Expert Opinion


Gary P. Carlson, Davis, California

Current Therapy in Equine Medicine, ed. N. R. Robinson Vol. II

Exercise requires a massive increase in the metabolic rate of working muscle. However, only a portion of the available energy can be effectively utilized for this work and the remaining energy accumulates as heat. This heat must be removed by the vascular system, which serves to transport heat from core areas to the skin where heat exchange with the environment can occur. One of the major problems facing the equine athlete is the dissipation of this excess body heat. When exercise is performed in a hot or humid environment, competing demands for heat dissipation and maximal exertion limit performance and may result in serious heat associated illness. For these reasons thermoregulatory processes are of particular relevance to the practitioner working with performance horses.

Core temperature is critical and is normally maintained within relatively narrow limits. The processes by which this is accomplished are controlled by the thermoregulatory center located in the midbrain. This center integrates input from the central as well as the peripheral nervous systems and is responsive to local concentration changes of specific electrolytes, hormones, and certain chemical mediators. The center establishes a set point or threshold temperature at which increased heat production (e.g. shivering) or increased heat dissipation (e.g. peripheral vasodilatation, vascular redistribution, and sweating) are brought into play. Alteration of this set point is one of the important adaptive responses to physical training in humans. Thermal balance represents careful regulation of factors responsible for metabolic heat production and heat loss or gain from the environment. Failure to maintain this balance may lead to progressive elevation of core temperature, altered CNS function, collapse, and death.

Heat is lost from the body by four basic processes: radiation, conduction, convection, and evaporation. The effectiveness of the first three processes as means of heat dissipation are all dependent upon a favorable temperature gradient between the subject and the environment. For this reason exercise in cool climates is rarely associated with thermoregulatory problems. As environmental temperature increases and approaches that of body temperature the first three of these processes become ineffective. Evaporative processes, particularly sweating, are the most important means of heat dissipation for the exercising horse. Race horses may lose up to 10 L of sweat in an attempt to disperse the excess body heat generated in a one-mile race that can be run in less than two minutes. Exercise performed by endurance horses is at a much slower pace but must be maintained for hours. On hot days rectal temperatures of those horses often exceed 40 degrees C, [104 degrees F] and sweat losses may exceed 10 to 14 L per hour. [1] Clinically normal endurance horses may accumulate transient fluid deficits of 20 to 40 L (4 to 10 % of body weight), and significant electrolyte deficits area associated with sweat losses of this magnitude. The combined effects of fluid and electrolyte losses in sweat may have profound physiologic effects as detailed in the subsequent section on the exhausted horse syndrome. Dehydration and hypovolemia have adverse effects on heat transfer and the rate of sweating, thus impairing the ability of the horse to dissipate excess body heat. Human athletic performance is compromised by net fluid deficits of 2 to 5 percent of body weight.

The effectiveness of evaporative thermoregulatory processes is a function of the rate of heat production, environmental temperature, relative humidity, and wind velocity. High relative humidity interferes with sweat evaporation and thus impairs the principal means of heat dissipation. A variety of heat stress indexes have been developed to assess these factors in human subjects, including effective temperature index, equivalences en sejour, and the wet bulb globe temperature index. There have been no comparable indexes developed for the horse. An arbitrary system using a combination of temperature in degrees Fahrenheit and relative humidity in percent has been proposed for use in the horse. In this system the sum of the relative humidity and temperature in degrees Fahrenheit is made. If the value is less than 120, there are no problems; if the value is greater than 150 care should be exercised. Values over 180 will severely compromise thermoregulation.


Heat stroke occurs when the normal thermoregulatory processes fail. It may occur when an unacclimatized horse is suddently exposed to a hot humid climate. Most often heat stroke occurs during transport in overheated, poorly ventilated trailers or with vigorous and protracted exercise in a hot and humid environment. Dehydration associated with sweat losses compromises thermoregulatory response and is considered to be a major contributing factor in the development of heat stroke. Obesity, long hair coat, heavy blanketing, extensive skin damage as the result of dermatitis or burns, and anhidrosis impede sweating or effective sweat evaporation and may also be contributing factors in the onset of clinical signs.

Horses with heat stroke may proceed from a state of restlessness and anxiety to depression, stupor, disorientation, collapse, and death. Sweating may be profuse in some cases but is generally less than anticipated for a given circumstance and the skin is hot to the touch. Heart rate and respiratory rate are elevated and pulse pressure may be weak. Rectal temperature is 40 degrees C or greater and may actually increase. These horses are in critical condition and emergency therapy is indicated.

Therapy. The first and foremost therapeutic objective is control of body temperature. This can be accomplished by rigorous sponging or spraying of cold water or alcohol over the large vessels of the distal extremities, head and neck. The horse should be placed in a shady area with good ventilation. Air movement is critical; advantage should be taken of any available wind or electric fans. Cold water enemas may be of benefit in extreme situations. Intravenous fluids are an important component of successful therapy. Sodium-containing polyionic fluids such as lactated Ringers solution are suitable, although saline fortified with potassium at 10 to 15 mEq perL may more closely match likely deficits. Fluids should be given rapidly to effect and 10 to 20 L or more may be required. Antipyretics such as dipyrone, phenylbutazone, and flunixin meglumine are generally given but care should be taken to restore hydration if these agents are to be used.


Anhidrosis (failure to sweat under appropriate stimulus) is a condition that occurs exclusively in horses maintained in the tropics, where there is persistently high temperature and humidity. Anhidrosis has been reported as a special exercise associated problem in individual performance horses moved from a temperate to a tropical climate. Recent epidemiologic studies indicate that the problem may also occur in stabled sedentary horses and horses native to tropical areas. There is no breed or sex predisposition, but the condition is definitely exacerbated by exercise. Further details are provided on P. 187.


1. Temperature- A horse's normal body temperature is 99 - 101 F.  Equine Veterinary Services

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