Myopathies in horses

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Myopathies and muscle diseases in horses aren’t uncommon. Just like humans, horses can suffer from numerous muscle diseases, the causes and variety of which are still partly unclear and insufficiently researched.

On one hand, there are inherited muscle diseases where the mode of inheritance and the specific mutations have been identified. On the other hand, there are muscle diseases that arise due to metabolic or hormonal disorders, improper feeding, or even muscle trauma.

In human medicine, a wealth of data and studies is available on these conditions, but in horses, especially concerning myopathies, there are ongoing discoveries. As a result, distinguishing between various diseases is becoming more feasible, leading to the identification of new causes. For instance, PSSM2 now serves as an umbrella term for several underlying muscle diseases. The number of known myopathies in horses is expected to continue rising in the years ahead.

Myopathies in horses with already established genetic causes include:

• Polysaccharide storage myopathy 1 (PSSM1)
• Hypercalaemic Periodic Paralysis (HYPP)
• Glycogen Branching Enzyme Deficiency (GBED)
• Malignant hyperthermia (MH)
• Myosin Heavy Chain Myopathy (MYHM) or Immune-Mediated Myositis (IMM)

Additionally, there are myopathies with causes that remain unexplained (genetic) or are related to feeding:

• Myofibrillar myopathy (MFM)
• Polysaccharide storage myopathy 2 (PSSM2)
• Recurrent exertional rhabdomyolysis (RER)
• Sporadic Exertional Rhabdomyolysis (SER)
• Tying up

Myopathies caused by poisoning or deficiencies include:

• Atypical pasture myopathy
• White muscle disease: Selenium deficiency in newborn foals

Myopathies caused by trauma:

• Fibrosing myopathy

Often, the distinction between a genetic cause and a feeding-related cause is not entirely clear-cut, as some of these myopathies can be effectively managed with appropriate feeding. We will now introduce the most important ones, starting with PSSM1.

PSSM1 – A heritage of the working horse?

Polysaccharide storage myopathy 1 is a hereditary muscle disease commonly found in cold-blooded horses and Quarter Horses. But it also affects warm-blooded horses to some extent.

This condition is caused by a mutation in a gene called glycogen synthase 1 (GYS1), responsible for excessive glycogen storage in the muscles of affected horses without proper breakdown. Glycogen, which stores glucose in the muscles for later energy production, doesn’t function as expected in horses with PSSM1.

DELETE Consequently, these horses often suffer from muscle tissue breakdown, known as rhabdomyolysis or “tying up.” This primarily affects the horse’s back and croup muscles and is associated with severe pain.

Symptoms typically manifest shortly after starting exercise and can vary from reluctance to move, muscle stiffness, and lameness to complete immobility. In severe cases, horses may adopt the “sawhorse posture,” with front legs forward and hind legs back, seeking relief.

The severity and frequency of the tying-up syndrome depend on whether the horse is homozygous for this mutation, inherited from both sire and dam, or heterozygous, inheriting the mutation from one parent. Homozygous horses tend to be more severely affected. The horse’s diet plays a significant role in whether it will develop the disease and have these excess glycogen deposits in the muscles.

A low-sugar diet and regular exercise can prevent the occurrence of sugar deposits in the muscles of affected horses. Exercise is crucial. Historically, tying up was colloquially called “holiday sickness” in Germany. It often occurred when working horses had a day off or when they were to be taken back to work on Monday morning after Sunday rest but were fed the same diet as a working day. It’s essential to note that the presence of tying up does not necessarily indicate the presence of the GYS1 mutation. Horses without this mutation can also be affected if they are fed a high-carbohydrate diet without sufficient exercise.

It is believed that this gene underwent a mutation around 1200-1500 years ago, long before the existence of today’s horse breeds. This suggests that working horses, in particular, might have been deliberately selected for this trait, as affected horses could perform well with minimal feed. In the USA, 87% of working horses and 72% of Quarter Horses are carriers of PSSM1. Additionally, PSSM1 is prevalent in European cold-blood breeds, such as Rhenish-German coldblood (68%) and Belgian draught (92.1%). It is also found in Freibergers (2%), Haflingers (23%), and Norikers (33-62%).

Diagnosing PSSM1 involves examining the DNA from blood or hair samples. If PSSM1 is detected, it is crucial to provide the horse with a low-carbohydrate diet and, most importantly, ensure sufficient exercise. A low-carbohydrate diet entails ample hay with low sugar content and mineral supplements, but under no circumstances should concentrated feed or sugary treats and fruit be given. Alongside appropriate feeding, regular and daily exercise is essential for these horses.