Horses that test negative for PSSM1 but have strong PSSM symptoms are tested for PSSM 2, now referred to as MIM.
Previously the only way to test for PSSM2 was with an invasive muscle biopsy, and many still choose to have this test done.
The equine genetic world is advancing rapidly and there is a company pursuing testing for PSSM2 . To date they have found a few different variants which predispose a horse for PSSM2 and a hair test is now publicly available (in the USA) to test for the P2, P3, P4 and PX (RER). They are working on other variants as well and can test for P5/P6, P8 and K1.
Please contact Equiseq.com for more info on testing. If you are outside the USA you can order a test from Equiseq and they will send you a card to send back to them with a dried blood sample. The tests are fairly costly at this point as all new genetic testing is at first but the company's main focus is still on finding and developing tests for more variants. They have your horse's best interest at heart.
Many breeders are still not testing for PSSM2 and most testing is being done because a horse is showing symptoms and the owner is looking for answers. In the future as costs come done breeders will routinely test for all of the PSSM variants.
The symptoms for PSSM 2 are very similar to PSSM 1 and are often more severe. We recommend testing first for PSSM 1 and then if that comes back negative, pursue a PSSM2 diagnosis.
There is more info on the Specific PSSM 2 variants P2, P3, P4, and also on PX in the main menu above Genetic Diseases > PSSM > PSSM2 > and click the variant you wish to see more info on.
BRIDGEquine has many tested PSSM2 horses in the database. We do not have specific source horses yet but can predict fairly well whehter a horse is more likely to be PSSM1 positive or PSSM2 positive. If you are wondering what to test for asking BRIDGE to analyze your pedigree is an excellent way to decide where to start testing.