The patient is a young Quarter Horse stallion who went off feed and became dull and lethargic over a couple of days. He had also lost a lot of muscle along his top line and gluteals. When Dr. Wright saw him, he was quiet and had a mild fever of 102. He also had edema (swelling) in his distal limb (lower legs) and sheath. His vaccine history was unknown at this time, and he was housed with many other horses of various ages at a busy training barn. His lungs sounded normal, and he had no cough or nasal discharge. Although he was not eating well, he was still passing manure and showing no signs of colic (abdominal pain).
Bloodwork showed no evidence of infection at that time, but the muscle enzymes were all abnormal, likely due to the rapid weight loss.
Due to the colt’s breed, age, location (busy barn with lots of horses in and out), and the rapid loss of muscle on the topline, a condition called Immune Mediated Myositis (see below) was suspected.
To confirm the diagnosis, a sample of hair was sent out to an equine genetics lab to see if the horse is positive for a gene known to cause this disease.
Treatment was initiated while waiting for the results of the genetic test, and consisted of a tapering, long-term steroid therapy. The colt began to show signs of improvement after two days and is expected to make a full recovery.
Horses can develop muscle diseases as a result of complications of an infectious disease or as a result of the immune system attacking skeletal muscle. Depending on the disease clinical signs vary from severe muscle damage to muscle atrophy and wasting.
Immune-mediated myositis (IMM) is a disorder in which the immune system of horses attacks the skeletal muscles causing rapid atrophy of the muscles along the topline. There is a specific inherited form of IMM in Quarter-horse related breeds. Immune-mediated myositis is rare in horses but it is the most common cause of rapid atrophy of the topline in Quarter and Paint horses. The gene mutation is present in about 7% of Quarter Horses and is most common in reining, working cow horses and halter horses (16-22% carriers). The effect of the mutation can lie dormant until an immune-system attack is triggered by exposure to a pathogen.
Horses that have had recent exposure to a respiratory disease are more likely to develop the disease.
Luckily, most horses recover without incident. Management strategies for horses with the gene or previous episodes include a high-quality protein feed in addition to free choice hay. Spreading out vaccines may be helpful if the horse’s episode appears to be brought on by vaccination.
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