Thermoregulation - Too hot to handle!

Article by Adam Jackson MRCVS

Exertional heat illness (EHI) is a complex disease where thoroughbred racehorses are at significant risk due to the fact that their workload is intensive in combination with the high rate of heat production associated with its metabolism.  In order to understand how this disease manifests and to develop preventative measures and treatments, it is important to understand thermoregulation in horses. 

What is thermoregulation?

With continuous alteration in the surrounding temperature, thermoregulation allows the horse to maintain its body temperature within certain limits.  Thermoregulation is part of the greater process of homeostasis, which is a number of self-regulating processes the horse uses to maintain body stability in the face of changing external conditions.  Homeostasis and thermoregulation are vital for the horse to maintain its internal environment to ensure its health while disruption of these processes leads to diseases. 

The horse’s normal temperature range is 99–101°F (37.5–38.5°C).  Hyperthermia is the condition in which the body temperature increases above normal due to heat increasing faster than the body can reduce it. Hypothermia is the opposite condition, where the body temperature decreases below normal levels as the body is losing heat faster than producing it.   These conditions are due to the malfunction of thermoregulatory and homeostatic control mechanisms.

Horses are colloquially referred to as warm-blooded mammals—also known as endotherms because they maintain and regulate their core body, and this is opposite ectotherms such as reptiles.  The exercising horse converts stored chemical energy into mechanical energy when contracting various muscles in its body. However, this process is relatively inefficient because it loses roughly 80% of energy released from energy stores as heat. The horse must have effective ways to dissipate this generated heat; otherwise, the raised body temperatures may be life threatening.

Transfer of body heat

There are multiple ways heat may be transferred, and this will flow from one area to another by:

1. Evaporation 

The main way body heat is lost during warm temperatures is through the process of evaporation of water from the horse’s body surface. It is a combination of perspiration, sweating and panting that allows evaporation to occur.

Sweating is an inefficient process because the evaporation rate may exceed the body heat produced by the horse, resulting in the horse becoming covered and dripping sweat. This phenomenon occurs faster with humid weather (high pressure).

Insensible perspiration is the loss of water through the skin, which does not occur as perceivable sweat. Insensible perspiration takes place at an almost constant rate and is the evaporative loss from skin; but unlike sweating, the fluid loss is pure water with no solutes (salts) lost. The horse uses insensible perspiration to cool its body.

It is not common for horses to pant in order to dissipate heat; however, there is evidence that the respiratory tract of the horse can aid in evaporative heat loss through panting.

2. Conduction

Conduction is the process where heat is transferred from a hot object to a colder object, and in the case of the horse, this heat transfer is between its body and the air.  However, the air has poor thermal conductivity, meaning that conduction plays a small role in thermoregulation of the horse.   Conduction may help if the horse is lying in a cool area or is bathed in cool water.  

The horse has the greatest temperature changes occurring at its extremities, such as its distal limbs and head.  The horse can alter its blood flow by constricting or dilating its blood vessels in order to prevent heat loss or overheating, respectively.

Interestingly, the horse will lie down and draw its limbs close to its body in order to reduce its surface area and to control conduction. There also have been some adaptive changes in other equids like mules and burros, where shorter limbs, longer ears and leaner bodies increase its surface area to help in heat loss tolerance.

3. Convection 

Convection is the rising motion of warmer areas of a liquid or gas and the sinking motion of cooler areas of the liquid or gas.  Convection is continuously taking place between the surface of the body and the surrounding air. Free convection at the skin surface causes heat loss if the temperature is low with additional forced convective heat transfer with wind blowing across the body surface.

When faced with cold weather, a thick hair coat insulates and resists heat transfer because it traps air close to the skin; thus, preventing heat loss. Whereas, the horse has a fine hair coat in the summer to help in heat loss.

4. Radiation

Radiation is the movement of heat between objects without direct physical contact.  Solar radiation is received from the sun and can be significant in hot environments, especially if the horse is exposed for long periods of time.  A horse standing in bright sunlight can absorb a large amount of solar radiation that can exceed its metabolic heat production, which may cause heat stress. 

