Polysaccharide Storage Myopathy (PSSM1) is a dominant autosomal hereditary condition that can cause a genetic form of tying-up of muscles, causing muscle damage and the inability to move. There are several forms of PSSM. One of these forms is PSSM1. The PSSM1 mutation causes a disruption in the amino acid sequence. This, in turn, disrupts an energy producing process that allows muscles to operate normally. At least 20 breeds have been identified with Type 1 PSSM. The prevalence of this mutation in Belgian horses is as much as 50%. About 8% of the Quarter Horse-related breeds also have this mutation.
Some horses make and store abnormal muscle glycogen (a form of energy) and cannot tolerate dietary starches and sugars. Horses with PSSM1 can be maintained with low-starch and low-sugar rations and precise exercise protocols. In some horses, symptoms may begin by 2 to 3 years of age. In others, symptoms can remain subclinical, or non-observable. Clinical signs can include skin twitching, stiffness, firm, painful muscles, sweating, weakness, and reluctance to move, even during light exercise. Occasionally, gait abnormalities, mild colic and muscle wasting may also occur. In many cases, horses that have tested positive have had no history of 'tying-up' or other symptoms associated with PSSM1.
Research conducted at Animal Genetics has identified several additional mutations associated with PSSM1. These DNA mutations form a haplotype that allow us to identify horses with PSSM1. A haplotype is a group of inherited genes. Further ongoing research may provide us with a more comprehensive assay for PSSM and enable us to better determine the severity of the disorder in all horses.
An additional genetic mutation in RYR1 gene (a mutation that causes malignant hyperthermia in horses) influences PSSM. This mutation can also increase the severity of the symptoms of PSSM1 in Quarter Horses and related breeds. The combination of PSSM and MH genetic tests are available at Animal Genetics.
Not all cases of tying up are caused by the PSSM1 mutations. These cases are currently being looked at. A horse that tests N/N for PSSM1 but exhibits signs of tying-up or muscle pain may be suffering from an another muscle disorder. Please contact us with any examples of this so we can include these animals in further studies. In many cases, horses that test positive for these mutations will exhibit only minor problems or may never exhibit any noticeable problems at all. Again, please contact us and let us know so we can add these animals to our current research on PSSM1.
Management of Horses That Test Positive for PSSM1:
Horses that test positive for one or two copies of the PSSM mutation should be carefully managed through diet and exercise to help prevent the onset of the disease. For many horses affected by PSSM1, strict control of diet and exercise can reduce, or even prevent the onset of symptoms related to PSSM1. Eliminating high-sugar foods in their diet and consistent exercise are two simple ways to help prevent the disease from developing. Although taking these simple steps may not be effective in every situation, research has shown that often they will provide positive results. It is always important to let your veterinarian know if an animal has tested positive for PSSM1.
|P1/P1||At Risk: Horse has two copies of the PSSM Type 1 gene mutation and may exhibit signs of the disorder.|
|n/P1||At Risk: Horse has one copy of the PSSM Type 1 gene mutation and may exhibit signs of the disorder.|
|n/n||Clear: Horse is negative for the PSSM Type 1 gene mutation.|