By: Full list of authors at the end of this article
Photography: Adobe stock images modified by Hyungsuk Kang
Healthy adult horses can balance accumulation and dissipation of body heat to maintain their body temperature between 37.5 and 38.5 °C, when they are in their thermoneutral zone (5 to 25 °C). However, under some circumstances, such as following strenuous exercise under hot, or hot and humid conditions, the accumulation of body heat exceeds dissipation and horses can suffer from heat stress.
Prolonged or severe heat stress can lead to anhidrosis, heat stroke, or brain damage in the horse. To ameliorate the negative effects of high heat load in the body, early detection of heat stress and immediate human intervention is required to reduce the horse’s elevated body temperature in a timely manner. Body temperature measurement and deviations from the normal range are used to detect heat stress. Rectal temperature is the most commonly used method to monitor body temperature in horses, but other body temperature monitoring technologies, percutaneous thermal sensing microchips or infrared thermometry, are currently being studied for routine monitoring of the body temperature of horses as a more practical alternative. When heat stress is detected, horses can be cooled down by cool water application, air movement over the horse (e.g., fans), or a combination of these. The early detection of heat stress and the use of the most effective cooling methods is important to improve the welfare of heat stressed horses.
There are more than 16.9 million horses in the world (Cross 2019) and approximately 0.24 million horses are registered globally for Thoroughbred racing (International Federation of Horseracing Authorities 2019), 11,444 horses globally for American Quarter Horse racing (American Quarter Horse Association 2021), and approximately 0.27 million horses are registered to compete at equestrian events (Fédération Équestre Internationale 2022). These include endurance, jumping, dressage, para-dressage, eventing, driving, vaulting, and reining [ed: the latter only until 2021].
Given this substantial number of competition horses, it is not surprising that welfare issues have been raised. Heat stress is a serious welfare issue, not only during horse competition including Thoroughbred and Standardbred racing, endurance events, Olympic competition, and other equestrian disciplines, but also leisure riding, transportation, and inappropriate housing and management under hot and humid conditions (Waran et al. 2007; Pritchard et al. 2006; Brownlow et al. 2016; Padalino et al. 2016; Brown-Brandl et al. 2005; Holcomb et al. 2014). Even though there are no clear data of economic losses in the equine industry related to heat stress, extremely hot and humid climate conditions have detrimental effects on the industry by reducing athletic and reproductive performance, increasing the risk of infectious and heat stress–related diseases and injury, and affects equestrian event management (Melissa 2011).
Although heat stress is an important welfare issue, there is no clear definition of heat stress in horses, and there is little data available regarding this condition. In relation to welfare in horses, heat stress can be defined as the inability of the horse to maintain body temperature within a prescribed temperature range (Caulfield et al. 2014; Marlin 2009; Spedding 2000). Understanding the impact of heat stress on horses will allow mitigation strategies to be developed and reduce the likelihood of adverse events across all levels of the equine industry. To help understand and improve the welfare of heat-stressed horses, we have used peer-reviewed publications papers to summarize information about what defines heat stress in horses and how to prevent injury and illness resulting from severe heat stress.
Thermoregulation in horses
Physical heat transfer: The normal body temperature range of a healthy horse is between 37.5 and 38.5 °C (Mealey 2019), when horses are in their thermoneutral zone (5 to 25 °C) (Morgan 1998). The body temperature of horses also fluctuates due to circadian and seasonal rhythms, where the minimum body temperature occurs in the early morning during the winter season and the maximum in the late afternoon during summer (Ayo et al. 2014; Giannetto et al. 2012; Kaseda and Ogawa 1993). Even though the body temperature fluctuates, keeping a balance between heat production (gain) and heat dissipation (loss) is essential to maintain the body temperature in a narrow range, and to avoid both cold stress (Mejdell et al. 2020; Cymbaluk 1994) and heat stress (Guthrie and Lund 1998).
However, horses have a comparatively lower body surface-to-mass ratio (1:90–100 m2/kg) than humans (1:35–40 m2/kg) which further reduces their ability for heat dissipation (Hodgson et al. 1993). This increases the use of energy required for dissipating accumulated body heat so that body temperature is maintained in the narrow range considered to be normal (Lindinger and Marlin 1995; Brownlow et al. 2016; Hodgson 2014; Tansey and Johnson 2015). To maintain a normal body temperature range, the horse uses four heat transfer mechanisms, thermal radiation, conduction, convection, and evaporation (Fig. 1; Noakes 2008; Guthrie and Lund 1998; Hodgson 2014). See Figure 1.
Thermal radiation: Heat exchange by thermal radiation occurs between the animal’s skin (or hair surface) and the surrounding environment by electromagnetic waves without direct physical contact (Kaviany 2011; Hodgson 2014). Thermal radiation, such as solar radiation or radiation from a fire, is not explicitly physiologically controlled by animals, but it has a significant role in thermoregulation. All physical objects subjected to a temperature above absolute zero (− 273 °C) emit thermal radiation which can be visualized with an infrared camera (Morgan et al. 1997; Meisfjord Jørgensen et al. 2020).
A human study has shown that 60% of human body heat can be dissipated by thermal radiation when there is a sufficient thermal gradient under shade (Wendt et al. 2007). The heat gain via radiation becomes greater than the heat dissipation under sunlight even when the ambient temperature is lower than the body temperature, as it depends on the amount of thermal radiation (W m−2), added to the ambient temperature (Jessen 2001). Under these circumstances, horses can still maintain their body temperature via other thermoregulatory systems, such as sweat evaporation. However, under hot and humid conditions this way of heat dissipation is limited (Wendt et al. 2007; Cheuvront and Haymes 2001; Holcomb et al. 2014).
