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What is the difference between PSSM1 and PSSM2?

PSSM1 is a muscle disease caused by a disorder of glycogen metabolism. Excessive activity of the enzyme GYS1 leads to abnormal accumulation of glycogen granules in muscle cells, which can lead to symptoms of exertional myopathy. Approximately 70% of horses with the GYS1 gene mutation develop symptoms, often showing exercise intolerance, muscle tension and cramping, and lack of coordination especially with gait changes.


PSSM2 was originally a collective term for horses with symptoms, and often tissue biopsies, that were similar to PSSM1, but did not have the GYS1 gene mutation. (Link: exertional myopathy and muscle diseases page and graphic). However, no mutations associated with glycogen storage have yet been identified in “PSSM2” horses, and this classification is no longer valid.


The six genetic variants (called P2, P3, P4, P8, Px, K1) known under the name of PSSM2 are disrupt muscle integrity by affecting the structure, stability, and/or function of muscle fibres. They are therefore now classified under the name Muscle Integrity Myopathy.


PSSM1 and Muscle Integrity Myopathies are all multifactorial disorders. The risk of developing symptoms is greatly influenced by environment conditions such as diet and exercise.

Should I test my horse for PSSM1 or Muscle Integrity Myopathy (PSSM2)?

Variants of the Muscle Integrity Myopathy have been found in nearly all breeds tested thus far.


In contrast, PSSM1 is found in a smaller selection of breeds. PSSM1 is common in stock breeds, heavy and light draft horses, and cobs. It is less common in warmbloods etc. but these breeds can also be affected by PSSM1.

If a horse is tested positive for PSSM1 and does not respond well to the PSSM1 management protocols (low sugar diet, regular exercise), it is recommended to also test them for Muscle Integrity Myopathy.


For this reason, it makes sense to have horses of all breeds tested for MIM (PSSM2) and additionally some for PSSM1.



Is the test validated? Why are you offering a test before it is published?

Even in human medicine, there is no requirement for genetic tests to be “validated” before being offered for commercial testing. There is not even any official process by which a test can be validated – only the method a laboratory uses to perform the test.


Sometimes it is suggested that validation is ensured by publication in a peer-reviewed journal.  While this does mean that other scientists have reviewed a manuscript, the reviewers only evaluate what is presented to them.  They do not do any testing of their own to see if claims in the paper are appropriate or substantiated.


It is common for tests to be offered commercially prior to publication in a peer-reviewed journal.  When the test information has been published in a patent all information is available for laboratories to run and evaluate the test.  For example, the Warmblood Fragile Foal Syndrome patent was published in 2011 and commercially offered, but the first peer reviewed publication came in 2015. 


 More complex disorders such as those caused by a combination of genetic variants (polygenic) or by a combination of genetic variants and environmental factors (multifactorial) may require thousands of samples to be tested and statistically analysed to better understand the contribution of each factor, as required for peer review.  Tests are therefore offered commercially, and data gathered for such statistical analyses for publication.

Why doesn’t any other lab offer the test?

generatio has the exclusive license to offer the Muscle Integrity Myopathy “PSSM2” panel in Europe. No other laboratories are allowed to offer the test in Europe without generatio’s permission.

How accurate is the test?

 The accuracy and precision of a test are ensured in many ways. 


  1. Internal controls: Previously tested samples of known genotypes are routinely rerun in the generatio laboratory, including controls on every test plate.
  2. External controls: generatio exchanges samples of known genotypes with partner laboratories when establishing tests, and routinely send samples to (and receive samples from) other labs to ensure that the results are accurate in each lab.
  3. Participation in ring comparison tests. Generatio is a member of the International Society of Animal Genetics (ISAG) and takes part in all of the ring comparison tests for horse and dog testing. ISAG organizes that the same samples are sent to laboratories around the world, results are submitted from the labs, and they are graded on the correctness of the calls. Generatio has always tested in Rank 1 of horse and dog comparison tests. Generatio is also a member of the IPFD initiative of Harmonization of Genetic Testing in Dogs.

What are the specificity and sensitivity of the test?

Specificity and sensitivity are parameters which are appropriate for many biochemical laboratory tests, but are less appropriate for genetic tests, especially for those which are polygenic and/or multifactorial. It also depends on how a trait is defined. Genetic tests can determine if an individual possesses a risk variant or not. It cannot predict how that variant will interact with other genetic variants the animal possesses, or its interaction with the environment.  In general, it is possible to identify further risk factors – for example, with the Muscle Integrity Myopathy (“PSSM2”) risk variants, situations where the animal’s muscles are likely to become stiff (being cold or wet, lack of movement) increase the risk of symptom development. Protective factors include keeping the muscles supple (keeping the animal warm, having regular movement, routine training when possible), optimized nutrition and the avoidance of stressful triggers.

My horse is symptomatic but tested normal for Muscle Integrity Myopathy (“PSSM2”). Is the test wrong?

There are over 300 genes identified in humans and rodents as being involved in neuromuscular disorders. It is clear that only a small subset have been identified in horses. There are currently nine genes that have been identified that lead to symptoms of exertional myopathy.


In addition to the six identified forms of Muscle Integrity Myopathy (“PSSM2”), PSSM1, MH, and MYH1 Myopathy can also cause similar symptoms:

  • PSSM1 (GYS1 gene) is most common in Quarter horses and related breeds, and heavy and light draft breeds, but is sporadically found in other breeds (warmbloods, ponies) as well.
  • Malignant Hyperthermia (MH) (RYR1 gene) is found in Quarter horses and related breeds.
  • MYH1 Myopathy (previously Immune-mediated myositis, IMM) is found in Quarter Horses and related breeds.


Research is on-going to identify new mutations causing exertional myopathy. For these reasons, testing normal for the Muscle Integrity Myopathy (“PSSM2”) variants does not mean that an animal does not have a hereditary muscle disease, just that the responsible mutation may not yet have been identified.


There are also non-hereditary causes of exertional myopathy (link to explanation about EM and muscle diseases) .

My horse is symptomatic but tests normal for the applicable genetic tests. What else could it be?

  • ECVM (equine complex vertebral malformation) is a congenital malformation of the C6 and C7 vertebrae which can also involve the first rib.  Horses with this defect may show muscle symptoms because of damage to the spinal cord.  This can only be diagnosed with special radiographs.

  • NEV (New Equine Virus) is a lentivirus that attacks the immune system but can also lead to horses having unclear muscle symptoms, including weakness laminitis, pseudocolic and/or muscle atrophy.  The virus has been found in horses in different European countries including Portugal, Germany, and the UK.  The virus was discovered by researcher at the company Equigerminal in Portugal and can be tested there.

  • Mineral deficiencies can also effect muscle function and cause symptoms of exertional myopathy. It is recommended to have a yearly blood test to ensure that trace minerals are within normal parameters. Selenium is of particular importance.

  • Toxic Plants can also cause symptoms of muscle disease. For example, Summer Pasture Myopathy/Atypical Myopathy is caused by ingestion of seeds of the Sycamore tree.

My horse’s blood work doesn’t have increased values of the CK or AST. How can it still have muscle disease?

Most horses with Muscle Integrity Myopathy (MIM, “PSSM2”) do not have increased levels of AST or CK in daily life. CK and AST are leaked into the blood when muscle cells are damaged. With Muscle Integrity Myopathy the defect is in the structure or function of the muscle fibrils, so the muscle cells themselves are not damaged or leaking unless the horse is under extra stress that pushes the muscle to the point of breakdown (rhabdomyolysis).