PSSM2 is an umbrella term used for a group of muscle diseases, 2 of these muscle diseases include something known as MFM (Myofibrillar myopathy) & RER (Recurrent Exertional Rhabdomyolysis).

Muscle biopsy showing a PSSM2 affected horse, notice the breaks in the muscle

What Types Of PSSM2 Are There?

There are currently 6 known variants of PSSM2 these include:

P2– A likely variant of MFM affecting the Myotilin protein in muscles
P3– A variant of MFM affecting the Filamin C protein in muscles
P4– A variant of MFM affecting the Myozenin 4 protein in muscles
P8– A variant of MFM affecting the Pyridine nucleotide-disulphide oxidoreductase domain 1 protein in muscles
K1– A muscle disease affecting the COL6A3 protein in muscles
PX- A likely variant of RER affecting the CACNA2D3 gene in muscles

With up to 32 genes in horses, research is still very much ongoing into more variants of PSSM2, P5 & P6 have recently been discovered but research is still in early stages. As more variants of PSSM2 are discovered and information is released we will add them to this list.

Is PSSM2 Hereditary?

There is debate amongst top scientists as to whether PSSM2 is hereditary or caused by environmental factors but there is still much research to be done, due to the amount of variants there are each variant would need to be looked at individually, and with more PSSM2 variants still coming to light research is not an easy task for scientists. But from the evidence we have seen in both PSSM databases Bridge Equine & our own PSSM & MFM awareness database we as owners, along with many other owners of PSSM2 horses, and some vets throughout Europe are confident that PSSM2 is in fact hereditary, we are seeing patterns with the same bloodlines continually appearing in these databases. What aids to this fact is some foals are born with severe cases of PSSM2, these cases are usually put to sleep as symptoms affect them from the moment they breathe.

Can A Horse Be Affected With Both PSSM1 & PSSM2?

Yes- In fact one of our Admin’s Christine has a horse affected with both PSSM1 & PSSM2

Can A Horse Be Affected With More Than One Variant Of PSSM2?

Yes, a horse can have up to 1, 2, 3, or even all variants of PSSM2

What Are The Symptoms Of PSSM2?

Symptoms of PSSM2 are very similar to those of PSSM1 and can occur before or after work these usually include:

What Breeds Are Affected By PSSM2?

ANY BREED can be affected with PSSM2 some breeds it has been detected in so far are:

  • Warmbloods
  • American Quarter horses
  • American Paint horses
  • Appaloosas
  • Thoroughbreds
  • All types of Arabians
  • Iberian horses
  • Native Ponies
  • Draught Horses
What Is PSSM2?
Karlo my n/p4 homebred warmblood, after becoming mysteriously lame as a 4 year old he ended up at Rossdales where he had scans, x rays, and ultrasounds which were inconclusive. The PSSM2 test came back n/p4

How To Test For PSSM2?

Hair strand test or a muscle biopsy although a muscle biopsy will not tell you the specific variant they are affected with.

Interpreting The Results Of A Hair Strand Test

When you receive your PSSM results back it can often be confusing as to what all the terminology means, but we are here to explain that for you.
The main confusion with PSSM results stands with the terms Heterozygous & Homozygous as this is the terminology that is usually used to determine whether a coloured horse will throw a coloured foal. Because of this people and also some vets automatically assume that if a horse is Heterozygous for PSSM it only means it has a 50% chance of having the condition.This is FALSE regardless of if your horse is Heterozygous or Homozygous they 100% have PSSM, the only time they would not have it in a test result is if their results come back N/N which means negative for that specific PSSM gene.


Means your horse IS affected and carries 1 copy of the gene, if a horse is Heterozygous for PSSM it means it has a 50% chance of passing on the PSSM gene to it’s offspring, an example of a Heterozygous PSSM result would be N/P2 or N/P4.


Means your horse IS affected and carries 2 copies of the gene, if a horse is Homozygous for PSSM it means it will have a 100% chance of passing on the PSSM gene to it’s offspring, an example of a Homozygous PSSM result would be P4/P4 or P2/P2.
Regardless of if a horse is Heterozygous or Homozygous for PSSM they should not be bred from, even with a 50% chance of passing on the gene it is still to much of a higher risk to take.

Where To Test For PSSM2?

Europe (PSSM2 Alone)
Cag (Center for animal genetics)

Europe (PSSM1 & PSSM2 combined test)
Cag (Center for animal genetics)

USA (PSSM2 Alone)

Is PSSM2 manageable?
PSSM2 is much trickier to manage due to the problem being in that of the horses muscle makeup, and the fact it is degenerative. It seems that some variants are more manageable than others. However some PSSM2 horses respond well to a PSSM1 diet but with some grass, along with lots of added protein and/or Tri aminos.

For more information on PSSM2 check out our YouTube video by clicking this link