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Muscle Atrophy in Horses

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Muscle Atrophy in Horses
Muscle Atrophy in Horses | Innovet Pet

As a muscle of a horse reduces in size, seemingly wasting away, it is well-known as muscle atrophy. As this sudden loss of muscle mass happens, the main concerns to a horse owner are if the muscle is going to regenerate, returning to its regular shape and size, and how to treat him to help him recover from this muscle wasting and atrophy disease.

Muscle-Wasting in Older Horses

As horses grow older, their bodies transform, oftentimes in ways owners do not expect. For example, muscular mass noticeably changes enough in some horses that specific areas of their bodies take on entirely different shapes.

According to a Kentucky Equine Research vet, Laura Petroski, B.V.M.S., muscle-wasting or muscle disease in horses, also referred to as sarcopenia, is typical in aged horses and defines generalized muscle loss which slowly results in weakness. Sarcopenia might be associated with “inflammaging,” a low-grade inflammatory state related to old age which makes horses more prone to disease and infection.

Sarcopenia is more frequently seen in larger muscle groups, like the ones alongside the topline and over the horse’s hindquarters, as well as the ones of his forearm. The vertebral chain might seem more prominent than normal, as might the horse’s hip bones.

Petroski adds that this neck atrophy condition usually occurs in older horses which aren’t exercised, incorrectly nourished, or have been diagnosed with PPID (pituitary pars intermedia dysfunction). Horses with neurological disease occasionally have muscle atrophy in horse’s shoulder, yet the collection of medical indications that neurological horses exhibit varies from sarcopenia.

In order to offset the sarcopenia’s development, aged horses ought to be given quality diets which align with their metabolism and workload. Because protein isn’t synthesized as effectively in older horses as in younger ones, it’s vital that older horses have accessibility to quality protein, which can be defined as protein containing a variety of essential amino acids.

High-quality forage along with a balancer pellet or a senior feed are good starting points for a sound diet. Depending upon the horse’s dental health and age, forage might be supplied in traditional ways, as hay or pasture, or as alternative forage sources, like hay pellets or Equine Hemp PCR - CBD Pellets for Horses.

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Petroski claims that a complete diet analysis offered by an equine nutritionist is going to ensure that all nutrient requirements are met, and all inadequacies are addressed.

Besides a fortified, well-rounded diet, supplementation with muscle- and immune-supporting nutrients might help animals with sarcopenia and inflammaging.

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Specific long-chain omega-3 fats, in particular, EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), decrease body-wide inflammation, as well as strengthen the horse’s immune system. In selecting a supplement that has EPA and DHA, search for a marine-derived source, such as fish oil, a direct essential omega-3 source. One proven EPA and DHA source is EO•3, developed by KER.

Vitamin E is a known antioxidant which supports immune integrity. Similar to omega-3 fatty acids, not every vitamin E supplement shares the same bioavailability. Various horse studies have exhibited that natural vitamin E is more efficient than the synthetic form. Beyond this, how vitamin E is generated matters. A natural-source vitamin E,  Nano•E, utilizes nanodispersion technology and liposome encapsulation to confer rapid bioavailability. Vitamin E is particularly critical for horses which don’t have accessibility to green, fresh grass. Preserved forages, even as they’re made underneath ideal conditions, go through vitamin degradation in storage; therefore, it’s crucial to supplement using natural-source vitamin E.

An additional nutrient which supports muscle health is a mineral called selenium, which also has vital antioxidant properties. The antioxidant supplement Preserve PS has natural vitamin E, magnesium, vitamin C, and selenium.

Besides nutrition, owners of sound, old horses may utilize exercise to boost health and fitness. By itself, age doesn’t preclude the horse from interacting in physical activity, and an exercise plan may keep the horse from developing muscle loss and might stave off metabolic problems.

If soundness impedes intense or moderate exercise, horses may become and stay fit through low-speed work, like slow trotting, walking, as well as tackling hills.

Work with a vet and farrier to assist in devising a suitable fitness plan which comfortably meshes with soundness and make certain to inform your nutritionist of any changes in workload.

