Summer is the primary season for many equine competitions, and intense exercise coupled with a high ambient temperature can quickly put a horse in the danger zone of heat-related illness.
“There are several things a person can do to prevent their horse from developing heatstroke, or hyperthermia,” said Jason Turner, New Mexico State University’s Extension equine specialist. “The most important things are to prepare your horse for a heat stress environment and to be aware of your horse’s body temperature while in that environment.”
Heatstroke may occur when a horse’s body temperature goes higher than the normal rectal temperature range of 99 to 100.5 degrees. The horse’s natural thermoregulatory mechanisms are capable of maintaining this normal body temperature except when overwhelmed by severe circumstances, such as disease or intense exercise in hot climates.
“Heatstroke is a serious condition that can be fatal if not dealt with quickly,” said Turner. “There are several signs that the horse is experiencing hyperthermia. How the horse is acting is one of the first visual signs.”
Is it lethargic? Is it sweating profusely? Or is there an absence of sweating altogether? Perspiration is the way animals reduce their body temperature when it increases above a normal range. The sweat that is produced evaporates and cools the body surface.
“Some horses may suffer from a condition called anhidrosis, a disorder where the horse does not sweat normally,” Turner said. “These horses are especially prone to hyperthermia if not managed appropriately. The specific cause of anhidrosis is unknown; however, it is thought that there is a physiological defect at the level of the sweat gland that inhibits sweating. A veterinarian can perform diagnostic tests that can confirm this condition if the owner suspects their horse might be afflicted.”
Clinical signs that the horse is experiencing heatstroke are elevated respiratory rate of 40 to 50 breaths per minute instead of the normal eight to 16; heart rate of 80 or more beats per minute compared to the normal resting heart rate of 36 to 44 beats per minute; and a rectal temperature over 103 degrees.
“Hyperthermia most often occurs as a result of inadequate physical conditioning, extreme hot and humid conditions, a weakened thermoregulatory system, or a combination of the three,” Turner said. “The heat index, which is the temperature plus humidity, gives a means of assessing the danger that extreme environmental conditions pose to horses performing intense exercise in such an environment.”
If the heat index is less than 130, which occurs at 100 degrees and 30 percent relative humidity, the horse’s built-in cooling mechanisms are usually capable of dissipating the excess body head generated during exercise.
“However, when the heat index is greater than 150, which occurs at 100 degrees and 50 percent or higher relative humidity, the horse will probably need assistance in order to prevent heatstroke,” Turner said. “Owners should proceed cautiously when, or seek alternatives to, exercising horse in situations where the heat index is greater than 170 or the relative humidity is above 75 percent, since these conditions severely diminish the effectiveness of the horse’s thermoregulatory systems.
“Prevention is the best medicine” also goes for heat stress for horses. With our normal low relative humidity, New Mexico’s arid climate is not usually a pre-disposing factor to heat stress in horses. However, one needs to consider the climate at the competition location. For example, a New Mexico-trained horse that is shipped to North Central Texas or Oklahoma in July or August is not acclimated to that area. Environmental conditions there increase the likelihood of heat stress in horses not accustomed to the heat and humidity.
“If it is possible, avoid strenuous exercise of horses when the heat index is near the danger zone,” Turner said. “This may require adjusting your training or exercise schedule to do intense work early in the morning or late at night when ambient temperatures are lower.”
If a horse must be worked in a heat index situation, Turner suggest owners view the NMSU Extension guide regarding helping horses handle heat stress located on College of Agricultural, Consumer and Environmental Sciences website at aces.nmsu.edu/pubs/_b/b-711/welcome.html.
“There are several means of relieving heat stress,” Turner said. “The primary goal is to lower the horse’s body temperature as rapidly as possible, and this is done by employing ‘active cooling’ methods that make the most efficient use of the heat loss mechanism that include evaporation, conduction, convection and radiation.”
Those methods includes cool water bathing, which can be done with a garden hose or a sponge and bucket; increasing air flow by placing the horse in front of a fan or in a natural breeze; shading by keeping the horse out of the sun; and drinking cool water by giving the horse water to restore the body fluids lost in sweat.
“Sweating results in a significant loss of body fluid, so it is important to monitor the horse and ensure that normal body fluid levels are maintained,” Turner said. “Horses that are moderately dehydrated, 4 to 9 percent loss of body fluid, will show decreased skin elasticity, poor capillary refill time of the gums, reduced saliva production, sunken eye sockets, muscle weakness and fatigue.”
Information from: NMSU