Like the old saying goes (or similar to it), you can lead a horse to an electrolyte replacement fluid but you can't make him drink. As most equestrians know, balancing a horse's electrolyte and fluid intake with the sweat they produce during exercise is an ongoing challenge.
"When horses sweat they lose more electrolytes per liter of sweat than humans do, which means that horses do not develop as strong of a 'thirst stimulus' as human athletes do," explained Hal Schott II, DVM, PhD, Dipl. ACVIM, professor of equine medicine at Michigan State University's College of Veterinary Medicine, during his presentation at the 12th Congress of the World Equine Veterinary Association held Nov. 2-6, 2011, in Hyderabad, India. "They simply do not have the same drive to drink while competing as humans."
Another reason that sweating competitive horses don't drink when their riders and veterinarians think they should is because of the fluid reserves in their gastrointestinal systems.
"Approximately 5% of their body weight is extra fluid--called a fluid reserve--in their intestines that can be used to replace fluid during endurance exercise," relayed Schott.
But what happens when this fluid reserve is drained and excessive electrolytes are lost during competition?
"When horses participate in prolonged exercise, the fluid lost in the sweat exceeds fluid replacement and a state of 'involuntary dehydration' ensues," said Schott. "The consequences of an involuntary dehydration may include metabolic problems, such as decreased gut motility and 'thumps' (synchronous diaphragmatic flutter), circulation problems, uncontrolled hyperthermia, and even 'exhausted horse syndrome.' "
Horses with exhausted horse syndrome most commonly develop colic signs associated with decreased intestinal blood flow and motility. In a few cases more serious multiple organ failure and death can occur due to prolonged decrease in blood flow to all the abdominal organs.
So how can riders prevent these problems?
"Multiple field studies of endurance rides have shown that most of the body mass loss due to sweating occurs in the first half of competition," Schott explained. "This does not mean horses stop sweating; rather, successful horses drink and eat at rest breaks later in the ride at a rate that matches further sweat fluid losses.
"A 'red flag' for impending problems is when horses lose interest in eating and drinking at vet gates or checkpoints," he continued. "Veterinarians examining horses will not be able to pick up on a decrease in appetite or thirst because they only see the horses for a short time. Thus, it is the riders' responsibility to monitor their horses' attitude, appetite, and thirst and, if there are concerns, contact the ride veterinarians before going back out on the trail."
A full summary of Schott's presentation titled "Challenges of endurance exercise: Hydration and electrolyte balance" will be available for free on the International Veterinary Information System.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.