Friday, December 16, 2011

PSSM: The Rest Of The Story

I noticed upon visting our blog half of the post was dropped. I have re-entered the entire post including Dr. Valberg's exerpts. Sorry for the inconvenience and I apologize for the lost comments at the end of the article. This post is intended to share the updates from people that have had years of education and experience and are recognized as the true experts in the field.
Update: PSSM in Horses
The gene that identifies type 2 PSSM one can only be found with a full muscle biopsy. Type 2 PSSM seems to be more prevalent in performance based horses.
With the more easily performed test for type 1 PSSM, we have seen the gamut of responses from armchair Veterinarians (generally those with performance bred horses) how relieved they are to find their horses PSSM free. They would lead you to believe all horses with PSSM are doomed to die,” and in the process would want you to believe their horse is PSSM free, merely because one test came back in their favor.
The more reasonable approach.
"There is more to this test that needs to be considered."
Many horse enthusiasts have taken the identification of the type 1 PSSM gene and have been able to better understand their horse’s behavior under stress. Unfortunately, some misinformed owners/breeders continue to believe every misstep can be tied to this one type of PSSM. They believe this one test will remove all things bad in the industry.

The more dangerous "expert" deliberately fails to address, there is more than one form of PSSM and there are additional genes that affect the severity of the PSSM condition. These same people want you to believe the elimination of type 1 PSSM will correct all the tying-up issues in the entire equine industry. They would like you to look past their breeding herd that has NOT been diagnosed PSSM negative with a muscle biopsy.
These people will show you a $35 test and ask that
you join them in their celebration their horse is PSSM FREE!
To date, a muscle biopsy is the ONLY
way to identify type 2 PSSM.

Type 2 PSSM is more prevalent in performance horses.
As seen by a plethora of posts in numerous forums by the self-proclaimed “experts”. These armchair Veterinarians seem to be practicing Veterinary Medicine without a license. These “experts” can be found on YouTube diagnosing PSSM to misbehaving horses merely by watching a naughty horse in a video. You will find the same people on Facebook proclaiming the demise of the equine industry to horses with PSSM. They have assigned every bad behavior and injury (from abscess to overwork) to PSSM. These “experts” have not tested their horses for type 2 PSSM or the additional genes that amplify PSSM symptoms.
In this scenario, some knowledge is more
harmful than no knowledge, by leading you to believe a
$35 test represents a clear bill of health.

Setting The Record Straight
As for us, at Smith Show Horses, we feel the industry needs to know all of the facts and you need to know that there are more undiagnosed cases of PSSM than the simple pulling of mane hair cannot detect.

Before you book your mare to a stallion represented as PSSM free, and if you are worried about symptoms associated with PSSM, ask to see their PSSM results. There are many breeders that will tell you they have the $35 test confirming their horse does NOT have PSSM.
A horse with a Negative type 1 PSSM test does not mean the horse is a negative for PSSM. Through Dr. Valberg's explanation, unless, a muscle biopsy has been done on a horse that tests negative for type 1 PSSM, you may be living with a false security.
Omission by ignorance is wrong...
...Omission by intention is deceitful.
Before any breeder can proclaim their horse PSSM freeALL of the tests need to be completed with negative results. It is much easier to point fingers at one part of an industry and compartmentalize all problems lead to one gene. The last time I saw someone pointing their finger at someone, there were 3 pointing back at them.
Update: PSSM In Horses
By Kentucky Equine Research Staff · September 29, 2011
At the 2011 Texas Equine Veterinary Association (TEVA) conference, one of the leading researchers in equine muscle problems, Stephanie Valberg, D.V.M., Ph.D.,of the University of Minnesota College of Veterinary Medicine, brought the audience of veterinarians up to date on polysaccharide storage myopathy (PSSM), one cause of tying-up, September 29, 2011.
What is Tying-Up?
When tying-up happens on a regular basis, it is termed recurrent exertional rhabdomyolysis (RER or chronic tying-up). RER can be caused by several muscle problems. According to Valberg, three genetically identifiable causes of muscle malfunction are
  1. Exertional Rhabdomyolysis type 1,
  2. PSSM type 1
  3. Malignant Hyperthermia. Dr. Valberg, also, warned that there might be other unrecognized causes of RER.

…“While some horses sporadically develop exertional muscle damage as a result of nutritional, training, or environmental factors, others develop RER in spite of a sound diet and environment,” noted Valberg. “Many of these chronic cases are due to an intrinsic and inherited dysfunction of muscle metabolism or muscle contraction.”

In short, horses with (any type of) PSSM are unable to normally store and use sugar in skeletal muscles. There are extenuating circumstances that cause the severity of symptoms exhibited by horses with PSSM.

There has been much progress made in diagnosing PSSM. In fact, more than one form of PSSM exists today. Only laboratory tests determine which specific genetic mutation is causing the tying-up. Type 2 PSSM can only be diagnosed with a muscle biopsy, which in most cases has not been done on an entire herd.

