Fibrotic myopathy occurs secondary to an injury of the semitendinosus or semimembranosus hamstring muscles, where the muscle cells start to join the connective tissue of the tendon attaching the muscle to the tibia.
“The muscle-tendon junction is the weakest part in the muscle and most susceptible to strain and tearing,” says Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR, of Michigan State University’s McPhail Equine Performance Center, in East Lansing. “This type of injury often occurs during exercise requiring sudden turns and sliding stops.”
You can spot affected horses just by the way they walk. Instead of the foot of the affected hind limb moving forward and touching the ground smoothly, it stops abruptly during the anterior phase (as the leg reaches forward) of the stride. This causes the leg to jerk suddenly to the ground, creating a characteristic hoof-slapping sound.
Myofibrillar myopathy (MFM)
Valberg and colleagues described this relatively new muscular disease in 2016.
“This condition involves disruption of the orderly alignment of myofibrils (muscle fibrils, the rodlike units of muscle cells),” she explains. “In normal muscle, the actin filaments are stabilized by a region called the Z disc and kept in alignment by another protein called desmin. In horses with MFM, a change in the Z discs disrupts the alignment of actin and myosin (myofibrils’ two main proteins) in some regions, and abnormal deposits of desmin accumulate in those breaks.”
This disruption can cause weakness and potentially inflammation. Horses with MFM can become stiff and sore with exercise.
“In Warmbloods MFM presents as an unwillingness to engage the hindquarters and to work in a frame.” Valberg says. “In Arabian horses MFM often causes muscle pain and stiffness after fit horses are rested for about two weeks and exercise is reintroduced.”
She adds that MFM appears to be a complex disease with strong diet, exercise, and potentially genetic components.
Horses can ingest the soil-dwelling bacterium Clostridium botulinum, which is capable of producing and releasing eight toxins in the gastrointestinal system, or they can ingest the toxins in forage. C. botulinum also can contaminate a wound, but the ingestion routes are more common. Once the toxin enters the bloodstream and reaches the neuromuscular junction, it prevents the nerve from releasing acetylcholine. This means the Na+ (sodium) channels located along the muscle cell membrane cannot open to initiate the cascade of events required for muscle contraction.
As a result, the signs of botulism include:
- Muscle tremors;
- Loss of tongue and tail control and an inability to hold food in the mouth;
- Weakness progressing to recumbency (down and unable to get up); and
- Paralysis of respiratory muscles and death.
Hyperkalemic periodic paralysis (HYPP)
One of the first genetic disorders discovered in horses, HYPP occurs in Quarter Horse-related bloodlines. Caused by a single point mutation in the gene that produces an Na+ channel protein (SCN4A), this mutation results in “leaky” Na+ channels.
“Specifically, high levels of potassium in the bloodstream change the electrical potential of the muscle cell membrane,” says Valberg. “This allows Na+ to more easily flow through the channel and make the muscle cell more likely to contract.”
HYPP horses show a range of clinical signs, from muscle tremors and third eyelid prolapse to weakness and/or collapse. Horses might breathe loudly due to paralysis of their respiratory muscles. Rarely, sudden death can occur.
“In its severest form, horses will dog sit and then go down on their side from weakness during a severe episode of HYPP,” says Valberg. “Horses are fully conscious; however, their muscles are paralyzed, and they are unable to move for 20 to 30 minutes.”
“Type 1 PSSM involves the abnormal accumulation of the storage form of glucose called glycogen, as well as an abnormal form of sugar or polysaccharide that cannot be broken down by the muscle cell,” says Valberg.
Type 1 PSSM results from a mutation in the gene that produces the enzyme glycogen synthase (GYS1), which is responsible for making glycogen. This GYS1 mutation results in an overactive enzyme that cannot shut off as it would in a normal muscle. The PSSM1 muscle cells constantly focus on making glycogen, which hinders their ability to release glycogen and burn it as fuel during exercise. This results in an energy deficit early in exercise before the body can burn fat as fuel. Without enough energy, the muscle might contract but not relax, leading to stiffness, pain, sweating, and muscle cell damage—all typical signs of tying-up.
“When horses that had been diagnosed with PSSM by muscle biopsy were examined for the GYS1 mutation, some of these horses did not have the PSSM1 mutation,” says Valberg. “The term type 2 PSSM or PSSM2 was given to horses with small clumps or ‘inclusions’ of normal glycogen without the abnormal polysaccharide that cannot be burned as fuel. Type 2 PSSM is not necessarily a specific disease but a classification for horses that have signs of a muscle disease, do not have the PSSM1 mutation, and have some abnormalities in the staining for glycogen in their muscle biopsy.”
More research is needed to determine whether PSSM2 has a single cause or, as Valberg suspects, different causes. For example, Quarter Horses diagnosed with PSSM2 classically have signs of tying-up, whereas Warmbloods diagnosed with PSSM2 show reluctance to exercise and poor impulsion without muscle damage, suggesting a separate underlying cause.
When things go wrong with your horse’s muscles, seek your veterinarian’s help, because many conditions affecting the muscular system cause similar signs. Some might be self-resolving (e.g., normal stiffness following training/competition), whereas others require immediate medical attention.