I brought a stallion from Tucson, Arizona, that had never sweat in his life, to Illinois. We put him on a feed additive specific for horses with anhydrosis. Later when we were working him, he started sweating. Anhydrosis is the inability of a horse to produce normal amounts of sweat. Some horses sweat normally in spring, but develop anhydrosis in the summer as heat, humidity, and workload increase.
Horses depend heavily on sweating to regulate body temperature.
Many horses raise their body temperature to around 104 degrees during training. Exposure to a temperature of 106 degrees may cause brain damage. Sweat is critical so that evaporation cools the horse. Sweat glands of horses with anhydrosis and of normal horses look the same under a microscope. Something is interfering with the physiological, chemical, or neural transmission.