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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). [1]
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. [1]
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. [1] Researchers have found that EMND is associated with severe vitamin E deficiency. [2]
EMND is a serious condition and over half of horses diagnosed with it are euthanized within three months of diagnosis. However, 45% of affected horses respond to Vitamin E treatment. [2]
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. [3]
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. [1]
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. [2] 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. [4]
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. [3]
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. [3]
Symptoms of EMND include: [3][5]
- 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
- Inability to engage the hind leg stay apparatus
- Raised tail head
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. [5]
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). [3]
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. [6] Researchers aren’t sure if VEM is a separate disease or if it is the early stages of EMND. [1]
Signs of VEM
Clinical signs of VEM include: [3][6]
- Loss of muscle mass
- Toe dragging
- Poor performance
- Weakness
- Muscle twitching
- Difficulty chewing
Treatment of VEM
The good news is that the atrophy experienced by horses with VEM is often responsive to Vitamin E supplementation. [1]
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. [3]
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. [1]
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. [2]
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 newborn foals.
NMD is caused by a dietary deficiency in selenium that begins in utero when the dam consumes a selenium-deficient diet. [3]
In some horses with NMD, there is also a Vitamin E deficiency, but this isn’t always the case. However, selenium and vitamin E seem to work together to prevent nutritional myodegeneration. [3][7]
Signs of NMD
Symptoms of NMD include: [7]
- Laboured breathing
- Rapid, irregular heartbeat
- Trouble swallowing
- Muscle stiffness
- Trembling
- Firm muscles
- Difficulty getting up
- Excess of myoglobin in the urine
- Sudden death if the myocardium is involved
Aspiration pneumonia is also a frequent complication associated with NMD. [7]
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. [7]
4) Myosin Heavy Chain Myopathy (MYHM)
Myosin Heavy Chain Myopathy (MYHM) (formerly known as Immune-mediated myositis) 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. [8][9]
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. [8][9]
Recently, a genetic mutation was discovered in horses with IMM. Genetic testing is now available to diagnose this condition in breeds that may be at risk. Early identification is important for stopping muscle loss with this disease. [1]
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. [1][9]
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. [1]
5) Type 2 Polysaccharide 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. [1]
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. [1]
Type 2 PSSM is usually diagnosed via muscle biopsy. [1] If you have your veterinarian do blood work, CK and AST levels are often elevated as well. [1]
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