PSSM in horses

Unfortunately, the term PSSM is increasingly being discussed. Not only because it is becoming more well-known, but also because more and more horses seem to suffer from it. PSSM stands for polysaccharide storage myopathy and is a genetic defect that causes muscle problems. There are two types of PSSM: type 1 and type 2. In this article, we explain the differences between these two types, how to recognize PSSM and what can be done about it.

What is PSSM?

As mentioned above, this disease is related to the storage of sugars. This is very normal and occurs in all horses. However, in horses with PSSM, the muscles absorb sugars much faster than they should. They also store glycogen in an abnormal way, which prevents sugars from being released, potentially damaging the muscle by not preventing acid build-up. This results in many symptoms that closely resemble tying up, such as muscle stiffness, sweating, and reluctance to move. Since there are two types of PSSM, we have explained the symptoms for each type below.

PSSM type 1: Accumulation of sugar molecules

Due to a genetic defect in a specific gene, the glycogen synthase 1 gene (GYS1), there is an abnormal accumulation of sugar molecules in the muscle cells. This causes horses with PSSM1 to quickly develop stiff and hard muscles, extreme sweating, or reluctance/inability to work. PSSM1 often occurs in attacks. During an attack, the horse often experiences muscle tremors (in the flanks and/or neck), appears tying up, tight in the belly, stands as if to urinate, and has dark urine.

A genetic test can determine if a horse has PSSM1. The test looks for abnormalities in the GYS1 gene. Genes come in pairs, which is why there are three possible outcomes:

• 2 normal GYS1 = no PSSM1
• 1 abnormal GYS1 = heterozygous PSSM1
• 2 abnormal GYS1 = homozygous PSSM1

PSSM1 and heredity

Since the GYS1 gene is autosomal dominant, symptoms are present even if one copy of the gene is abnormal. It is also an inheritable disease, so with heterozygous PSSM1, there is a 50% chance of passing it on. A horse with two abnormal GYS1 genes will always pass it on.
PSSM1 is more common in Quarters, Paint Horses, and various draft horse breeds. PSSM originated in draft horses, so any horse with draft bloodlines can have PSSM1.

Managing PSSM1

Proper stable management is crucial for horses with PSSM1, and it must be consistently maintained. This can often help keep PSSM1 horses symptom-free for longer periods. It is important for their feed to be low in sugar (including carbohydrates). This applies to concentrate feed as well as forage. Sugar-free hay is often the best option, and PSSM1 horses often cannot graze due to the unpredictable and varying fructan levels. PSSM1 horses often benefit from supplementation with a vitamin nugget/balancer, Vitamin E (natural form), magnesium, and salt. For intensive exercise, supplementing with a fat source for energy is recommended.

Exercise is also important for a horse with PSSM1. It should be moderate but preferably include daily exercise (in addition to free movement outside). This helps the horse burn sugar, reducing sugar build-up.

PSSM type 2: (chronic, progressive) muscle breakdown

The symptoms of PSSM2 are similar to those of PSSM1, but there is no defect in the GYS1 gene. PSSM2 involves a (chronic) muscle breakdown problem. Horses with PSSM2 may have characteristic hollows on the neck and hindquarters, where muscle has been lost. There are no clinical or radiographic abnormalities visible, which is the most deceptive part of this disease. These horses often show abnormal movement and varying lameness. PSSM can only be diagnosed through a test.

Medical explanation of PSSM2

Studies indicate that various genes are responsible for PSSM2. The identified genes are P2, P3, P4, and Px. Other genes have been discovered, but information is limited. There are still discussions about testing for PSSM2 since it has not been officially published in medical journals. Therefore, not all veterinarians recognize PSSM2 as a condition.

The PSSM2 genes are semi-dominant, meaning symptoms are typically present. Because genes are in pairs, there are several possibilities for PSSM2. Genetic tests use "n" for non-abnormal. If the gene is mentioned, an abnormal gene has been found. The possibilities are:

• 1 copy, for example, n/p2
• 2 copies, for example, p2/p2
• Combination, for example, n/p2 and n/p4

The more combinations and copies there are, the more severe the symptoms tend to be. However, even one abnormal gene can cause severe symptoms.

Of the four genes, P2, P3, and P4 are associated with severe muscle breakdown. Symptoms usually start to appear around 7 years of age. Even if a horse tests positive, symptoms may not be visible at that moment. They may be triggered later by vaccination, injury, or surgery. This is because a negative nitrogen balance is created (nitrogen is crucial for proteins) in such situations. A healthy adult animal has a nitrogen balance close to 0. This means nitrogen intake equals nitrogen excretion. After an injury, nitrogen excretion increases (muscle mass breaks down for wound healing), resulting in a negative nitrogen balance and protein loss.

Px seems to be linked to Recurrent Exertional Rhabdomyolysis (RER), which mainly occurs in Thoroughbreds. RER is a form of tying up caused by abnormal calcium regulation in muscle cells.

Managing PSSM2

Since the symptoms of PSSM2 are similar to those of PSSM1 (muscle stiffness, reluctance to move, difficulty collecting, sore muscles), dietary adjustments are similar. A horse with PSSM2 has an increased need for proteins and essential amino acids to compensate for muscle breakdown. It is also important to keep sugar levels in the diet low to reduce inflammation and to add extra vitamin E (natural form) and magnesium.

For exercise, a horse with PSSM2 should have daily free movement and regular training to support the muscles. The training should focus on keeping muscles supple. It is essential to find a balance between maintaining muscle flexibility and preventing muscle breakdown. Excessive or prolonged training is not beneficial for these horses.

Conclusion:

Unfortunately, more horses are found to have PSSM due to its hereditary nature. Genetic testing can provide clarity on this condition. The difference between the types is that PSSM type 1 involves an accumulation of sugar molecules in muscle cells due to a genetic defect. PSSM type 2, on the other hand, involves muscle breakdown. However, the symptoms of both variants are very similar. Many horses can live relatively symptom-free lives with an adjusted diet and exercise regimen, but finding the right balance can be a long process.

Extra vitamin E and proteins are especially important for these horses. It is also crucial to minimize sugars and provide controlled exercise. PSSM attacks can also be triggered by stress or changes in weather.

There are various websites where extensive information about PSSM can be found. Do you suspect your horse has PSSM? Consult these websites and your veterinarian for more information.

Ps. We have done our best to compile the information about PSSM as accurately as possible by using various sources. If there are still mistakes, please let us know!