Getting a complete history and doing a full lameness, neurologic, and physical exam are crucial as muscle atrophy is identified, according to Stephanie Valberg, D.V.M., Ph.D., of University of Minnesota College of Vet Medicine within her presentation at Texas Equine Vet Association conference in 2011.

The history ought to include all falls, injuries, medications, vaccinations, environmental issues, foaling records, and signs of disease.

Valberg adds that from the historical data, it ought to be apparent whether there’s focal muscle atrophy, generalized, or multifocal atrophy; whether your horse is losing muscle mass in spite of a healthy appetite; and whether there’s any connection between lameness and atrophy, back or neck pain, concurrent disease or trauma.

The neck muscle atrophy physical exam includes muscle mass palpation, muscle mass symmetry observation, and noting any pain, heat, swelling, or other irregularities in the muscles. Valberg claims that a full lameness exam should be done and explained that any indication of neurologic disease ought to be followed by a thorough neurologic examination.

Offset Sarcopenia development in Horses

Is Muscle Atrophy Permanent?

As far as prognosis and atrophied muscles treatment options are concerned, it’s oftentimes useful to determine if atrophy is caused by myogenic atrophy (wasting of the muscle cells) or wasting because of damage to the nerves that supply the muscle.

She said vets frequently use one or more of these muscle atrophied tests to help in treatment options and prognosis:

Diet inspection

The type and amount of feed the horse actually is eating and quality of green pasture, that contains vitamin E, ought to be determined when muscle atrophy results from diet.

Muscle enzymes

Oftentimes, myogenic atrophy is related to elevations in serum CK (creatine kinase) activity and if rhabdomyolysis is over 6 to 12 hours long, elevations in AST (aspartate transaminase) and LDH (lactate dehydrogenase) also may be present.

Managing and Feeding to Decrease Recurrent Exertional Rhabdomyolysis 

Numerous horses, mainly fillies, have recurrent muscle cramping (rhabdomyolysis) episodes even with light exercise. RER (recurrent exertional rhabdomyolysis) is witnessed in several horse breeds, which includes Paints, Quarter Horses, Thoroughbreds, Appaloosas, Standardbreds, Arabians, and Morgans.

A broad array of causes for recurrent exertional rhabdomyolysis have been proposed, which includes hormonal imbalances, electrolyte imbalances, lactic acidosis, and selenium and vitamin E deficiencies. Specific RER causes were identified in the horse, which includes excitation contraction-coupling or a disorder of muscle contractibility.

Recurrent exertional rhabdomyolysis is a typical occurrence in Standardbred, Arabian, and Thoroughbred horses. It mostly occurs in younger fillies that have a nervous disposition.

Around 5 percent of Thoroughbred racehorses develop recurrent exertional rhabdomyolysis within the racing season, oftentimes when they’re trained at a gallop yet held back from full racing speeds.

A few highly vulnerable individuals have repeated episodes, which results in persistent elevations in serum AST (aspartate aminotransferase), CK (creatine kinase), as well as poor performance. Most of those fillies are retired to broodmare careers.

Within other vulnerable horses, episodes might be extremely intermittent, and in turn, the phrase chronic intermittent rhabdomyolysis was utilized to describe the syndrome.

Stress and period of stall rest before exercising seem to trigger recurrent exertional rhabdomyolysis in vulnerable horses. Research of horse lymphocyte antigens offer a bit of support for a familial basis for recurrent exertional rhabdomyolysis in Standardbreds.

A chronic intermittent rhabdomyolysis diagnosis is based upon the clinical signs and history as well as observed elevations in CK and serum AST. Muscle biopsy findings within affected horses involve various phases of muscle necrosis, as well as regeneration along with centrally located myonuclei.

Lactic acidosis previously was thought to cause recurrent exertional rhabdomyolysis, and several treatments still utilized these days are directed at resolving lactic acidosis.

However, studies have shown that recurrent exertional rhabdomyolysis occurs more typically with aerobic exercise, and that within an episode, affected animals have low muscle lactate concentrations, as well as metabolic alkalosis.

