Older Horses: Decreased Thermoregulation During Exercise

Experience and instinct tell us to condition older horses carefully, keeping a close eye on how they handle their workouts. A team of researchers at Rutgers University confirmed these instincts when they examined senior horses' propensity for developing hyperthermia, or elevated body temperature, when exercising.

Led by Kenneth McKeever, PhD, FACSM (fellow of the American College of Sports Medicine), a professor in the Department of Animal Science, a team of researchers examined older horse's propensity for developing hyperthermia, or elevated body temperature, when exercising.

In previous studies scientists have examined horses' ability to thermoregulate during physical exertion, but they had not evaluated the older horse population specifically. In the current study researchers evaluated relationship between physical exertion, thermoregulation, and plasma volume in six younger (7 to 8 years old) and five older (mid-20s) unfit Standardbred mares working on a treadmill.

The horses exercised until their core body temperatures reached 40°C (104°F, the normal temperature for an average adult horse is 99-101°F or about 37.2-38.3°C). The investigators monitored the animals' core, skin, and rectal temperatures, as well as heart rate, every minute during and immediately post-exercise. They also collected blood samples from the horses before their body temperatures reached 40°C, at 40°C, and every five minutes until 10 minutes post-exercise, later analyzing the samples for packed cell volume, lactate, and plasma protein. Sweat loss was also estimated using body weight as a reference.

The researchers' key findings:

  • Older horses reached the 40°C core body temperature mark more quickly than younger horses (an average of 998 seconds, or 16.63 minutes, for the older horses compared to an average of 1,925 seconds, or 32.08 minutes, for the younger horses);
  • On average, older horses had higher heart rates and lost more sweat; and
  • Younger horses had greater plasma volume levels than older horses, which gave them greater cardiovascular and thermoregulatory stability (the former is due to larger blood volume and the latter to the greater reserve of body water for the production of sweat).

The team noted that lower plasma volume levels contribute to less efficient thermoregulation during exercise in older horses, likely because blood flow impacts the body's cooling mechanisms which, along with fluid balances, becomes less optimal with age.

"The study indicates that there are age-related changes in how the horse responds to exercise," McKeever explained. "The older horse has less fluid for the production of sweat, and the functional capacity of the cardiovascular system declines with age."

He stressed that owners should adjust the duration and intensity of exercise for older horses according to each animal's condition, particularly when heat and humidity are involved.

The study, "Age related decreases in thermoregulation and cardiovascular function in horses," was published in November 2010 in the Equine Veterinary Journal. The abstract is available on PubMed.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.