None of our horses at R2 Ranch carry this genetic disorder

Have you ridden a rocky with PSSM1?

Yes, the horse was largely able to perform! She still exhibited a lovely gait and personality. My only comment is that she "felt tense". We still cantered, worked gait, trail rode daily like normal. No tremors or other symptoms were observed during a several week observation. Horse was managed with a low starch diet. 


What is it: Polysaccharide storage myopathy (PSSM1) is characterized by the abnormal accumulation of the normal form of sugar stored in muscle (glycogen) as well as an abnormal form of sugar (amylase-resistant polysaccharide) in muscle tissue. By definition horses with PSSM1 have a distinctive genetic mutation in the gene.

Can you ride a PSSM1 positive horse? Yes, You must be mindful of their exercise routine. i.e. consistent days with same amount of exercise. decreased rations on days off. 

Do you have to do anything different with them? Yes, diet is a little bit more strict. Low carb and high fat.

Is it prevalent in the Rocky Mountain breed as it is in AQHAs?   NO

What brought this into our breed? Many theories exist. It is thought many wider bred or drafty breeds carried this disorder from before horses existed in the Americas- possibly a common Spanish ancestor for both AQHAs and RMH and possibly Drafts as well. It has prevailed through the creation of many American breeds we now have today. i.e. Morgans, most drafts, AQHAs, and some gaited breeds.

Is there a common ancestor in the RMHA breed? Yes, some may have noticed it was a Yankee bred horse within the RMH but it is very exclusively KING'S BLONDE and most likely RIP in his line that carried PSSM1.

*all known or discovered PSSM1 positive horses I personally researched have gone back to these ancestors*

Are all offspring from these horses 100% possible carriers (a carrier is a horse that is asymptomatic but also may exhibit some symptoms, denoted as n/+ as a carrier and +/+ as a homozygote)?

No- only genetic testing for the gene will tell you. The known originator (Presumably RIP (n/+)) is only passing the gene 50% of the time (resulting offspring directly from a PSSM1 single carrier horse) ... unless a predecessor carries 2 copies of the gene - +/+ (All offspring would be carriers n/+).

​What are the symptoms?  

​Horses with PSSM1 can exhibit symptoms without exercise. During an episode, horses seem lazy, have a shifting lameness, tense up their abdomen, and develop tremors in their flank area. When horses stop moving they may stretch out as if to urinate. They are painful, stiff, sweat profusely, and have firm hard muscles, particularly over their hindquarters. Some horses will try pawing and rolling immediately after exercise. Most horses with PSSM1 have a history of numerous episodes of muscle stiffness at the commencement of training; however, mildly affected horses may have only one or two episodes/year. Rarely, episodes of muscle pain and stiffness can be quite severe, resulting in a horse being unable to stand and being uncomfortable even when lying down. The urine in such horses is often coffee colored, due to muscle proteins being released into the bloodstream and passed into the urine. This is a serious situation, as it can damage the horse's kidneys if they become dehydrated.

Our Notes on PSSM1

Is there PSSM type 2? Yes

We do not know what causes PSSM2 and for the most part recommendations for type 2 PSSM have been the same as those for type 1 PSSM. We believe that there are probably several causes of PSSM2 and the best approach to managing PSSM2 may be to look at this in a breed specific manner.