Recurrent Exertional Rhabdomyolysis

Studies performed in England implied that a dietary sodium deficiency or low calcium: phosphorus ratio based upon urine creatinine clearance ratios may contribute to recurrent exertional rhabdomyolysis.

Subsequent research proved that most of the Thoroughbred racehorses that had chronic intermittent rhabdomyolysis possessed normal electrolyte ratios. More recently, an irregularity in excitation-contraction coupling was identified in Thoroughbreds and Standardbreds with recurrent exertional rhabdomyolysis.

The altered relaxation of muscle after a contractile twitch in impacted horses implies that irregular intracellular calcium regulation is an RER cause. Additionally, recent research proved elevated myoplasmic calcium concentrations within horses that have acute recurrent exertional rhabdomyolysis.

Prevention of further RER episodes in vulnerable horses ought to involve standardized day-to-day routines an environment which reduces stress. The diet must be adjusted to involve a balanced mineral and vitamin supplement, quality hay, and minimum of carbs like sweet feed and grain. Dietary fat supplements might aid in maintaining weight in nervous fillies without offering excessive carbs.

The usage of low acepromazine doses before exercise is thought to assist some excitable horses. Day-to-day exercise is critical, whether in the form of riding, longeing, or turnout. Horses, in the past, have been stall-rested for multiple weeks after an RER episode. It actually is counterproductive and raises the likelihood that the animals will develop recurrent exertional rhabdomyolysis while placed back into training. 

The original muscle pain typically goes away within 24 hours of acute recurrent exertional rhabdomyolysis and day-to-day turnout in a little paddock may be offered during this time. Subsequently, a slow return to performance is suggested as serum CK is inside normal range. In some horses, Dantrolene administered one hour before exercising is thought to be efficient at preventing recurrent exertional rhabdomyolysis.

Dantrolene is utilized to prevent malignant hyperthermia in human beings and swine by reducing the release of calcium inside muscle. Phenytoin also was advocated as a treatment for animals that have RER. Therapeutic levels differ; therefore, oral doses can be adjusted by observing serum levels to accomplish 8 ug/ml and not go over 12 ug/ml.

Phenytoin acts on numerous ion channels inside nerves and muscle, which includes calcium and sodium channels. Unfortunately, long-range treatment with phenytoin and dantrolene is costly.

Some animals that have recurrent rhabdomyolysis were discovered to suffer PSSM, or polysaccharide storage myopathy. It’s a glycogen storage disorder that is characterized by the accumulation of an irregular polysaccharide inside the muscle. To this date Paints, Quarter Horses, drafts, Appaloosas, warmbloods, draft crossbreds, and some Thoroughbreds were identified with polysaccharide storage myopathy.

Horses that have polysaccharide storage myopathy oftentimes have a sedate and calm demeanor. The majority of horses have a history of a number of episodes of tying up starting with training commencement; but mildly affected animals might only have a couple of episodes a year. Muscle atrophy, exercise intolerance, respiratory distress, and renal failure are less typical presenting complaints. Muscle enzyme elevations usually are discovered if blood samples are received and muscle enzymes might stay elevated for lengthy periods even while at rest.

The seriousness of rhabdomyolysis episodes may range from mild stiffness to serious pain that resembles colic in horses. Many horses were euthanized because of the seriousness of muscle damage. Diagnosis is based upon genetic testing or an examination of muscle biopsies. Distinctive features of those muscle biopsies are glycogen storage, subsarcolemmal vacuoles, and irregular PAS positive inclusions within fast-twitch fibers.

Oftentimes, muscle glycogen concentrations are 1.5 - 4 times normal. Frequently, serum CK activities are increased by 1,000 U/L or more four hours after fifteen minutes of exercise while at a trot.

Treatment of horses that have polysaccharide storage myopathy is based upon raising the oxidative capacity of skeletal muscle via slow training and offering a diet high in fat. The majority of polysaccharide storage myopathy horses have successfully competed as hunter and pleasure horses as their diets get changed to quality grass hay, no sweet feed or grain, and fat supplement. 

