An unusual but very severe syndrome of muscle damage in horses is called seasonal pasture myopathy.  Myopathy means muscle disease, and seasonal is part of the name because the disease commonly occurs in the fall and is mostly found in the Midwest but does occur in East Texas so i felt you should be aware of the syndrome.  The disease causes severe muscle damage and is fatal is almost 90% of the cases.  Horses appear to be stiff, have difficulty walking and standing, void dark urine, and eventually have difficulty breathing and most do not survive.  And until recently, the cause of the disease was unknown but Dr. Stephanie Valberg at the University of Minnesota found a toxin in the seeds of the box elder tree.  This is the reason for the location of the disease, as box elder is usually a Midwestern tree but it does grow in East Texas. 

Ingesting box elder seeds results in a breakdown of respiratory, cardiac, and skeletal muscles. Although all horses in the pasture are susceptible, they all do not develop the disease and it is not known why some are susceptible and some are not.  Young horses are more susceptible than older ones, and we don’t know why unless they are more adventurous and eat more of the seeds. Horses turned out in pasture for more than 12 hours a day are more susceptible, especially if they are in a pasture with little grass.  And of course the larger number of box elder trees in the pasture, the greater the likelihood of toxicity.  Regardless, this is a serious disease and if you have box elder trees near your horse pasture, you should consider removing the horses or the trees.