What are Heat Stress?
Heat stress, which is also known as heat exhaustion, is typically due to the loss of fluids and electrolytes during a period of time during exercise that exhausts the horse, such as due to high temperatures, the horse not being in good shape, and lack of sweating.
When a horse is exercising, either for a long period or for a short period at a high intensity, it can challenge his ability to move heat quickly from his body. In order to get muscular heat out of his body, your horse will sweat, which will pull heat from within his body to his skin (known as evaporative cooling). Approximately 70% of the heat of his movement is usually dispelled through this method.
When it is hot and even more so when it is humid, the ability of the horse to release the heat from his body is adversely impacted. He will sweat, but in many cases it is not enough for the heat that is building up. When your horse is exerting himself over a long period, fluid is lost and his electrolytes become unbalanced as he sweats. Over an extended period of time he will continue to dehydrate as the heat from his muscles is removed through sweating and evaporative cooling.
Also known as heat exhaustion, heat stress results from the loss of fluids and electrolytes in your horse.
Symptoms of Heat Stress in Horses
Should your horse be experiencing heat stress, you may notice the following:
- A high temperature (over 41 degrees C or 106 degrees F)
- A heart rate of over 60 beats per minute
- High respiratory rate (more than 80 breaths per minute)
- The mucous membranes may feel dry and tacky when touche
- Time for capillary refill is lengthened
- Typically, skin is dripping wet from extra sweat, but can be dry and warm
- Thumping or jerking of the diaphragm and flanks
- A stiff, odd gait
- Muscle soreness
If heat stress is not treated, neurologic signs like seizures can occur and your horse can experience serious damage to his heart, muscles, and kidneys.
Heat stress is different from heat stroke. Heat stroke can happen over a short period of time and in a variety of circumstances, for example:
- An unfit horse who works very hard in hot temperatures
- A horse confined to hot trailers with poor ventilation
In heat stress, the condition is typically due to fluid and electrolytes being lost over an extended period of time due to exercise.
Causes of Heat Stress in Horses
High temperatures and humidity are significant components of heat-related conditions in horses, both when they are working and not working. The muscles that horses use when exercising create a large amount of heat and their bodies are not able to quickly disperse that heat when humidity and temperatures are high. The heat will build up as quickly in horses that are not exercising or working but are enclosed in a trailer, kept on a small, dry lot without shade, or residing in a barn that is not well ventilated. Additional factors that can lead to heat stress are obesity, long coats, poor conditioning, an inability to sweat, prior experiences of heat stress or heat stroke, and heavy muscling.
Diagnosis of Heat Stress in Horses
Should you notice signs in your horse that point to the possibility of heat stress, you will want to contact your veterinarian. In the meantime, it is important to begin the process of assisting your horse and stabilizing his condition.
The veterinarian will note that your horse’s body temperature may be as high as 106% Fahrenheit and the physical examination will reveal skin that is warm and dry, as opposed to wet with the sweat that would normally be present after exertion. Additionally, lack of skin elasticity and possible digestive discomfort, along with other signs will point to the need for immediate treatment.
Treatment of Heat Stress in Horses
It is important that horses struggling with heat stress receive treatment right away. Should your horse show even the slightest sign of heat stress, you will want to move him to a shady area and if possible, in front of a fan. Once he is moved, you should remove bits and any other tack as soon as you can. Using a sponge or a hose, put cool water on the neck and body of your horse, focusing on the large veins of his neck and thin-skinned areas in his groin. Take rubbing alcohol and apply it to his back and neck (this will help encourage heat loss). If you have cooling blankets, use them and change them frequently. Upon your horse becoming stable, you will want to offer him small amounts of cool (not cold) water.
Should your horse be unwilling to drink or if he is dehydrated, large volumes of intravenous fluids (more than 60 liters) should be given, either as a balanced electrolyte solution or 0.9% sodium chloride. This will replace the fluids and electrolytes lost through sweat.
Antipyretic, anti-inflammatory drugs like flunixin and ketoprofen may or may not be effective in reducing the body temperature of your horse, but will protect the horse against heat shock proteins and deliver pain relief if necessary. In most cases, a minimum of one dose should be given. Should your horse be agitated and/or display neurological signs, a fast-acting sedative may be administered (for example, detomidine, romifidine, xylazine).
Glucocorticoids like methylprednisolone sodium succinate (at 2-4 mg/kg delivered intravenously) are recommended for horses with severe heat stroke and neurologic signs that are quickly progressing. Depending on how severe the clinical signs in your horse, your veterinarian may choose to offer additional treatment.
Recovery of Heat Stress in Horses
Once your horse is stabilized, you can move him to a cool area with good ventilation. Your veterinarian will likely recommend conducting blood work to determine if your horse is experiencing any kidney or liver problems. Should your horse experience heat stress, you will want to allow him to rest and make sure he gets no exercise for at least three to five days. It is thought that if your horse has experienced and recovered from heat stress or heat stroke, he is more likely to experience it again; therefore, it is important that he be monitored closely.
Heat Stress Questions and Advice from Veterinary Professionals
My horse is out of shape and we have been experience high temps and Hugh humidity. During a clinic he was sweating and his muscles were twitching in both of his back legs/stifle regions. Which I’ve never seen before and yet everyone told me he has EPM. I chalked it up to being out of shape and overworking in the heat.
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