“For clarity, the form of PSSM caused by a GYS1 (a specific gene) mutation is now termed type 1 PSSM. The location of the specific gene that identifies PSSM that is NOT caused by the GYS1 is yet unknown. That form of PSSM is now termed type 2 PSSM,” explained Valberg. Type 2 PSSM can only be diagnosed with a muscle biopsy.


Type 1 PSSM
Research has shown that the genetic mutation at GYS1 responsible for type 1 PSSM can be found in more than 20 breeds of horses. “It is estimated to have emerged as far back as 1,600 years ago, when the great horse was being developed from European draft and light horse breeds to carry knights with heavy armor into battle,” noted Valberg.

Dr. Valberg noted in her presentation that the highest prevalence of PSSM appears to occur in draft horses derived from Continental European breeds (90% prevalence of PSSM in Trekpaards, a draft breed, with 40% of tested Belgian Trekpaards being homozygous for the trait).

While the complete cycle of how the genetic mutation causes problems in the storage and use of muscle glycogen is under investigation, it is known that diet and exercise can be used to manage the problem in many horses.
In postulating why this genetic mutation has become more prevalent in some breeds, it has been noted that owners of horses with type 1 PSSM often describe their horses as having a calm and sedate demeanor.

Clinical signs of chronic PSSM
A small number of stock horses have both the GYS1 mutation AND a genetic mutation (RYR1) for malignant hyperthermia (MH), which results in more severe signs of tying-up and a limited response to diet and exercise changes. In horses with the RYR1 mutation, during an episode of tying-up they can experience excessively high body temperatures and sudden death can occur.
Blood or hair samples can be used to test for type 1 PSSM, MH, and other genetic diseases of horses. Muscle biopsies would be needed to definitively diagnose a problem other than Type 1 PSSM.

Type 2 PSSM
Some horses with PSSM do not have the genetic mutation, Type 1 PSSM and are called type 2 PSSM cases. There is ongoing research to learn more about these horses. Valberg noted there are cases of PSSM diagnosed by muscle biopsy in Quarter Horses do not have the GYS1 mutation.
"Type 2 PSSM also occurs in Arabians; however, in my experience this breed is distinct in that it often has amylase-resistant rather than amylase-sensitive polysaccharide but is negative for the GYS1 mutation,” said Valberg.
The diagnosis of type 2 PSSM is done with a muscle biopsy. Valberg advised that mild PSSM cases should receive a full physical examination to ensure that there are not other underlying causes for performance problems.

Managing A Horse With PSSM
Understanding the horse’s caloric requirements and ideal body weight are important factors, stressed Valberg. “Many horses with PSSM are easy keepers and may be overweight at the time of

“Regular daily exercise is important for managing horses with PSSM,” stressed Valberg. “Even 10 minutes of exercise has been shown to be extremely beneficial in reducing muscle damage with exercise. Once conditioned, some PSSM horses thrive with four days of exercise as long as they receive daily turnout. For riding horses with type 2 PSSM, a prolonged warm-up with adequate stretching is recommended.”

Here is the entire article from Dr. Valberg and links to more specific questions.
  • On a final note, we at Smith Show Horses feel there is benefit to the less expensive PSSM1 test, but also realize a horse that tests PSSM1 n/n could very well have PSSM2. Even though, ALL of our horse are symptom free we have tested our entire herd and encourage all breeders and mare owners to do the same. Dr. Valberg and the AQHA continue to search for the location of the gene that identifies PSSM2. As of this writing a muscle biopsy should be on file to declare your horse PSSM free.


  1. Please feel free to share this blog. There are so many people that are lead to beleive there is only one form of PSSM.
    People need to know all of the facts...not just the dressed up ones.

  2. ...also, please refer to our post in October regarding PSSM as well...

  3. If your horse had tested negative and was not showing any symptoms at all, are you advocating carrying out a muscle biopsy to eliminate type 2? Why would you do that if your horse had never exhibited a symptom and was n/n. My mare tested positive n/p1 as a 4 year old just put into work. She exhibited every symptom, tying up, colic like symptoms, intermittent lameness and savage tendencies when she had an episode (she savagely attacked my daughters head with her teeth). Her diet was changed, she had maximum turnout and regular exercise. She was unmanageable and after 2 years on veterinary advice she was put to sleep aged just 6. (All breed pedigree register Tiptoe Dazzelena)
    Have you had all your horses had a muscle biopsy?

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  22. Would there not be a more simple "test" to see whether a horse is prone to PSSM...? like... working the horse for 4 days in a row + feeding say 2 handsful of oats and then to continue the oats, but stopping the exercise... ? something like that. Surely horses with severe PSSM could be identified with some easy "experiment" such as this? Or would you only see it in Stabled horses?