How to support your horse with PSSM 1, PSSM 2 or MFM

Key terms and definitions

Optimum support for horses with PSSM 1

Even though the genetic defect cannot be repaired, good management, correct training and appropriate feeding can often have a very positive influence on the progression and symptoms. PSSM 1 was first discovered in Quarter Horses, but has since been identified in around 35% of all horse breeds, particularly in muscular, calm horse types such as Haflingers, Noriker horses and other cold-blood breeds. Symptoms can appear even in young horses. Affected horses require grain-free, low-sugar rations to remain capable of performing.

Symptoms: Light work is usually well tolerated, but more strenuous exertion leads to muscle damage and symptoms such as muscle pain, muscle cramps, stiffness, fatigue or complete refusal to work. Even a visit from the farrier can be enough to trigger such reactions.

Training: Besides the correct feed composition, management and exercise are also crucial. The environment should be as stress-free as possible, particularly during turnout. Regular but light exercise is important. Box rest, on the other hand, would be counterproductive for the sensitive muscles.

Feed: Horses with PSSM 1 need a low-carbohydrate, grain-free diet with plenty of hay and as little sugar as possible. Care should also be taken with sugar-rich grass and hay. Fat, on the other hand, provides good energy. In addition, an adequate supply of vitamin E (1,000 to 3,000 mg per day), vitamin C (>500 mg per day) and selenium (1.8 mg per day, but do not overdose) is important to support the antioxidant capacity of the muscle cells.

Unlike with PSSM 2, protein-rich feed is not essential for PSSM 1, but a high protein quality can also support the muscles here. Depending on energy requirements, either Faser-Light (for low energy) or Vito, MyoCare-Müsli or Getreidefrei-Mix (for high energy) from our marstall range are suitable, including for horses with EMS and Cushing’s (PPID).

The right feed for horses with PSSM 2

PSSM 2 cannot be clearly attributed to one or several specific gene mutations. Researchers are currently discussing mutations in the genes P2, P3, P4, Px, P8 and K1, which are determined via a commercial genetic test. The severity of symptoms may possibly depend on which mutation is present, whether several genes are affected and whether the mutation is homozygous or heterozygous. However, diagnosis via the genetic test currently available is highly controversial and not validated.

Scientific studies show that the loci P2, P3, P4 are not meaningful for diagnosing PSSM2 or MFM. A renowned research group therefore warns of 40-57% misdiagnoses when using the genetic test to diagnose PSSM2 or MFM (Valberg et al. 2023). According to the latest findings, the Px variant also occurs very frequently in healthy control animals of affected breeds. Similarly, K1 and P8 are present in healthy animals (Durward-Akhurst, et al, 2024).

The research group doubts that these 6 loci can be the cause of PSSM2, particularly since none of these loci influence abnormal glycogen storage (Valberg, 2025).

PSSM 2 affects all horse breeds, but particularly lighter-muscled breeds with a higher proportion of blood such as warmbloods and Arabians. Affected horses typically show first symptoms at around 8 years of age.

Symptoms: PSSM 2 often presents with muscle tension, gait irregularities, reluctance to move, shifting lameness and behavioural changes. In severe cases, painful muscle wasting can occur, which may also be visible as grooves or muscle dimples. The course of the disease is episodic and progressive. Episodes often occur after operations, periods of rest due to injury, vaccinations or other stressful phases, particularly in connection with feeding low in protein.

Feed: The genetic defect can cause the body to require more protein to "improvise" muscle build-up. Affected horses therefore often benefit from a ration with an increased content of small intestine digestible protein. High-quality amino acids, particularly essential amino acids such as lysine, methionine and threonine, are important for muscle build-up.

Protein-rich feeds such as MyoCare muesli or the targeted provision of amino acid feed supplements, such as Amino-Muskel, greatly support the maintenance and rebuilding of muscles.

Although grain-free and low-sugar feeding is not always strictly necessary for PSSM 2, it is advisable. Adding omega-3-rich linseed oil is recommended. In addition, these horses require a slightly increased intake of magnesium, sodium chloride (salt) and plenty of vitamin E (over 2,000 mg per day).

Antioxidants such as vitamin E and C are considered particularly important for horses with the P8 mutation. In general, however, they support all types of myopathies. Trace elements such as zinc, manganese, copper and selenium (1.8 mg per day) should be fed in sufficient but not excessive amounts.

As a rule, PSSM 2 horses do not need to be restricted when it comes to grazing. However, caution is advised with sugar-rich grass and hay where necessary.

MFM in horses

The muscle disease MFM is known from humans. Researchers suspect that some horses affected by PSSM2 could have an early form of MFM.

Symptoms: The symptoms of MFM are similar to those of PSSM2. Science has not yet been able to gain more precise insights here. Therefore, the current recommendations for feeding, management and training of horses with MFM align with those for horses with PSSM2. However, the focus here is on an increased level of high-quality protein and essential amino acids.