Equine anhidrosis is a performance-limiting condition that involves a reduced ability to sweat in horses. The condition most frequently affects horses living in hot and humid climates.
Anhidrosis occurs because the sweat glands fail to function normally when body temperature increases. Horses with anhidrosis cannot properly regulate their core temperature in response to bouts of exercise or high ambient temperatures.
Without intervention, equine anhidrosis may result in heatstroke or permanent damage to the sweat glands. This condition can be life-threatening in some cases.
The exact cause of anhidrosis is unknown, although hormonal and neurological factors may play a role. Veterinarians diagnose anhidrosis based on a physical assessment and an intradermal terbutaline sweat test.
There is no known cure for this condition. Some horses resume normal sweating when moved to a cooler climate for 10 days or longer. Strategies to manage the condition primarily focus on preventing affected horses from overheating.
What is Equine Anhidrosis?
Commonly referred to as non-sweating disease or dry coat syndrome, equine anhidrosis describes the impaired function of the sweat glands. Horses with this condition cannot facilitate normal sweating in response to expected stimuli. 
Equine anhidrosis is problematic in affected horses because sweating is the primary way the body dissipates heat when core temperature increases. Horses need to sweat to cool themselves down in hot weather or when exercising.
In these environments, sweat does not evaporate as efficiently. This limits how well horses can cool themselves.
Cases of anhidrosis have also been reported during the warmer months in horses living in temperate climates. According to anecdotal reports, horses that have recently relocated from a cooler climate to a warmer one may also be susceptible to developing the condition. 
In transient cases, horses resume sweating normally once the weather cools. However, anhidrosis can progressively worsen and result in the gradual degeneration of the sweat glands, rendering some horses unable to sweat normally again. 
Prevalence of Anhidrosis
A large proportion of anhidrosis cases are reported along the Gulf Coast of the United States. 
In a study of 4,620 horses on 500 farms in Florida, 2% of horses were found to have anhidrosis.  Of the farms studied, 11% reported at least one case of anhidrosis.
Horses with a family history of anhidrosis were more likely to develop the condition. Breeds including Thoroughbreds and warmblood horses were most frequently affected. 
A study of 834 Thoroughbred horses on four central Florida farms found that 6.12% had the condition. 
Training horses and non-pregnant broodmares were most often affected by the disease. Adolescent horses were not as likely to be affected.
The coat colour and gender of the horses assessed were not correlated with the condition.
Why is Sweat so Important?
When horses exercise their muscles produce heat, which raises their core body temperature.
This heat is absorbed into the bloodstream and subsequently moved into the lungs where some of it is released during exhalation. The skin also radiates excess heat away from the body. These processes are known as evaporative cooling.
In healthy exercising horses, 70% of their body heat is dissipated through sweat and 23% through evaporative cooling during respiration. 
In horses with normally functioning sweat glands, body temperature lowers as heat dissipates through the action of sweating.
However, horses with anhidrosis are unable to cool their body effectively. This can cause their body temperatures to rise above normal (greater than 37 to 39 degrees Celsius/100 to 102 Fahrenheit) and puts the horse at risk of heatstroke.
Anatomy of the Equine Sweat Gland
Horses have approximately 810 tubular-shaped sweat glands per cm in their skin. 
These glands are each associated with a hair follicle and are either apocrine (secrete sweat into the hair follicle) or eccrine (secrete sweat directly onto the skin).
The predominant type of sweat glands in horses is apocrine whereas humans have mostly eccrine sweat glands.
Sweat glands are located near blood vessels and are abundantly innervated with nerves. The activity of the apocrine glands is influenced by neurotransmitters including epinephrine (adrenaline), and norepinephrine (noradrenaline).
When the brain’s hypothalamus detects an increase in body temperature, it activates the production of sweat through neural signals. 
Sweat glands produce sweat consisting of multiple components including:
- Electrolytes: sodium, potassium, and chloride. 
- Proteins: glycoproteins, surfactants, and proteins associated with skin protective mechanisms.
Signs of Anhidrosis
Common clinical signs of acute anhidrosis include:
- Limited or no sweating in response to stimuli
- Coat remains dry and hot to the touch following exercise
- High respiratory rate (tachypnea)
- Elevated blood pressure
- Laboured breathing
- Increased rectal temperature
Signs of chronic anhidrosis include:
- Dry and flaky skin
- Hair loss (alopecia) that causes a patchy coat appearance
- Facial hair loss
- General fatigue
- Weight loss (anorexia)
- Decreased water consumption
- Poor performance
Exercise can increase a horse’s body temperature significantly, but their temperature should return to normal within 30 minutes after exercise has stopped.
The core temperature of horses with anhidrosis may fail to return to normal or take longer to return to normal compared to horses that do not have this condition.
