Heat stress is caused by a rise in internal temperature. Exposure to direct sunlight, minimal airflow, high ambient temperatures and humidity along with a long hair coat all increase the danger of heat stress. In addition, muscles produce about 80% heat and 20% mechanical energy, so when exercised, a horse’s metabolic rate increases substantially, creating thermal (heat) energy.
Dissipating excess heat
1. Sweating is one way horses get rid of excess body heat. When it’s really hot, a horse can lose up to four gallons of fluid (equal to 30 pounds of body weight) every hour. This may cause dehydration if the lost fluids are not replaced while exercising. If a horse loses over nine gallons of fluid, the result can be severe heat stress, and even death.
Sweat also contains important electrolytes such as sodium, potassium and chloride that are found in body fluids. When these are lost, electrolyte imbalances can occur, furthering complications within your horse. Another important component of sweat is latherin (it creates a white foam on your horse when he is worked). This foam aids in spreading the sweat over the body surface, increasing evaporation. Latherin can be depleted by excess sweating, but will be replenished over a few days if the horse is rested and does not sweat.
2. Panting is another source of heat loss. It’s indicated by rapid, shallow respirations (120 to 140 breaths per minute). Panting causes rapid and significant airflow over the nasal surfaces. This allows heat loss from the blood in the nasal mucosal capillaries through evaporation (changing liquid to a vapor), direct heat transfer (conduction), and heat exchange (convection). As long as the respirations are shallow and the heart rate is declining normally, panting does not indicate a problem. The horse is just cooling out.
Signs of heat stress
Symptoms include depression, lethargy, sweating all over the body, and depressed gut sounds. Heat that is not dissipated through sweating will either be stored, resulting in an increased body temperature (102.1°F to 103ºF), or lost via the respiratory tract, resulting in rapid breathing (30 to 40 breaths per minute). The heart rate will increase to 50 to 60 beats per minute and the gums will be dark pink. Your horse’s temperature should decline within 20 to 30 minutes – if it doesn’t, this is cause for concern and other cooling techniques should be implemented.
With increasing (moderate) heat stress, symptoms include:
• Respiratory rate of 40 to 50 breaths per minute.
• Heart rate of 61 to 80 beats per minute.
• Reddish pink gums.
• Rectal temperature between 103.1°F and 105°F.
• The horse will be depressed, uninterested in food, and may act uncoordinated (stumbling, loss of balance).
• He will be dripping sweat profusely, but this type of sweating is ineffective because evaporation is not occurring to cool the body. It is also dangerous because large amounts of fluid and electrolytes are lost.
When a horse is experiencing severe heat stress or heat stroke, symptoms include:
• Dry, hot skin.
• A respiratory rate above 50 breaths per minute.
• A heart rate of more than 80 beats per minute.
• Dark red or purple gums.
• A rectal temperature over 105°F
• The horse will be non-responsive to his/her surroundings, may stagger and even collapse and convulse. These signs can progress to coma and death.
Cooling off
With continued exercise, these signs can go from relatively mild to severe in a short time, so it’s very important to monitor your horse and listen when he tells you “enough”.
Here are some tips to keep your horse cool during exercise:
• Allow him to frequently drink small amounts.
• Allow rest periods in the shade.
• Excitation can increase internal temperature. If your horse is excitable, use a little Rescue Remedy to calm him down prior to a workout (you can also use some yourself).
• If your horse is in the moderate or severe zone, a veterinarian should be called in. At this point, rest and other cooling methods will not be enough, and IV fluids will be needed to reduce your horse’s core temperature, correct dehydration and balance his electrolytes.
After a workout
Ice and cool water are the typical methods for cooling a horse down after exercising. You can also try water mixed with rubbing alcohol or witch hazel extract, as these hasten evaporation.
Apply on the inside of the legs, the underside of the belly and neck. There are large superficial veins in these areas that aid in heat transfer. Avoid putting cold water on the large muscles of locomotion, such as the gluteals, as this can cause cramping.
In general, the cooler the air temperature, the less cold the water needs to be. Remove your horse’s tack and go for a walk, or place your horse in a breeze – this increases airflow and evaporation.
Two ways to prevent heat stress
1. Start by conditioning your horse during cooler weather and keeping him conditioned as the weather heats up. This allows his cooling mechanisms to become more efficient, which will help him acclimate in several ways.
Heat will dissipate more efficiently from respiratory heat loss. As well, conditioning will lower the horse’s resting temperature, and lower stored body temperatures in hot conditions. A fitter horse will have a smaller rise in temperature during a standard workout.
2. Another preventative technique is to keep track of the heat index. This is simply the temperature (F) + humidity (%). If the heat index is less than 130, a horse can generally cool out without hosing or sponging off – unless the humidity is over 75%. If the heat index is over 150, evaporative cooling is diminished and you should begin monitoring your horse closely. Anything above 180 means the horse cannot cool himself and should not be exercised.
Learning to recognize and avoid heat related problems will help you protect your horse from an uncomfortable and potentially dangerous situation. If you are ever in doubt about what stage of heat stress your horse might be at, call your vet. Heat stress can escalate quickly and have dire consequences. Listen to your horse, and keep cool!
After discovering at an early age that she had a real connection with animals, Kim Sergent set her sights on Veterinary School. She graduated in 1989 from the University of California, Davis and began practicing equine veterinary medicine in the San Diego area.
She began to discover health and behavioral problems which could not be treated medically and sought an alternative, a more complete approach to veterinary care. In 1997 she was certified for Musculo-Skeletal Manipulation by the American Veterinary Chiropractic Association, and began to integrate chiropractics into her veterinary practice. In 2002 she graduated from Colorado State University Veterinary Acupuncture School, and has more recently been studying herbology and homeopathy.
Dr. Sergent’s practice is based in Alpine and still services most of San Diego county. She is a member of the American Veterinary Medical Association, American Veterinary Chiropractic Association and the American Association of Equine Practitioners. www.acequine.com
