We’re all familiar with images of hot and steamy horses being doused in water at the end of a gruelling cross-country phase. But why exactly is cooling a horse after competition so important?
Heat is a major by-product of energy production and muscle function during exercise, with the amount of heat generated increasing in proportion to the intensity of a horse’s work.
Only 20-30% of the energy from the body’s stored supplies is used for muscle contraction and mechanical work — the remaining 70-80% is lost as heat during metabolism of these reserves. The resulting heat accumulation means that controlling body temperature during and after exercise is essential to avoid ill effects from excessive increases.
A horse’s normal core body temperature at rest should be between 37.5°C and 38.5°C. When this rises above 41°C to 42°C, side effects and signs of heat exhaustion can be seen. These can range from an excessively high heart rate and respiratory rate to dehydration, fatigue and poor recovery from exercise.
If left untreated, heat exhaustion can develop into heat stroke — a condition in which the body is unable to regulate its own temperature. This can ultimately lead to multi-organ dysfunction, including kidney and liver failure, abnormal heart rhythm, loss of coordination and, in severe cases, death.
Sweating it out
Evaporative cooling — losing heat through the skin by sweating — is the most important mechanism by which a horse controls his body temperature during and after exercise. Water from sweat evaporates into the air, taking heat energy with it.
In addition, about 30% of heat loss occurs through the lungs and respiratory tract during normal breathing.
These methods of heat loss are at their most efficient when the difference between the skin temperature and surrounding air temperature is greatest, such as in cool, dry weather.
Much of the research into the most effective ways to cool horses after competition has come from preparation for equestrian disciplines at the Olympics — notably those that took place in hot, humid conditions, such as Atlanta in 1996 and Hong Kong in 2008.
Difficulties experienced by horses during the cross-country phase at the Barcelona Games in 1992 highlighted the risks of heat stress in hot weather in high-level competition, despite the relatively low humidity at this event. Two horses collapsed, and a team of experts, led by Dr David Marlin, initiated research into thermoregulation (features, 7 July).
Evaporation of water from the skin surface during sweating is less effective in hot and humid conditions, because the surrounding air is already saturated with water and therefore the difference between air and skin temperature is minimal. Sweat starts to drip from the body but removes just 5-10% of the heat that would have been lost through evaporation.
In these extremes of climate, even the fittest horses struggle to lose enough body heat by sweating.
Core body temperature can rapidly climb to dangerously high levels towards the end of exercise and immediately afterwards.
Research has shown that applying cold water (9°C) to a horse after exercise in hot, humid conditions helps bring core body temperature back down to normal faster than using tepid (31°C) water. Cold water has no negative effect, such as increasing the risk of tying-up.
Shade and misting fans provided in rest areas were also found to help cool horses.
Fitness and acclimatisation to local weather are additional key factors in avoiding heat stress.
A fitter horse will experience the same core temperature rise as a less fit animal, but he will recover more efficiently and lose less water overall through sweating.
Dehydration and its deleterious effects on blood flow, particularly to the skin, will also make regulation of body heat less efficient. In warm weather, a horse can lose 3kg of bodyweight for each hour that he travels in a horsebox. Most (around 90%) of this is due to fluid losses and dehydration, so adequate preparation and correction of any pre-existing dehydration before a competition is vital to maximise his ability to cope with hot weather conditions.
Slosh and scrape
In normal warm weather, the ideal way to cool a horse at the end of a competition is to apply copious amounts of cold water over his whole body.
He should be offered water and electrolyte solutions to drink, and walked in-hand to promote good circulation through the muscles.
This should allow recovery with minimal side effects.
If a horse is slow to recover and his temperature continues to rise after exercise has finished, help get rid of excess body heat by repeating the application of cold water before scraping it off with a sweat-scraper and applying more.
Offering electrolyte solutions as well as plain water to drink has the additional benefit of replacing salts lost in sweat during exercise, which helps to restore normal circulation and promote recovery.
Rugs and sheets should not be put on until after the horse has fully recovered, as these insulate the horse and prevent heat loss from the skin.
Feeling the heat
While ambient temperature and humidity levels in the UK rarely reach those experienced in more tropical climates, signs of heat exhaustion are not uncommon in horses competing here in the summer — particularly if the weather suddenly becomes hotter.
Look for prolonged and delayed recovery (beyond 10 to 20 minutes after the end of the competition), sometimes accompanied by shaking, weakness, loss of awareness of surroundings and persistently elevated heart and breathing rates. Along with continued, excessive sweating, these signs suggest that the horse is exhausted and at risk of heat stress.
Ongoing increases in rectal temperature after the end of exercise are a strong indicator of heat stress and the potential to develop heat stroke. In its most severe form, this can leave a horse unable to sweat, reluctant to drink water or to walk, and can cause staggering, depression and collapse.
Urgent veterinary treatment, including intravenous fluids, is vital in these cases to prevent dire complications.
The case for ice boots, wraps and gels
Cold therapy — the application of ice packs, boots or massage, or cold spray and ice- and cold-water immersion — has been recommended to reduce inflammation and prevent tissue oedema (filling or “stocking up” of the legs).
The principle behind these treatments is to reduce blood flow to the area. There is some evidence of a positive effect on tissue metabolism and enzyme activity, including a reduction in those that initiate and promote arthritis, after 15 minutes, when skin temperature is reduced to 10°C or when joints are reduced to 30°C.
Cold therapy must be used carefully; significant skin damage — including burns — can occur if tissue is cooled below 10°C. Do not apply cold treatment to open wounds or directly over superficial nerves. Always use a damp barrier material, such as a clean cloth, between a solid cold pack source and skin to protect against damage.
The evidence for cold therapy in preventing injuries is sparse, but positive effects of cold water spa therapy (salt water at 5°C to 9°C) on tendon and ligament injuries have been shown. The general recommendation is to apply cold therapy for up to 20 minutes every two hours during the initial, potential injury period (24 to 72 hours).
Immersion of the lower legs in iced water has been suggested as a more efficient way of cooling tissues than the application of cold packs or crushed ice, but much of the research in this area is extrapolated from studies on humans and other species.
Further research into the benefits and methods of cold therapy in horses after exercise is still needed.
Ref: Horse & Hound; 14 July 2016
