Azoturia/Tying-up Syndrome
Jim Hamilton, DVM

Tying-up or azoturia is a muscle metabolism problem. The causes are several but the result is the same, muscle cramping. If the cramping is severe enough then the myoglobin released from the damaged muscle goes from blood to kidneys and into the urine, turning the urine a dark red color. Most cases of tying-up are not that severe and usually present as a horse that has been off for several days and is then exercised aggressively. In the middle of the ride (or sometimes right at the end) the horse's stride shortens and he cramps up. 
Treatment is usually routine in that anti-inflammatories, small doses of sedative, muscle relaxers and in some cases IV fluid therapy gets the job done. The horse is then put on a very low energy diet for at least 1 week and blood muscle enzyme levels are obtained to assess the damage. The bigger challenge is to figure out why it happened and to prevent it.
One of the more common causes of azoturia in mares is the hormonal effect of coming into heat (estrus). I have seen many young athletic mares who are on a good nutritional and exercise program tie-up only because they were in heat. In fact, this past weekend we had a large driving competition here in Southern Pines and a mare that was part of a two horse team came into flaming heat on the day of the endurance phase. She was very fir and on a good nutritional program but shortly after completing cross country, she had a severe episode of azoturia.
Some of the other common causes are too high a level of protein and energy in the diet and poor electrolytic supplementation in hot weather.
Prevention involves use of potassium salt and baking soda supplementation in the feed, making sure that dietary energy and protein levels are not too high and that you be aware of predisposing factors such as estrus in your mare or a lameness that is overlooked. Last but not least, you will have fewer problems in general if the horse is kept on a "regular" consistent exercise program.