Strep Immune-Mediated Myopathies in Horses

October 22, 2007 (published)

Strangles is a common upper respiratory tract infection in young horses. Most of the time strangles is a mild disease.  Strangles is caused by the bacterium strep equi and usually causes nasal discharge, fever, and swollen lymph nodes, especially under the throat.  However, complications can develop after strangles infection. An unusual complication of strangles is muscle damage that can occur at the same time as the upper respiratory infection or just afterwards.  These horses develop a stiff gait and muscles over the back and hips become painful.  This disease is extremely severe as most of these horses will be unable to rise after just a few days due to the severe muscle damage, and they may need to be euthanized.  It is believed this syndrome is related to toxic shock syndrome in people, which is also caused by a strep bacterium. 

These horses can be treated with penicillin, anti-inflammatories for pain, cortisone, and fluids, but most do not recover.  Another complication of strangles is an immune-related disease called purpura hemorrhagica.  After horses recovers from strangles, they may develop stiff and painful muscles in this syndrome but they also may colic.  Colic in horses has multiple causes but in this instance it is related to the horse’s reaction to the bacterium that decreases blood supply to the intestine.  Another muscle disease related to strangles can also cause muscle atrophy.  Muscle atrophy is a loss of muscle mass and when it’s related to strangles, the muscle loss develops over a very short period of time.  If your horse develops painful swollen muscles, colics, or loses a large amount of muscle quickly after an upper respiratory infection, contact your vet immediately         

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