Thermoregulation & Dehydration in the Horse
Dr Joan Deetman BSc BVMS GDEd - FEI 2* Vet
The horse’s average body temperature of 38C may vary according to the ambient
temperature, humidity and exercise. A stable core body temperature must be maintained
for the body’s organs, muscles, cells and nerves to function properly. If the body
temperature rises above 41C, oxygen supply to the tissues will be inadequate to maintain
normal metabolic function resulting in tissue damage.
In order to maintain a stable core body temperature, heat must be lost from the body
during and after exercise. Heat loss occurs via:
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Evaporation – most effective method, removing up to 2/3 of body heat. Horses rely
primarily on evaporation of sweat to cool the body. High humidity will decrease the
effectiveness of sweat cooling the body.
Conduction – heat transfer from the body to the skin (will allow heat to then be
transferred to the environment).
Radiation and Convection (using air flow) allow heat loss from the skin to the
environment. These methods need a temperature gradient and become ineffective
when skin temperature equals the ambient temperature.
Respiration – horses do not pant like a dog however respiration rate will increase
with exercise helping to remove heat from the body. If heat loss is ineffective,
respiratory rate may increase to a level above heart rate (inversion).
Unlike humans, the sweat of the horse contains a higher concentration of electrolytes
relative to the concentration of electrolytes in plasma. The horse produces massive amounts
of sweat in exercise; this large loss of body water plus electrolytes must be replaced in order
for the horse to recover effectively. If not, consequences may range from slow recovery to
serious metabolic illness.
It therefore makes sense that effective recovery from exercise will only occur if oral
rehydration fluids contain electrolytes with a composition similar to that of equine sweat. It
must be stressed that these remedies must be FLUIDS – water loss must be replaced in a
dehydrated horse. Electrolyte pastes alone, without concurrent water replacement, may not
achieve recovery and must not be given alone to a dehydrated horse.
In mammals, the thirst reflex is normally stimulated by changes in blood osmolality - the loss
of sweat causes decreased blood volume and increased blood osmolality. A unique situation
occurs in the horse where massive loss of sweat and electrolytes (esp. sodium) decreases
blood osmolality and inhibits the thirst reflex (adipsia).
Along with heat production and electrolyte loss, gut function can be affected with slowing of
gut movement (ileus) causing a build-up of fluid and gas which further reduces the
inclination to drink. The end result is dehydration where water intake and reduced intestinal
absorption cannot replace the fluid loss from sweat, urine, faeces and respiration.
A dehydrated horse in a hot, humid environment cannot rid itself of generated heat, leading
to heat stroke. This condition may also occur where horses are transported in poorly
ventilated floats, in obese horses, horses with long coats and where the horse fails to drink.
Riders should be very aware of their horse’s water intake and strive to achieve rehydration
through the use of solutions spiked with electrolytes or once the horse has drunk
sufficiently, then, and only then, use electrolyte pastes.
After any activity that causes dehydration e.g. exercise, floating - fresh clean water and
water spiked with electrolyte solutions can be offered. Offering a solution of water,
electrolytes and a small amount of molasses immediately after unloading your horse from
the float after a day of competition is a great way to replace your horse’s losses.
It is important to realise that the horse’s electrolyte requirements can differ depending on
the activity undertaken. The electrolyte requirements of a racehorse will be different to the
electrolyte requirements of an endurance horse. For example, an endurance horse after
competition will, in most cases, be in a state of metabolic alkalosis. The endurance horse
must not be treated with or supplemented with electrolytes containing bicarbonate.
Electrolyte supplementation should be part of the training horse’s diet as studies show
horses will drink better if electrolyte supplementation is already occurring before exercise.
Salt contains sodium and chloride and will therefore only supply part of the electrolytes
required. There is a confusing array of electrolyte supplements available so riders should
discuss their horse’s electrolyte needs with their vet.
Prevention and Control
Diet, Water & Electrolytes
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Know how much water your horse is drinking daily and during competition. Normal water
intake in a resting horse is between 20-40 l/day. Use water buckets that will allow
measurement of water intake.
Know what electrolyte solutions are appropriate for your horse in your sport.
Always offer clean water plus electrolyte spiked water. Use pastes only after the horse has
drunk. Add salt and appropriate electrolytes to your horses diet.
Be observant of changes in water intake and urine output.
Take water with you to competitions – don’t assume your horse will drink any type of water.
Providing adequate roughage in the diet will allow the large colon to supply a reservoir of
electrolyte rich fluid for absorption during exercise. Adequate roughage, water and
electrolytes should be available and intake monitored.
Body Condition and Acclimatisation
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Check for signs of dehydration – pinch the skin on the point of the shoulder (not the neck) to
test skin recoil which should be immediate. Dry, tacky mucous membranes, slow capillary
refill (blanch the gums), dark pink mucous membranes, sunken skin around eyes, depression,
unwilling to eat and not drinking are all signs the horse is dehydrated.
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Slowly acclimatise your horse to environmental conditions and high workloads. Fatigue will
quickly overcome an unfit horse with a long coat ridden hard on a hot day. Regular steady
work of increasing intensity will improve and adapt the musculoskeletal and cardiovascular
systems.
Strapping
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Take sponges, buckets, spray devices, etc to facilitate cooling your horse at a competition
Endurance riders take drums with water pumps, hoses and spray nozzles to cool their horses
at rides. Ice boots are used by event riders, use only on legs.
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Cool down properly after competition – get off the horse and loosen gear, walk it until
respiration slows to near normal then start strapping procedures. Riders will delay recovery
by not dismounting and not walking the horse out may only serve to increase body heat.
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Strapping when the horse’s temperature is high should consist of copious amounts of water
hosed or poured on especially around the head, neck and legs. Look for the large superficial
skin vessels which can be cooled directly and carry cooled blood into deeper tissue. DO NOT
pour ice water onto the horse of this may cause constriction of blood vessels and actually
prevent cooling.
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Scraping off the water will allow the skin to cool and therefore allow more heat to be lost
from the body. Very important in humid weather - warm water must not be allowed to sit
on the horses skin. Walk the horse while the respiration and heart rate recover, strap again,
scrape and walk as many times as necessary. Monitor heart and respiratory rates.
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Spray mists of small volumes of water are a very effective way of cooling the horse, followed
by scraping. Fanning the horse, allow it to rest and seeking shade will effectively contribute
to cooling.
Remember the mechanisms of heat loss used by the horse – evaporative (sweating), radiant and
convective (cool water, shade, cool air flow) and use them to allow quick and effective recovery for
your horse.