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Affected breeds

Muscle integrity myopathy (MIM, formerly "PSSM2")

Muscle Integrity Myopathy (MIM, formerly PSSM2) is found in all breeds.


The six variants of muscle integrity myopathy (MIM, formerly "PSSM2") have been found in nearly all tested breeds, although some variants are not found in specific populations and some variants are more common in some breeds than others.


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 MIM.

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


Breeds in which PSSM1 is commonly found:

  • Stock breeds - Quarter horses, Paint, Appaloosa, POA, etc.
  • Heavy Draft – Belgian, Percheron, Trait Comtois, etc.
  • Light draft-type – Haflinger, Edelbluthaflinger, Noriker, South German Draft, Black Forest Draft, etc.
  • Cobs – Tinker, Gypsy Cob, Irish Cob, Gypsy Vanner, etc.


Breeds in which PSSM1 is less common:

  • Warmbloods, Huzule, Rocky Mountain Horse, Morgan, Friesian, Connemara Pony, Arab-Barb


Breeds in which there are no documented purebred cases:

  • Thoroughbred, Arabian, Standardbred, Iberian (Andalusian/PRE, Lusitano), Icelandic, Welsh

Malignant Hyperthermia (MH)

Malignant Hyperthermia is found in Quarter horses and related breeds (Paint, Appaloosa etc.).

MYH1-Myopathy (IMM)

MYH1-Myopathy (Immune Mediated Myopathy, IMM) is found in Quarter horses (especially working cow and cutting lines) and related breeds such as Paint and Appaloosa.