This muscle disease, which was only discovered in recent years, occurs primarily in Quarter Horses and cold-blooded animals and is characterized by the storage of excessive amounts of glycogen (animal starch stored in cells) in the horse's muscles. It is known as polysaccharide storage myopathy (PSSM).
As the disease has not been known for very long, its causes have not yet been fully researched. Since the disease was initially only detected in Western horses, it was long considered to be a breed-specific condition. It is now known that all other breeds can also suffer from this metabolic disorder.
The muscle cells of affected animals show increased insulin sensitivity, which means that significantly more glucose (sugar) can be transported from the blood into the muscle cells and stored as glycogen. The breakdown and consumption of glycogen proceed normally.
Two different types of the disease can be distinguished in terms of occurrence and heritability:
This form mainly affects heavily muscled horse breeds such as Quarter Horses, Paints, Appaloosas, Haflingers, some cold-blooded breeds, but also ponies and heavy warmbloods. A hereditary predisposition (= genetic predisposition) has been proven for this type, which occurs in 90% of cases. With the help of a genetic test, it is now possible to determine exactly to what extent a horse is a carrier and transmitter of the PSSM gene. However, a positive test does not mean that the affected horse will necessarily develop PSSM; with appropriate feeding and husbandry, there is only a predisposition to develop the disease.
The exact triggers of this form of PSSM are still largely unknown. It occurs in about 10% of cases in lighter breeds (thoroughbreds, Arabians, light warmbloods) and no hereditary link has yet been proven.
With a low-carbohydrate and low-grain diet, affected horses can remain symptom-free for a long time. Sick animals often have a high proportion of easily digestible carbohydrates in their diet due to sugary feed such as carrots, apples, molasses, broken grains, or similar. For this reason, the disease often only occurs when young horses are broken in and suddenly receive large amounts of concentrated feed.
The first signs of the disease usually appear within 10 minutes of the start of exercise. Affected horses refuse to move, sweat profusely, and tire quickly. In mild cases, only a stiff gait and a stretched posture can often be observed. More severe cases are characterized by anxiety, muscle cramps and stiffness, or even recumbent posture in recurring episodes. If movement is not stopped immediately, the symptoms worsen, leading to colic (see corresponding article) and colic symptoms (see corresponding article). Severe fluid loss can also lead to kidney damage.
A general examination and a detailed preliminary report from the horse owner can provide the veterinarian with valuable information about the disease.
However, as the symptoms are similar to those of various other muscle diseases, a definitive diagnosis can only be made with the help of further tests. In acute cases, a blood test is advisable, as specific muscle enzymes are already significantly elevated even at rest. A second blood sample taken after light trotting shows massively elevated enzyme levels, which are a clear indication of PSSM but do not yet provide definitive proof.
Only a muscle biopsy (removal of muscle tissue) can clearly diagnose both type 1 and type 2 PSSM. However, the biopsy cannot prove heredity. After staining, excess glycogen appears as a dark area in the muscle fibers. The muscle tissue of affected animals has 1.5 to 4 times the glycogen content of healthy horses.
A genetic test for PSSM type 1 has been available for some time, but is currently only performed by one laboratory in Germany. However, the test only detects genetic predisposition and not the clinical disease. This test is already mandatory in some horse breeding associations.
If an acute attack occurs, the horse should be stopped immediately, covered warmly, and placed in a stall. Painkillers should be administered if the horse is in severe pain. Dehydration and fluid loss should be compensated for with appropriate infusion solutions to protect the kidneys. The next step in treating a horse with PSSM must be a change in diet. A high-fat diet that is free of rapidly available carbohydrates can prevent excessive glucose uptake into the muscle cells.
The prognosis is favorable if the feeding and training regimen is adhered to for life. With an appropriate change in diet and additional regular exercise, up to 90% of all affected horses can return to their original performance level.
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