Heat stress and heat stroke are extremely dangerous conditions for your horse. Often seen in the summer months in sports such as eventing, jumping, combined driving or other strenuous activities, especially in areas where the humidity is high, heat problems can also occur simply when riding an unfit horse at your own barn when you push him too hard or he has to stand in a poorly ventilated trailer for too long. There is a difference in the causes of heat stress and heat stroke. Heat stroke can occur over a relatively short period of time. Heat stress, also known as heat exhaustion, usually results from protracted fluid and electrolyte loss during exhaustive exercise.
As the horse continues to work, heat builds up in his muscles. When the number of your horse's respiration is faster than his heart rate per minute, this is called an inversion. This is a sign of high internal body temperature and the respiratory tract is attempting to dump some of the heat load. To remove the heat, your horse sweats, pulling heat from the interior of his body to his skin in a process known as evaporative cooling. Around 70% of the heat of locomotion is normally dissipated from the body using this process. Warm air temperature and high humidity prevent a horse from adequately dissipating internal heat from his body.
However, other horses are susceptible to heat problems as well. Horses with a full winter coat are at risk since the hair keeps in the body heat during cold weather. Heavily muscled horses, such as Warmblood breeds and Quarter Horses, are at greater risk of retaining heat in the working muscles than leaner-breed horses such as Arabians or Thoroughbreds (thus the preference for these breeds in endurance racing). This is because they have a lower ratio of body surface area for cooling relative to their body mass that's generating the heat. An overweight horse with abundant fat layers beneath his skin cannot dissipate heat effectively. Transporting a horse in an enclosed van in hot weather can contribute to dehydration and heat stress. Additionally, a horse which was shipped to a warmer climate and has not been acclimated to exercise in hot and humid conditions is ill-prepared to deal with the added stress of the new environment no matter how fit he is. Most horses need at least three weeks in a warmer climate to allow their bodies to adapt and dissipate heat more efficiently.
Know your horse’s usual temperature, especially after exercise. A rectal temperature over 103.5° is a sure sign the horse is overheating. Also know your horse’s respiration rate and heat rate and check them if you think your horse may be stressed. Finally do the capillary refill test to check blood flow and the pinch test to test for dehydration.
After any exercise here are some steps to help your horse cool out. As you finish a workout, bring your horse to a walk. Hop off and spend a minute or two walking him so blood flow continues to flush metabolic waste products and heat from his muscles. In warm weather, copiously bathe his head, neck, and legs with cool water. Large blood vessels in these locations flush heat to the skin surface, and rapid evaporative cooling is achieved by continual sponging of these areas. Apply cool water and as it heats up, scrape it off of major muscle groups, such as over the loin and hindquarters. Offer a bucket of water to your horse immediately following exercise. Find a shady spot for an overheated horse, preferably with decent air circulation from a light breeze or fan. An enclosed space with stagnant air adds to heat retention. Fans are helpful for convective cooling--as the air flows across the horse's body, it pulls heat off the skin. In severe cases, severe dehydration might need to be treated with Intravenous fluids. This can also help to cool the internal organs and muscles. Talk to your veterinarian about checking your horse’s acid-base balance and electrolyte status and correct if necessary.
Be aware of the condition called anhidrosis. Some horses in hot, humid climates can lose the ability to sweat due to overworked sweat glands that lose their ability to sweat and cool himself. The horse’s skin will be dry and hot to the touch. There may be sweaty areas under the mane and saddle or in the groin area, but no moisture when you touch it. Stop exercise immediately and restrain the horse from further physical exertion. Move the horse to a cool location and start aggressive cooling techniques immediately. This condition can easily proceed to heat stroke.
So enjoy the summer with your horse. Just be aware that too much sun, humidity and exertion can cause problems for your horse.