Heat stroke is a serious issue with horses that owners should be aware of and watch for whenever they are exercising their horses. Backyard horses or horses that are only worked or exercised occasionally are at a much higher risk that horses that are used routinely. In addition horses that are overweight or have respiratory problems are more susceptible to heat stroke than horses of the same age that don't have weight problems or breathing issues.
Heat stroke is more problematic in the summer months, but it also happens in the spring and fall, especially on hot days or days where the humidity is very high. The combination of high humidity and heat is far worse than either condition separately for both horses and riders, so care must be taken under these conditions.
Heat stroke occurs when the horse is not able to correctly cool his or her body. When the body heat rises, horses will naturally increase their respiratory rate and begin to sweat both functions that work to cool the body through evaporation and increased oxygen to the body cells. When the horse is not able to cool his or her body due to high humidity, exercise and heat the body responds by excessive sweating as well as rapid heart rate and respiration. Often a horse that has heat stroke will pant and breath through the mouth, stand with the legs extended to the side and appear uncoordinated, stagger and even collapse.
The quicker that the owner responds the faster the horse will recover from heat stroke. At the first signs that the horses respiratory rate is higher than the heart rate, the rider should immediately get the horse out of the sun and into a cooler area. If at all possible cold or cool water should be run over the horse's body or the horse should be led into a stream, lake or pond so that the body can be cooled quickly and efficiently. If there is no water available get the horse to the coolest location out of the sun and fan the horse to help with evaporation of the sweat and cooling of the body. Never walk or jog the horse until they have had the respiratory rate and heart rate returned to normal for at least 30 minutes. Horses that have abnormal heart rates and respiratory rates and stop sweating or collapse will need immediate veterinary attention.
There is no evidence to suggest that cooling the larger muscles of the legs and back with cold water will lead to cramping or muscle spasms, especially in horses that are in advanced stages of heat stroke. Do not simply place wet towels over the horse unless you are constantly removing them and re-cooling them in water. A wet blanket or towel will pick up and hold the heat and humidity close to the body, completely reversing what you are trying to accomplish.
Always provide lots of water and time to rest for horses that are exercised in the hot weather or if they are only used occasionally. Try to avoid riding in the heat of the day and exercise in the cooler morning and evening hours.