The colour of the hair coat in horses can also impact body temperature due to differences in solar radiation absorbance (McCutcheon and Geor 2014; Cobb and Cobb 2019). In a study by Cobb and Cobb (2019), it was found that the black and white stripes of the Zebra coat had different temperatures when the animal was standing in full sun. It has been reported that solar radiation absorbance black coat is twice as much as white coat (Maia et al. 2015; Laible et al. 2021), as also reported that the temperature of the black stripes of the Zebra was higher (44 and 56 °C) than the white stripes (36 and 42 °C) (Cobb and Cobb 2019).
Convection: Convective heat transfer is caused by the movement of a gas or liquid (Guthrie and Lund 1998), such as wind over the skin or breathing air in the lungs. The efficiency of this heat exchange depends on the temperature gradient between the body surface and the surrounding gas or liquid and the viscosity determines how rapidly the warmed gas or liquid is replaced by the cool gas or liquid (Willoughby 2002; McCutcheon and Geor 2014; Jefferson et al. 2018; Kaviany 2014). For example, body heat is transferred from the surface of the horse into the cooler surrounding air and heats up the air (conductive heat transfer), but when the heated air is quickly replaced by wind, the body heat can be dissipated more quickly to the replaced cool air (Mostert et al. 1996; Wendt et al. 2007). As a result, faster gas movement (air and wind) can increase the heat exchange by convection.
A horse with long hair has poor body heat dissipation through convection because the hair traps the warmed air and impedes replacement by cooler air (McCutcheon and Geor 2014). The different coat colour of the Zebra can increase convection (Cobb and Cobb 2019). As it was aforementioned in the ‘thermal radiation’, the black stipe has higher temperature than the white stripe, and the temperature gap, between the warmer air near black stirp and the cooler air near white strip, may cause slight airflow and it increases convective heat transfer (Cobb and Cobb 2019). Also, increased blood flow can help convective heat dissipation via the transfer of the body heat away from working muscle (McCutcheon and Geor 2014) through increased blood flow to the periphery. Increased respiratory rate results in heated air being exhaled from the lungs more quickly and being replaced by cooler air with inhalation. It was reported that a high respiratory rate (> 200 breaths/min) can dissipate 25% of the metabolic heat production of exercising horses (Mejdell et al. 2020).
Conduction: Conduction is defined as heat transfer through molecular interactions, whereby heat is exchanged between surfaces through contact when the surfaces have different temperatures (Ezekoye 2016; Kaviany 2014). Horses can emit body heat to the surrounding air when the air temperature is lower than body temperature via conduction and convection, but this heat flow can reverse when the air temperature is higher than that of body temperature (Wendt et al. 2007; Kaviany 2014).
In regard to convection, the body heat transferred from the surface of the horse to the cooler surrounding air heats up the air, and the heated air can be replaced by wind. This enhances the conductive heat dissipation to the surrounding air (Mostert et al. 1996; Wendt et al. 2007). In horses, skin thickness and hair formation, length and density can affect conductive heat transfer (Guthrie and Lund 1998). The efficiency of conductive heat exchange depends also on relative humidity of the air, as water has a high heat conductivity (Romanovsky 2018). Conductive heat transfer can be the most effective method to cool down the body temperature of horses when water that is cooler than body temperature is applied to the body (Marlin et al. 1998; Takahashi et al. 2020).
Evaporation: Only equidae, bovidae, and primate species have sweat glands that allow them to use the evaporation of sweat as the primary form of thermoregulation (McCutcheon and Geor 2014). One gramme of water, such as sweat on the skin or water from the respiratory tract, absorbs approximately 2397 kJ of body heat when it is vaporized (Ingram and Mount 1975).
Even though both humans and horses use sweat evaporation as a primary thermoregulation method under hot ambient temperature, it was reported that the sweat rate (L/h/m2) was three times greater in the exercising horses than in humans in similar exercise intensity (Kingston et al. 1997). Approximately 70% of heat loss from a horse during exercise, is via evaporation when humidity is low (Guthrie and Lund 1998). The sweat of horses, but not humans, is hypertonic and contains abundant Na+, K+, Cl−, and latherin, a protein that decreases surface water tension and makes the sweat spread to help evaporation (Eckersall et al. 1982; Hodgson 2014). Evaporative cooling also occurs via the respiratory tract of horses. Expelled air is always of body temperature with a humidity of 100%.
Although horses are not considered to be panting, breathing frequency and the volume of air intake can increase tenfold and up to 18-fold during strenuous exercise relative to that of a horse at rest, and this elevation in breathing enhances evaporative cooling via the respiratory tract (Franklin et al. 2012). In cold circumstances, the respiration rate decreases and becomes deeper in order to decrease heat loss via respiration and maintain gas exchange in the lungs (Mejdell et al. 2020). The efficiency of heat dissipation through evaporation relies significantly on relative humidity, as evaporation is increased when relative humidity is lower due to a difference between the vapour pressure on the body surface and the atmosphere (Geor and McCutcheon 1998; McCutcheon and Geor 2014; Girard et al. 2008). The efficiency of sweat evaporation is further reduced when the conditions are hot and humid, and eventually sweat runs off the horse. The heat loss from the sweat running off the animal is only 5 to 10% of that through evaporation from the skin (Guthrie and Lund 1998; McCutcheon and Geor 2014). Latherin, a protein in horse sweat, makes a bubble-like foam on the skin and this prevents sweat from dripping off the coat thereby enhancing evaporation (Eckersall et al. 1982; Hodgson 2014)... To read the complete article you need to be a subscriber
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