Heat stress in horses

Heat stress in horses

Heat stress in horses

In Australia and in the tropics of Asia horses endure severe extremes in temperature and humidity. This can dramatically affect the horse's health and performance if not managed correctly. High temperature, high humidity, lack of air movement, poor ventilation, dehydration and exposure to direct sunlight all increase the danger of serious heat and sun related problems for humans and horses alike. Horses are no exception, especially when they are expected to perform at intense levels.

Signs of Heat Stress

During exercise, there is a significant increase in the amount of heat produced by working muscles. Muscles cannot transform energy into movement with 100% efficiency. Horses transform energy to movement at approximately 25% efficiency. As a result, some of the energy is lost in the form of heat. The rate of heat production by working muscles is proportional to how hard the muscles work. Therefore the faster a horse goes the more heat it produces. The amount of heat a horse produces in a 160 km endurance race would be enough to boil approximately 770 litres of water. That's approximately 7.7 litres per mile. Fortunately for the horse, it is able to dissipate around 97% of the heat it produces during an endurance race in cool-warm conditions!! If not, its body temperature would increase by around 15°C/h. In response, a horse increases its sweating rate, moves more blood to the capillaries under the skin and increases its rate of breathing in an effort to release this build up of heat.


Commonly observed signs of heat stress are:

  • Profuse sweating
  • No sweating
  • Rapid breathing rate - panting (>20 breaths / min)
  • Rapid heart rate (>50 beat/min)
  • Skin that is dry and hot
  • Unusually high rectal temperatures (>38°C)
A simple pinch test can basically determine whether a horse is dehydrated as a result of heat stress. When you pinch the horse's skin on the neck, it should resume its original position immediately. If the skin takes a while to resume to its normal position it could be assumed that the horse is somewhat dehydrated.

Who is at Risk?

Obviously horses that perform at high levels such as thoroughbreds, standardbreds, endurance and other performance horses will be influenced by heat stress during their training and competition. However, high performance horses are not the only types of horses to be at risk. Foals, especially the very young, have very poor thermo-regulating ability. They can overheat simply standing in the very hot sun. Add to this some activity, such as following an upset mare up and down a fence line, or weaning, and a serious problem can develop quickly. Horses stabled in poorly ventilated barns, which are also fed a poorly digestible feed will produce a lot of heat during digestion, which can seriously affect horses prone to heat stress. Additionally any horse that does not have access to salt and electrolytes will be at greater risk of suffering heat stress.

Warning Signs and Treatment

Horses that are breathing with great difficulty, appear distressed, become weak, develop diarrhoea or signs of colic, or stop sweating are in serious distress and need immediate attention. A veterinarian should examine the horse as soon as possible and provide medical treatment. You should immediately get the horse into shade and hose or sponge it with cool or even cold water. Direct the hose to the insides of the legs and the head and neck areas where large blood vessels are located near the surface. Use fans and encourage the horse to drink. Offer water, both plain and with electrolytes, and let the horse choose. It may take an hour or more to get all the vital signs back to normal. Horses that have seriously overheated tend to be more susceptible to overheating in the future. Horses that have suffered a serious episode should have 10 to 14 days of rest with some turnout and a gradual return to work. If a horse develops these symptoms in tropical environments such as Asia, they should be sent to higher elevations to be rested, as it is generally cooler and less humid. Thus horses can recover from heat stress more quickly Most horses adapt to summer weather if given time to adjust gradually. Use a little common sense and keep a close watch on horses for signs of distress. Horses that are overweight and not used to regular hard physical activity are at the greatest risk.


Anhidrosis (anidrosis, or incorrectly anhydrosis), from the Greek meaning "without sweating", is a condition primarily of horses in - and failing to adapt to - hot, humid climates. Although imported horses are reportedly most frequently affected, it can also occur in locally bred animals, and there appears to be no age, sex or significant breed predisposition. While there is an inherited component to this disease in cattle, this has not yet been established in the horse. With constant humidity & high ambient temperatures, persistently elevated blood adrenaline levels may result, leading to "conditioning" or insensitivity of the sweat glands to the affects of adrenaline. In turn, a progressive atrophy of these secretory glands may occur. Thus, initially horses may be seen to sweat copiously after exercise, but this gradually decreases over time until sweat patches are found only under the mane & tail. Although blockage of the sweat glands occurs in horses with anhidrosis, this is regarded as a secondary effect. The skin of affected animals is often scurfy, inelastic, and there is frequently alopecia (hair loss), most marked around the face. Horses that are dry-coated lose the ability to effectively dissipate heat through the evaporation of sweat on the skin's surface, and their core body temperature can rise dramatically, especially during exercise. In an attempt to reduce internal body heat, respiratory system stimulation occurs, and this in turn leads to dyspnoea (difficulty breathing) and, if pronounced, acute respiratory alkalosis, and possibly death. These animals are very susceptible to heat stroke, and exercise intolerance may be so severe that the animal is incapable of any form of work. Recent research suggests that the addition of certain amino acids and minerals in conjunction can aid and reduce the incidences of bouts of anhidrosis.


Nutrition contributes to managing heat stress?

