Equine Hyperthermia Part I

Summer is the primary season for many equine competitions, and intense exercise coupled with a high ambient temperature can quickly put horses in the danger zone for heat-related illness. Before going into greater detail, it is necessary to define some basic terms related to thermoregulation—or the regulation of body temperature. The normal rectal temperature of the horse ranges from 99 to 100.5°F. 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.

Hyperthermia (heatstroke) occurs when the core body temperature starts to rise because these regulatory systems can no longer effectively cool the horse. Heatstroke is a serious condition that can be fatal if not dealt with quickly. The most common clinical signs include an elevated respiratory rate of 40 to 50 breaths per minute (normal at rest is 8 to 16) that does not slow when at rest, a heart rate of 80 or more beats per minute (normal at rest is 36 to 44) that does not slow down after a few minutes of rest, a rectal temperature of over 103°F, lethargy, and/or profuse sweating or an absence of sweating altogether. Some horses may suffer from a condition called anhidrosis, a disorder where the horse does not sweat normally. 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. Your veterinarian can perform diagnostic tests that can confirm this condition if you suspect that your horse might be afflicted.

Hyperthermia most often occurs as a result of inadequate physical conditioning (poor fitness), extreme hot and humid conditions, a weakened thermoregulatory system, or a combination of the three. The heat index (HI), which is the temperature (in °F) 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 HI is less than 130 (e.g., 90°F and 20% relative humidity), then the horse's built-in cooling mechanisms are usually capable of dissipating the excess body heat generated during exercise. However, when the HI is greater than 150 (e.g., 100°F and 60% or higher relative humidity), the horse will probably need assistance in order to prevent heatstroke. Owners should proceed cautiously when, or seek alternatives to, exercising horses in situations where the HI is greater than 170 or the relative humidity is above 75% since these conditions severely diminish the effectiveness of the horse's thermoregulatory systems.

In order to maintain a normal body temperature, the horse must dissipate heat that is produced as a result of normal body processes (e.g., digestion and muscular exercise). There are four main mechanisms that allow for this: evaporation, conduction, convection, and radiation. Evaporation (sweating) is the most important cooling mechanism for the horse because it removes heat as water (sweat) changes from a liquid to a gas (water vapor). Conduction occurs when heat is transferred from a hotter object to a cooler object by direct contact, such as using an ice pack on a sprain. Convection is heat exchange that occurs when an air current moves over the skin to pick up heat and/or moisture from the skin and carry it away. Radiation occurs when infrared rays carry heat from a hotter object to a cooler object. The heat that we feel from the sun is an example of radiation. We will describe the practical significance of these mechanisms later in this guide when we discuss means of relieving heat stress.

References: Jason Turner and Sandra Barraza, New Mexico State University