How the horse regulates its body temperature

The horse must regulate its heat production and heat loss using thermoregulatory mechanisms.  There are many peripheral thermoreceptors that detect changes in temperature, which leads to the production of proportional nerve impulses. These thermoregulators are located in the skin skeletal muscles, the abdomen, the spinal cord and the midbrain with the hypothalamus being instrumental in regulating the internal temperature of the horse.   A coordinating center in the central nervous system receives these nerve incoming impulses and produces output signals to organs that will alter the body temperature by acting to reduce heat loss or eliminate accumulated heat.  

The racehorse and thermoregulation

The main source of body heat accumulation in the racehorse is associated with muscular contraction.  At the initiation of exercise, the racehorse’s metabolic heat production, arising from muscle contraction, increases abruptly.  The heat production does alter the level of intensity of the work as well as the type of exercise undertaken.  

During exercise, the core body temperature increases because heat is generated and the horse’s blood system distributes this heat throughout the body. Hodgson and colleagues have theorized and confirmed via treadmill studies that the racehorse has the highest rate of heat production compared to other sporting horses. In fact, the racehorse’s body temperature can rise 33°F (0.8°C) per minute, reaching 108°F (42.0°C). But what core temperature can the horse tolerate and not succumb to heat illness and mortality?  The critical temperature for EHI (exertional heat illness) is not known, but studies have demonstrated that a racehorse can be found to have core temperatures between 108°F - 109° F (42–43°C) without any clinical symptoms. Currently, anecdotal evidence is only available, suggesting that a core temperature of 110°F (43.5°C) will result in manifestation of EHI with the horse demonstrating central nervous system dysfunction such as ataxia (incoordination).  In addition, temperatures greater than 111°F (44°C) result in collapse. 

Heat loss in horses

A horse loses heat to the environment by a combination of convection, evaporation and radiation, which is magnified during racing due to airflow across the body. However, if body heat gained through racing is not minimized by convection, then the racehorse’s body temperature is regulated entirely by evaporation of sweat. This evaporation takes place on the horse’s skin surface and respiratory tract.  

The horse has highly effective sweat glands found in both haired and hairless skin, which produces sweat rates that are highest in the animal kingdom.   Efficient evaporative cooling is present in the horse because its sweat has a protein called latherin, which acts as a wetting agent (surfactant); this allows the sweat to move from its skin to the hair.

Because of the horse’s highly blood-rich mucosa of its upper respiratory tract, the horse has a very efficient and effective heat exchange system.  Estimates suggest this pathway dissipates 30% of generated heat by the horse during exercise.  As the horse exercises, there is blood vessel dilation, which increases blood flow to the mucosa that allows more heat to be dissipated to the environment. When the respiratory tract maximizes evaporative heat loss, the horse begins to pant. Panting is a respiratory rate greater than 120 breaths per minute with the presence of dilated nostrils; and the horse adopts a rocking motion. However, if humidity is high, the ability to evaporate heat via the respiratory route and skin surface is impaired. The respiratory evaporative heat loss allows the cooling of venous blood that drains from the face and scalp. This blood may be up to 37°F (3.0°C) cooler than the core body temperature of 108°F (42.0°C). And as it enters the central circulatory system, it can significantly have a whole-body cooling effect. This system is likely an underestimated and significant means to cool the horse.

Pathophysiology of EHI in the thoroughbred

Although it is inconsistent to determine what temperature may lead to exertional heat illness (EHI), it is known that strenuous exercise, especially during heat stress conditions leads to this disease.  In human medicine, this disease is recognised when nervous system dysfunction becomes apparent.  There are two suggested pathways that lead to EHI, which may work independently or in combination depending on the environmental factors that are present during racing/training.

1. Heat toxicity pathway

Heat is known to detrimentally affect cells by denaturing proteins leading to irreversible damage.  In general, heat causes damage to cells of the vascular system leading to widespread intravascular coagulation (blood clot formation), pathologically observed as micro thrombi (miniature blood clots) deposits in the kidneys, heart, lungs and liver.  Ultimately, this leads to damaged organs and their failure.

