To dehydrate means to lose water or moisture from a living body in excess of what is being replaced, creating a net fluid deficit. In horses, dehydration occurs when fluid losses — through sweating during exercise, elevated respiratory rate, illness, or environmental heat — exceed water intake. A horse that is dehydrating shows a progressive series of signs ranging from mild performance decline to life-threatening organ dysfunction.
The skin pinch test is a standard field assessment: pinching a fold of skin on the neck and releasing it should result in the skin snapping back within one to two seconds; a delayed return indicates the horse is dehydrating and requires intervention. The gum color and capillary refill time are also evaluated — pale, dry, or tacky gums with a refill time over two seconds are consistent with significant fluid loss. A horse begins to dehydrate measurably at roughly three to five percent bodyweight loss in fluid.
Horses dehydrate more rapidly in hot, humid weather or during heavy exercise, because sweat is the primary cooling mechanism and horses produce large volumes of it under exertion. The risk compounds in winter, when horses often reduce water intake when water temperature drops. A horse that is dehydrating and not treated can develop colic from poor gut motility, reduced gut motility, and impaired kidney function. See full clinical guide for clinical detail on causes, stages, and treatment.
Further Reading
The physiology of fluid loss is described in the dehydration article on Wikipedia. For veterinary guidance on fluid and electrolyte disorders in horses, see Metabolic Disorders of Horses in the Merck Veterinary Manual.