It happens innocently enough. A healthy, seemingly fit horse is competing at a local horse show on a hot, humid summer weekend
or participating in a 25-mile competitive trail ride or perhaps simply working cattle as part of a regular day. It has been
hot all week, but the horse has had access to plenty of water and even had a salt block available. The horse has been sweating
heavily all morning, but its owner made sure to take numerous breaks to offer it water. Even though it hasn't shown much interest
in water, the owner is certain his horse will drink when it needs to, so they continue to ride or jump or work.
Water is 20 times more efficient than air at cooling a horse. The proper method to use is spray (or sponge), scrape and repeat.
Meanwhile largely unnoticed metabolic and physiologic processes are occurring that soon can make this equine athlete an emergency
situation that you will have to deal with. Research has shown that during a 50-mile endurance ride in ambient temperature,
the average horse will do enough muscular energy and produce enough heat to melt a 150-pound block of ice and then bring it
to a boil. At a temperature rise of almost 2 degrees F per hour (without sweating) a horse would soon reach the critical temperature
of 42 C, and a crisis would ensue. The need to remove this heat from the performing/working horse is crucial.
Horses lose heat through evaporative cooling, utilizing sweat and through respiration. About 1.5 to 4 gallons of sweat can
be lost each hour during strenuous work in hot conditions. Add slowly progressing dehydration to this picture, and the athletic
horse begins a dangerous process that, if not altered, can result in heat stress, heat stroke and possible death. The horse
working on a hot day in our example is beginning to become dehydrated. Usually dehydration causes the plasma sodium (Na) concentration
to increase, and this rise in sodium is a trigger for the thirst response in horses. However if a horse is becoming systemically
dehydrated but is also sweating heavily, then the overall loss of Na in the sweat can offset the drop in plasma volume, and
the sodium concentration can stay the same or actually increase. Horses in this situation are losing significant electrolytes
and are beginning to experience hydration problems, but they might not be thirsty and might not drink when water is offered.
It is important to stress to owners that all exercising horses must drink even if they don't appear to be thirsty.
Even though the old saying goes: "You can lead a horse to water but you can't make him drink," a good number of endurance
and event riders will tell you differently. Horses that will be exercising heavily in heat and humidity must be taught to
drink at every opportunity — especially during the early stages of exercise.
Dr. Harold Schott and researchers at the Veterinary Medical Center at Michigan State University have shown that offering horses
slightly hypertonic water (0.9NaCl) during exercise will stimulate the thirst center and will result in that horse drinking
more overall water than if non-salty water had been used. Their research would also support offering exercising horses water
at ambient temperature because experimental horses drank more water at room temperature than either warmer or cooler water
when offered. The consequences of dehydration are far too serious to ignore possible ways to encourage horses to drink.
Once dehydration begins to take place, the horse's body loses the ability to sweat efficiently. Without enough sweat, the
horse cannot continue to utilize evaporative cooling to dissipate the heat that it is still producing. As the body dehydrates,
there is a loss of plasma volume, so blood becomes less fluid. The cardiovascular system becomes less efficient at transporting
oxygen throughout the body, and it must begin to work even harder to support the working horse.