Heat Stress in Horses: A Life-Threatening Situation

By: Rochelle Jensen BVSc and Joan Norton VMD DACVIM

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Hot weather can have devastating effects on horses, especially when increased temperatures are coupled with high humidity. It is important to appreciate the potential for serious problems associated with horses over-heating during exercise and in hot, humid conditions. Left untreated, heat stress can have life-threatening consequences for horses. In this article, Drs Joan Norton and Rochelle Jensen from the University of Queensland Equine Specialist Hospital explain how it can develop, how to recognise the signs and what you can do to avoid its devastating effects.

Dissipating heat

Heat stress is the result of increasing heat storage in the horse’s body, due to insufficient dissipation. In the horse, sweating is the most important means of thermoregulation and accounts for two thirds of heat dissipation. Sweat glands provide the water that, through evaporation, cools the skin. Body heat is also lost when the blood is shunted towards the skin through the processes of convection and thermal radiation. A small amount of heat is also lost via the respiratory tract.

Environmental factors

Environmental factors, such as temperature and humidity, have profound effects on ability of sweat to evaporate and reduce body heat. When the moisture content in the air is high, as happens in hot, humid climates, evaporation slows. Hot sweat remains on the skin and acts as insulation, preventing the heat from dissipating. The body’s response is to produce even more sweat and the horse becomes dehydrated, develops electrolyte imbalances and still fails to cool its core body temperature.

Contributors to heat stress

Hot weather is not the only factor which contributes to heat stress. A horse that is exercised at a level higher than that of its conditioning or fitness level is also at risk of developing heat stress. Older horses suffering from Cushing’s Disease will not shed out their winter coats. This extra fuzz traps heat in and prevents cooling. These horses should be body clipped to help them keep cool (don’t worry, clipping the hair now won’t affect its ability to grow back once the weather turns cooler). Horses with known respiratory problems such as Inflammatory Airway Disease or Heaves should also be monitored closely. Hot dry conditions can exacerbate their airway disease and decreased air exchange may contribute to higher body temps. Finally, foals who are being treated with a macrolide antimicrobial (erythromycin, azithromycin or clarithromycin) for Rhodococcus pneumonia should be kept in out of the sun. There have been reports of idiosyncratic hyperthermia in foals on erythromycin (though we should be cautious with all three) and drug induced heat combined with high ambient temps is a recipe for heat stroke.

Horses should undertake a training program that is suitable to their level of exercise or competition. When there is a requirement for horses to perform in hot or humid conditions, the horse should be acclimatised gradually. Most horses require a minimum of three weeks to allow for acclimatisation, and this is an important consideration to take into account when there is an unseasonal heat wave and when traveling from cooler to hotter climates. Diagnosis of heat stress is usually based on clinical signs and a history of exercise during hot or humid weather. Horses showing signs of heat stress require immediate attention.

Signs of heat stress

Signs of heat stress are associated with the deleterious effects of hyperthermia, dehydration and electrolyte imbalances. Horses with early heat stress may exhibit signs as subtle as a loss of enthusiasm for their work. Horses with more severe signs of heat stress are fatigued, have rapid, shallow respiration, elevated heart rate (i.e. above 60-80 beats per minute at rest) and a rectal temperature above 41 degrees Celsius. As the core temperature increases, the horse’s respiratory rate will increase. The horse may develop rapid, shallowing respiration, which looks like panting, or the horse may develop a slow, deep respiration with marked breathing effort. They may develop irrational behaviour, such as kicking, and will appear agitated. Horses with worsening heat stress may stagger and seizure. Even if they recover from the initial episode, signs of muscle damage or organ dysfunction can occur up to three days later.

Early detection

Monitoring of rectal temperature, pulse rate and respiration rate will aid in the early detection of heat stress.

• Heart rate should drop to 44-52 beats per minute within 15 minutes of exercise.
• Normal respiration for an adult horse is 8-12 breaths per minute (normal foals are between 20 and 40 breaths per minute). However, horses that are heat stressed can have respiratory rates of 120-140 breaths per minute.
• Respiration rate can be determined by watching the horse’s chest move in and out (inhalation and exhalation is counted as one breath) or, if the horse is breathing heavily enough, the nostril flare can be counted. Remember that respiration rate is also affected by excitement, respiratory disease or changes in the acid-base (pH) balance, which could be caused by diet and/or disease.
• The normal rectal temperature for an adult horse is 37.5 degrees Celsius to 38.6 degrees Celsius. Healthy foals less than one month of age can have a rectal temperature as high as 38.8 degrees Celsius. To take a horses rectal temperature, approach from the side. Raise or move the horse’s tail and insert the thermometer into the anus. Tilt the thermometer to the side, so that you are measuring the temperature of the rectal wall and not the temperature of faeces sitting in the rectum.

