The set of mechanisms that allows horses to maintain a desired temperature is known as thermoregulation. Thanks to this phenomenon, the horse can maintain a body temperature of around 37.7 °C, with a variation of more or less 2 °C.
The skin, tissues, fat, and hair play a major role in the horse’s thermoregulation. The coat insulates most of the cold, while skin disperses the heat. This is why thermogenesis (hair thickening) occurs in the winter: to increase the insulating capacity of the horse’s coat. Once the cold has passed, the horse will gradually shed his thick coat in order to regain his summer coat.
Scheme of vasoconstriction and vasodilation of arteries
2. Reactions to heat
When the heat returns, the horse’s body reacts in order to eliminate the extra heat.
The circulation of heat in the horse’s body can then be represented by an equation that represents the exchanges between his body envelope and his environment:
GAIN = LOSSES
M – W = R ± C ± K ± E
M = metabolic heat production
W = mechanical work
R = heat exchange by radiation
K = heat exchange by conduction
E = heat exchange by evaporation
Four components are at the origin of this heat exchange:
- Radiation is the flow of heat between two things without direct physical touch, using two forms of electromagnetic radiation. There is short-wave radiation from the sun and long-wave radiation absorbed by the horse’s body in contact with its surroundings.
- Convection is the movement of molecules in a fluid that transports heat; hot particles rise and cold ones sink. This sort of transmission occurs all the time between the horse’s body and the surrounding air. When the air is chilly, the coat’s aim is to prevent too much heat loss, but in the summer, the coat is thin to facilitate heat loss.
- Conduction is the exchange of heat through contact with the ground. When it is hot, vasodilation occurs to let heat to escape. When it is cold, blood is pushed away from the extremities to prevent heat loss by conduction.
- Evaporation is the primary method through which horses lose heat. When the vapor pressure at the skin’s surface reaches a maximum (saturation of skin sweat), sweating overcomes evaporation. The sweat then evaporates without cooling.
Over-exertion in humid heat can lead to a loss of evaporative efficiency in the horse. This can lead to myositis, where vasodilation causes blood to pool in the blood vessels under the skin and the irrigation of the vital internal organs becomes less efficient.
Weather factors also influence horses’ choice of heat removal mode. The most effective mode is determined by the temperature:
- Low temperature (10°C): the skin temperature will be approximately 25°C to 28°C (with a temperature difference with the ambient air of 15°C to 18°C). Convection and radiation would be sufficient to dissipate the heat load imposed by light to moderate exercise.
- Moderate temperature (25°C): 50% of the heat load is dissipated by radiation and convection, and the remaining 50% by evaporation.
When the skin temperature reaches the ambient temperature, evaporative cooling becomes the only method of heat loss.
➡️ For more information about myositis, we recommand this article.
Thermoregulation in horses during effort
When a horse exercises, 20% of its muscle cells are used for training, while the remaining 80% are transformed into heat. This is because the horse’s exertion boosts blood flow, which transfers heat from his core to the surface of his skin.
As a result, during training, the horse’s body temperature can rise by 1°C every minute, reaching 45°C. The horse then uses convection to reduce the temperature from his muscles to his skin. The horse will begin to sweat if vasodilation is insufficient.
Vasodilation is an increase in the size of the diameter of the vessels induced by relaxing of the muscles that make up the blood vessel walls.
1. In winter
In winter, clipping allows the horse to evaporate his sweat more easily. A horse that is exercising can lose a lot of water through sweating, which can cause him to catch a cold. A thoroughbred can lose up to 10 liters of water during exercise in summer. It is thus recommended to clip horses that are still working throughout the winter, as long as they are covered with a blanket to compensate for the loss of hair insulation.
2. In summer
During hot weather, the shower can be useful to cool your horse down after exercise. According to a study in the Journal of Equine Veterinary Science, the most effective method is to shower the horse continuously while he is steady at ambient temperature for 2 minutes. Another effective method is to apply cold water (10°C) intermittently to the horse, alternating with walking for 10 minutes.
