A normal horse’s temperature sits between 37.5 and 38.5°C, with most horses sitting in the 37.5-38.2 °C range. Illness and exercise can increase the body’s core temperature and cause problems with the normal physiological functioning of the body.
With exercise, the temperature rise is usually short-lived as the body utilises various biological methods to reduce heat and return the core temperature to an ideal level. (Increases in body temperature because of illness relate to the release of inflammatory mediators and the loss of thermoregulation, and can cause heat stress in the horse — but that will not be discussed in this article).
Heat stress should be thought of as the inability of the horse to lose excess body heat. If the heat cannot be effectively dissipated and the body temperature stays high, critical thresholds are surpassed and death can occur.
There are two major contributing factors to a horse becoming heat stressed; firstly, the body generates too much heat, and/or the body is compromised in its ability to lose this heat into the environment. Initially, the body generates heat during normal physiological activities such as eating and exercise, with the amount of heat being related to the type and the amount of chemical reaction occurring at a cellular level.
When horses are asked to exercise at high intensities, or over prolonged periods, the chemical reactions that occur in the muscles are inefficient and produce large amounts of heat relative to the amount of energy produced to make the muscles function. The body relies on several pathways to remove this heat from the body, but when these processes are overwhelmed, the system can fail. The usual physiological pathways that are activated when the core temperature rises include sweating, allowing the heat to be transferred into the sweat; dilation of skin blood vessels to allow heat to pass through the skin, and an increase in respiratory rate to transfer heat from the lungs into the exhaled air.
The environment can also facilitate the removal of the generated heat. This occurs if the ambient temperature and the humidity are low enough to allow transfer of heat from the hotter body to the cooler/drier environment. In environmental temperatures between 5°C and 25°C, the body can readily deal with maintaining an ideal temperature during intense activities.
When the temperature in the environment is high, heat is not readily able to be transferred from the body as the gradient (temperature difference) between the two is not big enough. Think of it like a glass of hot water: if the glass is put into a refrigerator, it will cool down quickly; if left in a hot room the water will remain hot for a much longer time.
Humidity also plays a major role in the horse’s ability to deal with heat stress as sweating is the predominant way a horse loses heat. If the humidity is high, sweat remains on the skin and the heat is not removed. If the air is dry or there are light winds blowing, the sweat is evaporated, removing the heat and cooling the horse down.
There are heat stress guides available that rely on adding the temperature (in Fahrenheit) and the relative humidity percentage together to give a figure which enables the rider/handler to predict the likelihood of the horse getting heat stress.
The Celsius temperature can be converted to a Fahrenheit value by multiplying it by 9/5 and adding 32. So, the formula is F = 9/5 x C + 32.
If the Heat Guide (temperature in Fahrenheit + relative humidity) is:
A) 120 or less: Exercise should not be a problem as the horse should easily be able to deal with generated heat.
B) 120-150: There is some compromise to the horse’s ability to cool down so program in some time to allow the horse to recover between exercises and do not over-exert.
C) 150-180: There are significant risks associated with intense work and overheating, so reduce work intensity and implement cooling strategies where possible.
D) 180+: Horses should NOT be exercised in these conditions. If exercise is undertaken, appropriate and adequate cooling procedures should be utilised frequently and without delay.