Prevention

Most of the research has been done on exercising horses and has universally shown that horses exercising in both high temps and high humidity have higher core body and blood temperatures, higher heart and respiratory rates, lower cardiac output, worse sweat response, longer recovery and a decreased ability to dissipate heat from their bodies. And while the horses in these studies were being run on treadmills at increased ambient temps you can imagine that even a horse just standing in a sun baked field would experience the same changes.  While we can’t bring our horses into our homes to enjoy the cool refreshment of central air, we can take some simple steps to keep our horses cool and prevent heat stress.
1)   Shade- Temperatures in direct sunlight are going to be much higher than in the shade, so make sure your horses have the option to get out of the sun. Run in sheds, large trees or even keeping them in the barn can help keep them cool.
2)   Water- In high temperatures, and even more so in high humidity, more water is lost through sweat and increased respiration. Horses should always have access to fresh water but it’s even more important in this weather. So make sure your water troughs and buckets are full (and clean) at all times. If you’re not able to make it to the barn to refill them throughout the day, think about adding extra buckets to your stalls or paddocks.
3)   Electrolytes- Offering a salt or mineral block, placing an E-lyte powder on their feed or administering an electrolyte paste will not only replace important electrolytes lost through sweat but can also increase your horses’drive to drink plenty of the aforementioned water.
4)   Baths- Cold hosing a horse and allowing them to air dry can help lower body temperature. The cold water will feel great and their wet hair will cool the skin through evaporation. For extreme conditions, water can be mixed 50/50 with rubbing alcohol to increase the evaporative effects. ** but make sure never to get any alcohol mix anywhere near your horses eyes!!**
5)   Ventilation- We all know how good a cool breeze feels. Placing fans in your barn aisles and in front of stalls can help keep things cool. Make sure barn doors and windows are open to let the air flow.
6)   Rest- Though we spend all winter dreaming of the days when we can ride across the fields, basking in the sunshine, extreme temperatures may be a good excuse to hit the beach and give your horse a break. We discussed earlier that horses exercised in heat and humidity have much higher stress markers and much slower recovery. If you don’t have to train, don’t risk it. If you are getting ready for a big event, or use your horse in your work, plan activities early in the morning or in the evening when the temperatures are cooler.
7)   Protection from the Elements- The hot weather is caused by the sun’s rays, which also causes sunburns. Make sure to apply sunscreen to white noses which can become burnt just as easy as I turn into a lobster at the mere mention of UV rays! Warm weather also brings along biting insects which irritate and agitate your horses. Use fly sprays, fly masks and sheets to help keep your horse comfortable and bug-free.

Treatment 

Treatments include many of the same steps as prevention. Obviously you should immediately stop whatever activity you are doing and get the overheated horse out of the sun, under a hose and in front of a fan. Offer fresh water. Extreme cases, or cases that do not respond to the above measures should be seen by a veterinarian immediately. Prolonged high body temperatures can lead to systemic inflammation, denaturation of proteins and even multi-organ failure. You veterinarian can help lower your horses core temp by lavaging the stomach with cool water (via nasogastric tube) and administering cool fluids intravenously. Bute or Flunixin can also be administered, not to lower the body temperature but to prevent some of the ill effect of the inflammation induced by excessive heat.

Anhidrosis

Horses most at risk of heat stress are those who suffer from anhidrosis. Anhidrosis (from the Greek meaning “without sweating”) is a condition primarily of horses in which the horse fails to sweat effectively in response to heat or exercise. It occurs almost exclusively in horses living in hot or humid climates, and the incidence is increased when horses are bred in temperate climates and transported to the tropics.

The condition, also known as ‘Puffs’or ‘Dry Coat’, manifests as an inability to produce sweat. Consequently, the horse will pant with marked nasal flaring when hot and their core temperature will increase. Despite an increasing body temperature, the horse will not sweat adequately, producing only small patches beneath the mane, between the legs and under a halter. Long standing cases may develop a sparse hair coat with scaling, particularly around the face, neck and shoulders.

The exact cause, or set of causes, of this condition is unknown. Hypothyroidism, hypochloremia, elevated blood adrenalin and exhaustion of sweat glands have been suggested as potential causes. Prolonged stimulation of the sweat gland receptors may decrease their responsiveness to stimulation. Anhidrosis occurs with varying degrees of severity from partial sweat production to complete absence of sweating.

Diagnosing anhidrosis

Definitive diagnosis of anhidrosis requires intradermal skin testing using injections of ß2 agonists, such as terbutaline or salbutamol. Serial dilutions are injected into a shaved region over the neck and the dilution at which a horse produces sweat provides some information about the severity of anhidrosis.

Treatment

There is no proven treatment for anhidrosis and horses who suffer from this condition should be considered for relocation to a cooler climate or be housed in a temperature-controlled stable. A return to normal sweating may occur, but recurrence of the condition is almost always observed if the horse is returned to a hot, humid environment.

In summary

The thermoregulatory system of a horse is capable of maintaining core body temperature within a specified range in the face of extreme heat during exercise or during hot, humid weather. However this system can become overwhelmed in extreme circumstances. The prognosis following an episode of heat stress is variable depending on severity. An understanding of the process that leads to heat stress and recognising the early signs are essential to ensure the welfare and continued performance of your horse. 

About the Authors: Joan Norton VMD DACVIM is an Equine Internal Medicine Specialist at UQ Vets Equine Specialist Hospital at the University of Queensland in Gatton. She is board certified in Large Animal Internal Medicine by the American College of Veterinary Internal Medicine. Her areas of interest are emergency and critical care and neonatal intensive care.

Rochelle Jensen, BVSc, graduated from James Cook University in 2012 and completed an Equine Internship at UQ Vets Equine Specialist Hospital at the University of Queensland in Gatton in June 2014.

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