We have also seen that a horse exercising during the summer can lose a large amount of water through the sweat. If the horse does not drink enough, this can lead to dehydration. To prevent this risk, electrolytes are useful supplements to help restore water balance during exercise.
Sweating causes the horse to lose a significant proportion of the minerals present in the electrolytes, which are sodium, potassium, and chlorine. Electrolytes are thought to be the vehicle that transports water throughout the body. As a result, they have a significant impact on the horse’s musculoskeletal and neurological functioning. It is consequently critical to keep a horse hydrated and electrolyte-supplemented during activity. Both ingredients must be provided at the same time, since electrolytes without water, and vice versa, can worsen dehydration.
In order to better understand the mechanism of thermoregulation in the context of satisfactory recovery, this diagram illustrates the distribution of cardiac output when the horse is at rest, during exercise and during recovery.
Distribution of cardiac output in resting, exercise and recovery situations
Source: Equine Veterinary Education
It can be seen that the cardiac output is highly concentrated on the muscles during exercise, at 80%, while the skin is only at 5%. This trend is reversed during the recovery phase, when the skin takes over to dissipate the heat produced during the effort by sweating.
Thermoregulation in horses after exercise: which parameters to monitor?
As seen previously in the bar graph, the horse must thermoregulate after being exposed to intense exercise in order to effectively remove heat excess. However, sometimes the effort and weather conditions are such that the horse’s metabolic heat is not dissipated into his environment.
Indeed, racehorses have one of the highest heat production rates of all sport horses, with a production of 1250 kJ/minute.
1. Unusual sweating
The first mode of heat regulation in the horse is sweating, which alone can dissipate up to 30% of the metabolic heat produced by exercise. However, excessive sweating can be a sign of poor thermoregulation in the horse. If sweat covers the whole body, cooling cannot take place and hyperthermia can worsen.
Hyperventilation is also a parameter to be monitored. Under normal racing conditions, the horse naturally hyperventilates in order to rebalance the oxygen level in his body. The horse then holds the same level of respiration as when he has just finished exercising, the only difference being that he lowers his breathing rate and slightly increases the volume of air entering his body. This phase only lasts a few minutes when the effort takes place in mild weather conditions.
When the effort occurs in humid heat, the horse may adopt a “panting” type of breathing. The horse’s panting does not result from heat regulation via the mouth, as in cats or dogs, but is expressed via the breathing rate. This can result in a breathing rate of more than 120 breaths per minute, a dilation of the nostrils, coupled with minimal thoracic excursions. A pendulum movement may also occur, due to the repeated excursions.
These signals may be the precursors of a condition called exertional heat illness. It is therefore important for the veterinary service to monitor the horses carefully after the race in order to detect these signs and act quickly to cool them down.
The horse therefore has a great ability to regulate his body heat. The horse has a range of options to dissipate the heat load before, during and after exercise. However, it is important to monitor these means of thermoregulation to ensure the well-being of these athletes during particularly difficult weather conditions, such as in summer.
Key words: thermoregulation, cold, heat, body
Wallsten, H., Olsson, K. and Dahlborn, K. (2012). Temperature regulation in horses during exercise and recovery in a cool environment. Acta Veterinaria Scandinavica, 54(1). Available at: https://doi.org/10.1186/1751-0147-54-42.
Brownlow, M.A. and Mizzi, J.X. (2020). Thermoregulatory capacity of the thoroughbred racehorse and its relationship to the pathogenesis of exertional heat illness. Equine Veterinary Education, 34(4), pp. 214–221. Available at: https://doi.org/10.1111/eve.13433.
Takahashi, Y. et al. (2020). A comparison of five cooling methods in hot and humid environments in thoroughbred horses. Journal of Equine Veterinary Science, 91, p. 103130. Available at: https://doi.org/10.1016/j.jevs.2020.103130.