How, Where, and Results explained
PSSM1 and PSSM2 are both types of hereditary equine muscle disease. We know that only one parent has to have a copy of a mutated gene to pass it on. So whether your buying, breeding, or currently own a horse displaying symptoms, testing for PSSM is more important than ever- But how do you test for PSSM?
As of 2020 there are currently 3 available methods of testing for PSSM these are:
Hair Strand Test
A hair strand test is performed by genetic testing centres, samples can be taken by yourself or a vet.
Hair strand testing is used to diagnose many genetic diseases including PSSM1 & PSSM2. Alongside a blood test the hair strand test is one of the most reliable methods of testing, it will also give you the most information like whether your horse is Homozygous or Heterozygous for PSSM, along with the specific variant of PSSM (in PSSM2 cases) which testing via muscle biopsy is unable to determine.
To do a hair strand sample you need to pull 30-40 tail or mane hairs from your horse. This can be done by either pulling hairs individually or pulling out a few hairs at a time. It is of the utmost importance that each hair contains the root, the root is the white bit of skin at the top end of the hair- the root of the hair is what contains the genetic material of your horse and what genetic testing centres need for hair strand testing.
Once you have pulled the right amount of hair it then needs to be banded and taped to a piece of labelled paper, before being placed into a clear zip lock bag labelled with the horses name. Hair needs to be 100% dry before placing it into the bag, otherwise it could cause bacteria to grow and could contaminate your horses sample.
A muscle biopsy is invasive meaning that it can only be performed by your vet. Muscle biopsies are used to diagnose changes in the muscle which are then classified into a type of change consistent with a muscle myopathy like PSSM1 or PSSM2.
Muscle biopsies for PSSM are taken from either the Semimembranosus or semitendinosus muscle which are located in the hind end. (As per diagram below)
Your vet will numb the area and sedate the horse before making a small incision and cutting away a piece of muscle, they will then re-stitch the area and provide some pain relief. As this is an invasive method a small scar may remain from where the biopsy is taken.
The sample will then get shipped to a pathologist where it will be stained to help see structures in the tissue and cells, the pathologist will then look at these slides under a microscope and describe what they see. At the end of the pathologist’s report they will try to sum up into a diagnosis what they think their findings mean, this diagnosis could be PSSM1 or PSSM2.
However this is as far as it goes, muscle biopsies are unable to determine what specific variant your horse has in cases of PSSM2, it is also unable to determine if your horse is Heterozygous or Homozygous for the mutated gene, the muscle biopsy can only tell you that your horse is positive for PSSM1 or PSSM2 for more information further genetic testing (like a hair strand test) would need to be done.
Muscle biopsies are also not the most reliable method as just like a photograph they are a snapshot in time, if a horse is not symptomatic enough at the time the muscle biopsy is taken, the biopsy will not necessarily show changes. This does not mean the horse doesn’t have PSSM, it could still have PSSM but just not be badly affected yet.
A blood test can only be done by your vet, and can be used to diagnose both PSSM1 & PSSM2.
The blood can either be put onto a blood card, or sent in the tube to a genetic testing lab to diagnose PSSM1 or PSSM2 where the DNA can be extracted from it and used to give a diagnosis the same as the 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/p1 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 P1/P1 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 can I test?
Below is a list of hair strand testing labs in the UK & Europe
UK & Europe
PSSM1 & PSSM2 Combined Test
USA & Rest Of World
For more information on the “How To Test For PSSM” check out our YouTube video by clicking this link