Tying up Disorders in Horses
Horses with a history of tying up or have other continuing symptoms of muscle dysfunction may have one of a number of chronic exertional myopathies. These disorders can be caused by a single gene mutation, polygenic (multiple genes are involved), or an interaction between their genes and environment. These disorders are diagnosed through a combination of careful veterinarian examination, blood tests, exercise tests, muscle biopsies or genetic screening. Here we will focus on some of the more common disorders seen in horses. Polysaccharide storage myopathy Type 1 (PSSM1)– This disorder is most commonly seen in draft horses and stock horse breeds. It may be seen as frequently as 6-10% of Quarter Horses, Paint and Appaloosas. Horses of halter-type breeding are more commonly affected with race bred horses having lower incidence rates. PSSM1 is caused by a single gene mutation in the gene – glycogen synthase 1. Essentially these horses become “glucose sponges”, storing excessive carbohydrates as glycogen in the muscle when readily available glucose is fed to them. Unfortunately, this glycogen is not an advantage as it isn’t easily used for energy by the muscle. Instead it causes disruption of normal energy metabolism. These horses must be fed diets low in nonstructural carbohydrates and receive regular exercise. Most commonly bouts of tying up in these horses follow periods of rest or confinement with the onset of light exercise. Fortunately, a simple genetic test using either blood or hair roots can confirma horse to have PSSM1. Polysaccharide storage myopathy Type 2 (PSSM2)- PSSM2 is a relative new comer in identification and unfortunately far less is known in comparison to PSSM1. At this point in time, the causative genes have not been identified, nor has the metabolic pathway causing the disorder been discovered. These horses have abnormal glycogen in the muscle, and can only truly be diagnosed with a muscle biopsy. Poor handling or analysis of the biopsy can result in false positives, and many horses are diagnosed through clinical history and examination. In Quarter Horses, more horses with abnormal muscle biopsies will be PSSM1 compared to PSSM2, but in warmblood breeds the reverse is true, with a greater incidence of PSSM2. In Quarter Horse bloodlines, PSSM2 is more common in western performance horses, which include reiners, cutters, barrel racers and some pleasure and halter horses. Recurrent exertional rhabdomylosis (RER) – This muscle disorder is seen much more frequently in racing breeds – Thoroughbreds, Standardbreds and racing Quarter Horses. Some Arabians and warmbloods may also have RER. RER horses do not have abnormal glycogen, but have an alteration in the muscle’s ability to contract and relax. Fillies are more affected, especially those with nervous or excitable temperaments. Bouts of RER are more common when horses are excited or restrained from traveling at a faster pace than the horse desires. Diagnosis is based on clinical symptoms and history, as well as blood tests to monitor elevation in muscle enzymes present in the blood. Malignant Hyperthemia (MH) – This is a genetic disorder with a genetic mutation of the ryanodine receptor 1 gene. Horses may not show symptoms, or may show bouts of ER as well as high body temperatures. This disorder can be severe, with death following an episode of ER. Luckily MH does have a genetic test to screen for positive horses. Unfortunately, MH is also associated with PSSM1 and some unfortunate horses will have both mutations. These horses are much harder to manage and have more severe bouts of ER. If you suspect your horse has one of these disorders, it is important to work with a veterinarian to obtain an accurate diagnosis.
–Oklahoma State University Extension
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