Heat tissue damage depends on the degree of heat as well as the exposure time to this heat. Mammalian tissue has a level of thermal damage at 240 minutes at 108°F (42°C), 60 minutes at 109°F (43°C), 30 minutes at 111°F (44°C) or 15 minutes at 113°F (45°C).  This heat damage must be borne in mind following a race requiring suitable and appropriate cooling methods, otherwise inadequate cooling may lead to extended periods of thermal damage causing disease. 

The traditional viewpoint is that EHI is caused by strenuous exercise in extreme heat and/or humidity.  However, recent studies have revealed that environmental conditions may only cause 43% of EHI cases, thus, suggesting that other factors are involved.

2. Heat sepsis pathway

In some instances. a horse suffering from EHI may present with symptoms and clinical signs similar to sepsis like that seen in an acute bacterial infection. 

A bacterial infection leading to sepsis causes an extreme body response and a life threatening medical emergency.  Sepsis triggers a chain reaction throughout the body particularly affecting the lungs, urinary tract, skin and gastrointestinal tract.

Strenuous exercise in combination with adverse environmental conditions may lead to sepsis without the presence of a bacterial infection— also known as an endotoxemic pathway—causing poor oxygen supply to the mucosal gastrointestinal barrier. Ultimately, the integrity of the gastrointestinal tract is compromised, allowing endotoxins to enter the blood system and resulting in exercise-induced gastrointestinal syndrome (EIGS).

However, researchers have observed that EHI in racehorses is unpredictable as EHI may develop in horses following exercise despite “safe” environmental conditions.  Even with adequate cooling and resuscitative therapies, tissue damage that occurs demonstrates that thermoregulatory and inflammatory pathways may vary, and hyperthermia may be the trigger but may not necessarily be driving the condition.

Diagnosis of EHI

The diagnosis of EHI is based on the malfunctioning of the central nervous system.

Initially, hyperthermia reduces the blood flow to the cerebrum of the brain, leading to a decrease of oxygen to that area—also known as ischemia. As a result, the clinical signs are:

• Extreme restlessness

• Confusion

• Substantial headache

If this hyperthermia continues, then the blood-brain barrier (an immunological barrier between circulating blood that may contain microorganisms like bacteria and viruses to the central nervous system) begins to leak plasma proteins, resulting in cerebral oedema (build up of fluid causing affected organ to become swollen). If treatment is not initiated at this point, then neuronal injury will result especially in the cerebellum.

EHI follows and involves serious CNS dysfunction.  The clinical signs associated with EHI are:

• Delirium

• Horses unaware of their surroundings

The final stage of EHI occurs when the swollen oedematous brain compresses vital tissue causing cellular damage. The clinical signs of end-stage EHI are:

• Collapse

• Unconsciousness

• Coma

• Death

Definition of EHI

EHI most commonly occurs immediately after a race when the horse is panting, sweating profusely and may be dripping with sweat. The most reliable indication of EHI is clinical signs associated with the dysfunction of the central nervous system in the presence of hyperthermia. Researchers have provided descriptions of levels of CNS dysfunction, ranging from level 1 to level 4.

Level 1 – The earliest recognizable signs of CNS dysfunction

The horse becomes restless, agitated and irritable. There is often head nodding or head shaking. The horse is difficult to restrain and will not stand still.  Therapeutic intervention such as cooling can resolve these clinical signs, but if the horse is inadequately cooled then the disease can escalate. 

Level 2 – Obvious neurological dysfunction

Often misdiagnosed as colic symptoms, the horse becomes further agitated and irritable with the horse kicking out without any particular stimulus present. This stage is dangerous to all handlers involved as the horse’s behavior is unpredictable. 

Level 3 – Bizarre neurological signs

At this stage, the horse has an altered mentation appearing vacant, glassy-eyed and “spaced-out”.  In addition, there is extreme disorientation with a head tilt and leaning to one side with varying levels of ataxia (wobbly).  It has been observed that horses may walk forward, stop, rear and throw themselves backwards.  It is a very dangerous stage, as horses are known to run at fences, obstacles and people. Horses may also present as having a hind limb lameness appearing as a fractured leg with hopping on the good limb.  These clinical signs may resolve with treatment intervention.