Electrolytes -A racehorse can lose up to 10 litres of sweat per performance (work/race). This fluid isn't just water - it contains a lot of salt. These salts, when broken down into their chemical components, are referred to as electrolytes. These are typically groups of different salts that contain such electrolytes as sodium, potassium, chloride, magnesium and calcium. Electrolytes govern the transfer of water through cell membranes into or out of the cells. Thus, they function in getting the nutrients in and the waste products out. They are responsible for getting nerves to fire and muscles to contract. Essentially all of the physiological actions in the body require electrolytes. And importantly, they need to be present in the fluids in the appropriate amounts for these biochemical reactions to proceed in an orderly manner. If we don't provide at least a minimum electrolyte replacement, horses show up with such medical conditions as metabolic alkalosis, inefficient transport of oxygen and energy substrates, poor tissue perfusion, thumps, muscle spasms, exertional rhabdomyolysis, cardiac arrhythmias, gastrointestinal stasis, anhidrosis, kidney impairment, and poor recoveries. (Actually, poor heart and respiratory recovery is one of the key signs that can lead you to recognise the problems associated with the task of accomplishing thermoregulation.) The point is, most of these problems mentioned stem from the resulting dehydration and electrolyte imbalance.

The real question becomes, how much better could the horse do if it were in a state of ideal electrolyte and fluid balance? How many of the horses that fade in the last third, or have prolonged recoveries after the event, could be winners if their electrolytes and fluids were balanced and at appropriate levels? Thus, it is essential to correctly manage and supplement horses diets with electrolytes. In the commercial world of equine nutrition there is a plethora of electrolytes available. It is wise to carefully examine the labels of these products as many contain vast amounts of fillers and incorrect rations of electrolytes. Research conducted on horse sweat and sweat loss has resulted in products such as HYGAIN® REGAIN®being formulated.


Low Heat Feeding

Many equestrians would know about cool feeds, but what we are referring to is feeds that do not provide much heat as a result of fermentation in the hindgut. This is done by processing grains by, processes such as Micronizing. This enables nearly all of the starch in the grain to be digested in the small intestine, which results in little or no heat produced. Thus the hindgut only has to digest the fibre in the horse's diet, which in turn dramatically reduces the amount of heat generated by fermentation. Fat is digested quite efficiently in the horses' small intestine and does not produce any heat whilst being digested. The problem however is that high fat feeds in the tropics can quickly go rancid and mouldy. Anti oxidants and mould inhibitors can be added to these feeds, which provided they are stored correctly will greatly prolong the effective use by date of the feed. Elevated environmental conditions reduce the horse's appetite. Micronizing improves feed efficiency, thus less feed is required to be fed to the horse. Also the Micronizing process actually enhances the palatability of grains and is a useful tool to help keep horses eating during times of environmental stress. HY GAIN FEEDS extensive range of horse feeds are designed to reduce thermoregulatory stress in horses due to their micronized grains and high fat feeds. HYGAIN®'s feeds also contain elevated levels of electrolytes to ensure horses receive optimal levels of electrolytes.


Should I work My Horse Today?

A practical test to determine whether it is safe to work your horse is the "effective temperature" test, used to help determine the environmental conditions most likely to result in heat related illness in an exercising horse. This test combines ambient temperature with relative humidity. "When the sum of the ambient temperature in degrees Fahrenheit and the relative humidity is around 150, the rider should use caution in exercising the horse so heat build-up doesn't become critical". Most riding activities involving long or intense exercise should be postponed when figures approach 180. To convert Celsius into Fahrenheit F =( 9 * C / 5) + 32


How does a Horse Chill Out?

The single most important means the horse has for getting rid of the enormous heat load generated during exercise is evaporation. This accounts for approximately 65% of the heat dissipation. Sweat is evaporated off of the skin surface and cools the horse. The lungs account for approximately another 25%. The capacity of the respiratory tract to dissipate heat from the body becomes very important under conditions of high humidity and high temperature when evaporative conditions are not favourable. High humidity makes evaporative cooling less efficient. The combination of high temperature and high humidity combined can lead to serious trouble quickly. Direct sunshine on a clear day intensifies the problem.


What to do?

  • Hose horses with cold water. Hose the horse down then take it for a 1-minute walk, then repeat hosing. This will encourage the dilation of capillaries close to the skin, which will increase the evaporation of heat from the horse.
  • Encourage horses to drink cool water (small amounts frequently). If you are able to monitor the amount of water your horse drinks it will give you a good idea of how much water it is consuming. Horses working in hot/humid conditions should drink approximately 50-70Litres of water per day.
  • In severe cases vets have been known to give cold-water enemas or drenches to cool the horses core body temperature down to approximately 38°C. The critical temperature, one that is characteristic of a life-threatening situation, if maintained for any length of time, is 40-41°C.
  • Supplement electrolytes daily. 60g of HYGAIN® REGAIN® and 60g of Salt.
  • It is important not to overlook cool-down periods following exercise bouts, even when environmental temperatures are well within normal parameters.
  • Ensure that the horse has plenty of ventilation and access to a cool breeze as convection helps cool horses quicker. If none is available fans / air conditioners can be used to produce an artificial breeze. (Remember poor ventilation in stables can lead to respiratory problems).

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