Some younger Quarter Horses recently were identified with polysaccharide storage myopathy which developed moderate to serious rhabdomyolysis without any related exercise. These younger halter horses were on a diet high in grain, and serum CK normalized while changed over to a lower carb ration.

Muscle Wasting Issues and Prognoses

Vitamin E concentration

This may be measured in serum samples; but, variability in serum levels may be very large and pooling of multiple samples often is suggested to accurately evaluate deficiency.


Diagnostic ultrasonography may be helpful in identifying the depth and extent of focal muscle atrophy and cervical facet lesions, as well as pelvic fractures, according to Valberg. She says that the look of muscle on ultrasound may change because of the way the animal is standing and if the muscle is under tension; therefore, it’s vital that the animal is standing squarely and evenly bearing weight.


Animals that have focal areas of neck muscle atrophy ought to have radiographs done of the cervical spine to search for spaces of osteoarthritis which may impinge on the neck motor nerves.

Nuclear scintigraphy

According to Valberg, this modality is helpful for identifying spinal and pelvic fractures which can’t be viewed on radiographs. These fractures may lead to damage to motor nerves close to the injury and produce neurogenic atrophy.

EMG (Electromyography)

Even though not readily obtainable in most vet practices, Valberg stated that EMG testing permits the extent of muscle atrophy to be assessed and offers details that may be diagnostic for the muscle atrophy cause. Horses that have abnormalities within the electrical conduction muscle system or denervation of motor units exhibit irregular spontaneous electrical activity.

Muscle biopsy

Assessment of muscle shapes, fiber sizes, and types within a biopsy sample may determine if regeneration will be expected or if there’s neurogenic atrophy which may respond well to therapeutic intervention like vitamin E, EPM (equine protozoal myeloencephalitis) treatment, or rehab therapy, explained Valberg, whose laboratory at the University of Minnesota provides many kinds of muscle biopsy services.

Myogenic atrophy

Valberg claimed that 1 to 5 percent of muscle mass goes through remodeling on a day-to-day basis. She said that if a negative nitrogen balance happens, net protein withdrawal from skeletal muscle masses begins within 48 - 72 hours. With malnutrition, 30 to 50 percent of the muscle mass might be lost within the initial 1 - 2 months. A few myogenic atrophy issues are caused by disuse atrophy, malnutrition, Cushing’s disease, secondary to serious rhabdomyolysis, immune-mediated myopathy, and PSSM.

Neurogenic atrophy

According to Valberg, denervation takes away the usual low-level tonic neural stimulus that’s needed to sustain muscle fiber mass. Full muscle denervation results in over a 50 percent muscle mass loss within a 2-3 week time period. Some neurogenic muscle atrophy in horses causes involve EMND (equine motor neuron disease); cervical vertebral osteoarthritis; EPM; post-anesthetic myoneuropathy; or pelvic, lumbar, or thoracic fracture.

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Options for Treatment

Valberg talked about some options for treatment which are obtainable for some of the issues causing muscle atrophy:

  • cervical vertebral osteoarthritis (corticosteroid injection of articular facets)
  • EMND (vitamin E)
  • EPM (ponazuril)
  • PSSM (exercise and diet modification)
  • Cushing’s disease (pergolide)

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Diagnostic Work-Up for Atrophy
Muscle Atrophy in Horses
Myopathies in Horses
Denervation Atrophy in Horses with Fibrotic Myopathy


Approved by:
Dr. Sara Ochoa
Doctor of Veterinary Medicine, St. Georges University

Sara Redding Ochoa, DVM was raised in north Louisiana. She graduated from LA Tech in 2011 with a degree in animal science. She then moved to Grenada West Indies for veterinary school. She completed her clinical year at Louisiana State University and graduated in 2015 from St. George’s University. Since veterinary school she has been working at a small animal and exotic veterinary clinic in east Texas, where she has experience treating all species that walk in the hospital. In her free time, she likes to travel with her husband Greg, bake yummy desserts and spend time with her 4-legged fur kids, a dog Ruby, a cat Oliver James “OJ”, a rabbit BamBam and a tortoise MonkeyMan.

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