Level 4 – Severe CNS dysfunction

There is severe CNS dysfunction at this stage of EHI with extreme ataxia, disorientation and lack of unawareness of its surroundings. The horse will continuously stagger and repeatedly fall down and get up while possibly colliding with people or objects with a plunging action. Unsurprising, the horse is at risk of severe and significant injury.  Eventual collapse with the loss of consciousness and even death may arise.

Treatment of EHI

In order to achieve success in the treatment of EHI, it is imperative that there is early detection, rapid assessment and aggressive cooling. The shorter the period is between recognising the condition and treatment, the greater the chance of a successful outcome.  In particular settings such as racecourses or on particularly hot and humid days, events must be properly equipped with easily accessible veterinary care and cooling devices. It is highly effective if a trained worker inspects every horse in order to identify those horses at risk or exhibiting symptoms. 

If EHI is recognised, veterinary intervention will be paramount in the recovery to prevent further illness and suppress symptoms. It will be important to note any withdrawal periods of any non-steroidal anti-inflammatories (NSAIDs) and analgesics before returning to racing. There are a number of effective ways to cool the horse with easily accessible resources.

Whole body cooling systems

Cooling the horse with ice-cold water is an effective way to draw heat from the underlying tissues. In addition, cooling the skin redistributes cooled blood back to the central circulatory system thus reducing thermal strain with the cooling of core body temperature.

The system that works best for horses due to its size is spray cooling heat transfer. It is ideal to have two operators to spray either side of the horse. It is recommended to begin at the head and neck followed by the chest and forelimbs then the body, hind limbs and between the legs. Spray nozzles are recommended to provide an even coverage of the skin surface.   

Dousing is another technique in which horses are placed in stalls and showered continuously until the condition resolves. Pouring buckets over the entire body of the horse is not recommended as most of the water falls to the ground, thus, not efficient at cooling the horse. 

Because most horses suffer from EHI immediately after the race, the appropriate location for inspection, cooling systems and veterinary care should be in the dismounting yard and tie-up stalls.  There must be an adequate supply of ice to ensure ice-cold water treatment. 

When treating a horse with EHI, there must be continuous and uninterrupted cooling until the CNS dysfunction has disappeared. 

When the skin surface temperature decreases to 86°F (30°C), cutaneous skin vessels begin to disappear; CNS function returns to normal, and there is the normalization of behavior. Cooling can be stopped, and the horse can be walked once CNS abnormalities have resolved. It must remain closely monitored for a further 30 minutes in a well-ventilated and shaded region. It is important that they are not unattended.

Scraping sweat off of the horse must only be done if the conditions are humid with no airflow.  However, if it is hot and there is good airflow, scraping is unnecessary because the sweat will evaporate.

Cooling collars

During strenuous exercise, there is a combination of heat production in the brain, reduced cerebral blood flow, creating cerebral ischaemia as well as the brain being perfused with hot blood. It is believed that cooling the carotid artery that aids in blood perfusion of the brain might be a strategy to cool the brain. A large collar is placed on either side and around the full length of the horse’s neck and is cooled by crushed ice providing a heat sink around the carotid artery; and it is able to pump cooled blood into the brain. 

Another possible benefit of this device is the cooling of the jugular veins, which lie adjacent to the carotid arteries.  The cooled blood in the jugular veins enter the heart and is pumped to the rest of the body, hence, potentially cooling the whole body. In addition, it is thought that the cooling of the carotid artery causes it to dilate, allowing greater blood flow into the brain. 

Provision of shaded areas

Shaded areas with surfaces that reflect heat, dry fans providing air flow and strategically placed hoses to provide cool water is an important welfare initiative at racecourses in order to minimize risk of EHI and treat when necessary. 

Conclusion

The most effective treatment of EHI is the early detection of the disease as well as post-race infrastructure that allows monitoring of horses in cooling conditions, while providing easily accessible treatment modalities when they are needed.  

Evaluating the horse’s central nervous system dysfunction is essential to recognise both the disease as well as monitoring the progression of the disease. CNS dysfunction allows one to define the severity of the condition. 

Understanding the pathophysiology of EHI is essential. It is important to recognise that it is a complex condition where both the inflammatory and thermoregulatory pathways work in combination. With a better understanding of these pathways, more effective treatment for this disease may be found.