeks or months. Muscles affected can lose approximately half their mass within two weeks after the injury. 
9) Amino Acid Deficiency
All horses need adequate protein in their diet. Protein is made up of molecules called amino acids, some of which can be produced by the horse’s body and some of which are essential and must be consumed in the diet. 
Adult horses at maintenance need anywhere from 540-720 grams of protein per day. Fortunately, many horses can get all the protein they need from good-quality grass or hay alone. 
Working horses need between 699-1004 grams of protein per day, depending on the amount of work they are engaged in. These horses need more protein to help build and repair muscle. Exercising horses usually need supplemental protein from legume hay, concentrates, or protein supplements. 
In addition to meeting baseline protein requirements, horses also need to obtain adequate amounts of individual essential amino acids to support muscle protein synthesis.
Researchers believe that lysine and threonine are the most limiting amino acids in the equine diet. These are the amino acids that horses are most likely to be deficient in, resulting in impaired protein synthesis.
10) Pituitary pars intermedia dysfunction (PPID)
PPID (commonly referred to as Equine Cushing’s Disease) is one of the most common endocrine disorders in older horses. Muscle atrophy is a common sign of this disease, occurring in 50% of all PPID horses.
Other signs of PPID include:
- Excessive hair growth
- Weight loss
- Secondary infections
- Regional fat deposits (i.e. cresty neck)
- Excessive thirst and urination
- Excessive sweating
- Neurological signs. 
Researchers believe that muscle atrophy in PPID horses is related to decreased protein synthesis or increased protein degradation.
Sarcopenia (age-related progressive muscle loss) is a condition that frequently occurs as horses get older. It is usually observed in muscles along the topline, hindquarters, and forearm. The backbone and hip bones of horses with sarcopenia may become more prominent as well.
Horses no longer being exercised may be more prone to this condition.  Older horses that haven’t been properly nourished or that have PPID are also more prone to sarcopenia.
Though researchers don’t fully understand why sarcopenia develops, they do know that there is a reduction in the regenerative capacity of muscles in aging animals and people. 
Sarcopenia may not be reversible, but feeding good quality protein and senior feeds that are more easily digested can help to slow down muscle loss. Providing plenty of turnout and light exercise can also improve muscle tone in older horses.
Evaluating Muscle Loss in Horses
Horse owners should regularly monitor muscle mass of performance and leisure horses. Muscle loss should be investigated at an early stage when treatment will be most effective. 
Your veterinarian will evaluate your horse by having them stand square. This will reveal whether muscle loss is symmetric (affecting both side) or asymmetric (affecting one side). Horses with muscle weakness tend to have a camped under stance and may shift weight in their hind legs. 
Your veterinarian will also likely palpate muscle for tone, atrophy, and pain, and complete a full physical and neurological examination as well.
Researchers have recently developed a muscle atrophy scoring system (MASS) to evaluate muscle loss in horses. This is important for athletic horses because even a minor reduction in muscle mass can affect their performance. MASS evaluations show promise when applied to the neck, back, and hindquarter regions. 
Other useful methods for determining the cause of muscle atrophy in horses include serum or plasma Vitamin E concentration tests, genetic testing for type 1 PSSM and IMM, and muscle biopsy for VEM and EMND. 
Treating Muscle Loss in Horses
Treating muscle loss in horses will depend on the specific cause of the problem, but for many conditions, attention to diet and exercise is key.
Sarcopenia (loss of muscle) cannot always be prevented, but younger horses with muscle loss benefit from amino acid supplementation. Protein supplements for horses should provide the amino acids, lysine, methionine and threonine, commonly deficient in the equine diet.
Feeding a digestive health supplement can also help horses better utilize the nutrients they are obtaining in their diet. Consider adding a probiotic supplement with yeast to your horse’s diet to improve feed efficiency.
Also, ensure your horse’s diet is balanced, and there are no vitamin or mineral deficiencies, which can impact muscle protein synthesis.  Obtain a hay analysis and feed a balanced vitamin and mineral supplement to address common deficiencies.
Oxidative stress also contributes to muscle and tissue degeneration, especially in horses that do not get adequate antioxidants in their diet. Vitamin E is an essential antioxidant that is commonly deficient in equine diets for horses receiving cut hay. Have your veterinarian test serum Vitamin E levels and feed a supplement if levels are below 3 ug/ml. 
Routine exercise helps to strengthen your horse’s muscles and improve muscle mass. Exercise increases muscle protein turnover, improves aerobic capacity, boosts stamina and supports overall conditioning.
If your horse is losing muscle mass, ask your veterinarian about introducing appropriate exercise to support muscle growth. Exercise may not be appropriate for all horses, including horses that need stall rest to facilitate recovery from an injury.
For horses with conditions such as MFM, regular exercise combined with stretching and long, slow warm-ups is important. 
If your horse is experiencing muscle loss of any kind, evaluate recent dietary or management changes and get your veterinarian involved, especially if other symptoms are involved.
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