PSSM is a congenital genetic muscle disease. There is excessive storage of long-chain sugar molecules (polysaccharides) in the muscle cells. This leads to a dysfunction of the muscle cells. As a result, affected horses often show sympotms similar to those seen in a crossbreed.
Horse breeds particularly frequently affected by PSSM are Quarter Horses, Paint Horses and various cold-blooded breeds. But also other breeds like Haflinger, Freiberger, different warm blood breeds can be affected by PSSM.
We have specialists who can diagnose the cause of your horse's symptoms, assess the severity and prognosis of the disease, and provide you with treatment options if your horse is affected by PSSM.
PSSM – What do I need to know
How PSSM develops
PSSM is a genetic muscle disease. A distinction is made between PSSM type 1 and PSSM type 2.
Horses affected by PSSM type 1 have a known genetic mutation in the gene encoding the enzyme glycogen synthetase-1 (GYS1). The mutation is inherited in an autosomal dominant manner and can be either homozygous (inherited from both parents) or heterozygous (inherited from only one parent). Homozygous affected horses carry the mutated gene twice. Are therefore more affected by the disease than heterozygous affected horses. The genetic test for this disease has been validated by several scientific studies and is reliable.
PSSM type 2 disease involves horses that also show typical clinical PSSM symptoms and changes on muscle biopsy, but do not have a mutation in the GYS1 gene. There are indications that the disease called Myofibrillar Myopathy (MFM) may be a special form of PSSM type 2. PSSM type 2 is also suspected to have a genetic cause, i.e. a mutation in one or more genes in the genome. However, the responsible gene alterations behind PSSM type 2 (in contrast to PSSM type 1) are not yet known. The recently commercially available genetic test for PSSM type 2 is not reliable (see below).
Typical symptoms of PSSM
Horses suffering from PSSM show different degrees of muscle pain depending on the severity of the disease. The signs of the disease often occur repeatedly and are often triggered by stress, especially after days of rest. Typical signs are muscle stiffness, reluctance to move, muscle tremors, and profuse sweating. Affected horses often ame the urinary position but do not urinate, and they may show signs of colic. The croup and back muscles can be hardened and painful. Severely affected horses may become recumbent. In such cases, the discharge of dark urine can often be observed, as a large amount of red-brown muscle degradation must be excreted. Some diseased horses show only minor symptoms, e.g.B. a reduced willingness to perform, pain in the back muscles, weakness of the hind extremities or slight muscle wasting. Especially in phases in which the horse shows symptoms, increased muscle values can be measured in the blood test. However, horses with PSSM disease often show slightly elevated muscle enzyme activities without showing symptoms, or they show elevated muscle levels after exercise.
How PSSM is diagnosed?
The diagnosis of PSSM differs for type 1 and type 2 of the disease. An elevation of muscle enzymes in the blood may indicate the presence of muscle disease, but does not indicate the underlying disease. A reliable diagnosis of PSSM type 1 is made by genetic detection in a hair or blood sample. If genetic analysis is negative for PSSM type 1, PSSM type 2 is a possible cause. In order to diagnose PSSM type 2, a muscle biopsy is necessary. Specific staining technique can provide evidence of muscle degradation. PSSM-typical deposits of polysaccharides can be detected. In MFM, deposits of a cell protein can be detected. Ultrastructural changes in the muscle fibers can be visualized. The genetic test for PSSM type 2 which is offered is not conclusive (see below)!
Comment on the genetic tests
A genetic component with one or more mutations (gene changes) is suspected in both variants, PSSM type 1 and type 2. Such mutations can be detected by blood or hair analysis in the laboratory. However, it is important to know that genetic tests in the field of veterinary medicine are unfortunately not subject to any control. This means that a laboratory can at any time offer a genetic test that can detect a certain mutation, but without having to prove that this mutation actually causes a disease pattern. It is therefore important that genetic tests are used for diagnostic purposes only if they have been validated by scientific studies showing that a mutation is also associated with a disease pattern.
The Genetic test for PSSM 1 has been available for many years and has been validated by many scientific studies. The result can therefore be interpreted as safe and reliable be considered.
For some time now, a commercial Genetic test for PSSM type 2 offered that detects mutations in different genes. Depending on the test provider, different genes can be. variants can be examined. The most common variants offered are P2 (mutation in MYOT gene), P3 (mutation in FLNC gene) and P4 (mutation in MYOZ gene), but other tests are also offered. The result shows in each case in which gene which mutation was examined. However, two independent scientific studies, conducted by a recognized expert in the field of equine muscle disease, have failed to demonstrate any association of these gene mutations with typical clinical signs and muscle biopsy results. In one study, warmbloods were. Arabian examined. Here it was shown that 40% of the investigated horses were falsely diagnosed with PPSM type 2 due to the presence of a mutation in the genetic test, but did not show PSSM typical changes in the performed muscle biopsies. On the other hand, 66% of the horses that had PSSM typical changes in the muscle biopsy did not have mutations in this genetic test and thus were not detected by the genetic test (Valberg et. al. 2020). In a second study, Quarter Horses were examined and 57% of the healthy Quarter Horses were falsely diagnosed with PSSM type 2 and 40% of the Quarter Horses suffering from PSSM type 2 were not detected by the genetic test (Valberg et. al. 2022). The offered genetic test should therefore not be used for the diagnosis of PPSM type 2 and should also khave an impact on breeding or purchase examination. We recommend that horses with clinical signs of PSSM and negative PSSM type 1 genetic test should always undergo a muscle biopsy.
Who examines my horse?
Your horse will be examined by a veterinarian specializing in equine medicine, and all examinations and treatments will be discussed with the appropriate senior veterinarian. For special examinations other specialists of other fields are consulted if necessary. Veterinary students assist veterinarians during their clinical rotations, but are never solely responsible for your horse. Our team of senior physicians consists of proven specialists with the highest level of qualification in veterinary medicine, the diploma of the ECEIM (European College of Equine Internal Medicine) and/or the ACVIM (American College of Veterinary Internal Medicine).
The senior residents of the Department of Equine Medicine of l. after r. Dr. med.vet Meret Wehrli Eser Dipl ECEIM, Prof. Dr.med.vet. PhD Colin Schwarzwald Dipl. ECEIM, Dipl. ACVIM (Clinic Director), Dr. med.vet Hannah Junge Dipl ECEIM PD Dr.med.vet. PhD Angelika Schoster Dipl ECEIM, Dipl ACVIM
Treatment of PSSM
Horses with an acute PSSM episode are treated like horses with cross stroke. In order to prevent acute relapses, affected horses must be fed a special diet for the rest of their lives. They also require an exercise regime adapted to the disease. There is no cure. Medications are also not very effective to prevent acute relapses.
Based on the findings, we create an individual treatment plan for your horse in consultation with you. You can obtain the necessary medication directly from us or, depending on the agreement, from your private veterinarian.
You will receive a detailed report, in which all findings are summarized, the treatment is listed, as well as important information about the disease and aftercare are listed.
We will also inform your private veterinarian so that follow-up examinations can be done at home if possible.
Prognosis is good, but lifelong feeding and exercise management is necessary. If the feeding and exercise recommendations are followed, most horses with PSSM can be used normally as riding horses.
A cure is not possible. It is recommended to test horses that are to be used for breeding for PSSM beforehand.
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