Some horses affected by anhidrosis may continue to sweat in specific areas of their body including the throat latch, under the mane, under the saddle or halter, and in the groin area. However, other areas of their body that normally produce sweat will remain dry in these horses.
Prior to the onset of anhidrosis, some horses sweat profusely. In other horses, the sweating response diminishes gradually with the onset and progression of the condition.
Causes of Anhidrosis
Why anhidrosis occurs in horses is not yet well understood and is currently being investigated. However, the condition is believed to result from a combination of neurological and hormonal imbalances that influence the function of the sweat glands. 
Abnormal Function of the Sweat Glands
Research indicates that anhidrosis involves a gradual failure of the sweat glands to function normally.  Changes in the cellular structure of the sweat glands have been observed in horses with anhidrosis. 
In horses that sweat normally, the sweat glands are activated in response to nerve signals (neurotransmitters). Anhidrosis may result from sweat glands not responding properly to these signals.
Side Effects of Macrolide Antibiotics
Macrolide antibiotics (used to treat Rhodococcus equi infections) have been shown to inhibit sweat production in foals and induce anhidrosis.  Drugs in this class include:
Some foals treated with the macrolide erythromycin experience hyperthermia (elevated body temperature) as a side effect. These foals develop anhidrosis and are unable to dissipate heat. This effect can persist for at least 10 days after erythromycin treatment and can be life-threatening.
The mechanism is believed to be related to the inhibition of chloride secretion. Macrolide antibiotics work by inhibiting protein synthesis in bacteria to kill these pathogens. However, they can also affect chloride channels in epithelial cells, blocking the ability of sweat glands to transport water and salt.
Horse owners should be aware that foals treated with macrolide are a risk of hyperthermia. Provide adequate shade if the horse is outdoors and maintain a cool environment indoors to prevent major complications associated with drug-induced anhidrosis.
Other Proposed Causes
Hypothyroidism, low chloride concentrations, and abnormally elevated epinephrine levels have also been proposed as potential causes of anhidrosis.
These factors can influence the activity of the sweat glands.  However, no direct association between these observations and the development of anhidrosis has been proven in research studies.
A decrease in the amount of chloride excreted in urine is a consistent finding in horses with anhidrosis.  This does not necessarily indicate that chloride deficiency plays a role in the condition.
No definitive risk factors for anhidrosis have been identified other than exposure to hot and humid environmental conditions.
Researchers are currently investigating the role that genetics plays in the development of the condition. 
A study from the University of Florida involving 200 horses found that horses from families with a history of anhidrosis are more likely to be affected. 
Scientists discovered a gene that encodes a portion of a potassium channel protein that has a possible function in sweat gland outflow and may play a role in the development of anhidrosis.
Consult with your veterinarian if you suspect your horse has anhidrosis. Other conditions that produce similar signs – such as infections and respiratory diseases – must be ruled out.
A diagnosis of anhidrosis is typically made based on an assessment of clinical signs and a physical examination.
In some cases, a veterinarian will complete an intradermal terbutaline sweat test to quantify the sweating capacity of horses suspected of having anhidrosis.
This test involves administering injections of various dilutions of the drug terbutaline into the skin to promote sweating in a local area.  Terbutaline is a Beta2-agonist, similar to epinephrine.
By pressing absorbent pads against the skin, sweat is collected from the injected area for 30 minutes following the injection.
Horses with normally-functioning sweat glands should respond to all dilutions whereas horses with anhidrosis will not sweat in response to any level of terbutaline. Horses with partial anhidrosis may respond to more concentrated dilutions but not to weaker solutions.
Treatment of Anhidrosis
There is no known cure for this condition. Treatment of anhidrosis involves emergency actions to prevent hyperthermia in affected horses and long-term management strategies to help horses recover from the condition.
When a horse is displaying acute symptoms of anhidrosis, it is critical to lower their body temperature quickly to prevent heatstroke and potential death.
Affected horses should be moved to a cool environment immediately. Some strategies to lower body temperature include:
- Relocate the horse to a shady areas outside
- Move the horse to an air-conditioned stall
- Use fans to move air over the horse
- Use misting fans to apply cold water
In some cases, ice may be used to cool the body temperature quickly. A veterinarian may also need to administer fluids intravenously to ensure the horse’s electrolyte levels remain balanced.
Long-Term Recovery and Management
Supplements containing the amino acid L-tyrosine may work for horses with moderate symptoms. This amino acid is involved in the synthesis of catecholamines (including epinephrine) and may influence cellular response to epinephrine in sweat glands.
It should be noted that improvements with such supplements were seen only after exercise intensity was reduced for 3 weeks. It is unclear how much of the improvement can be attributed to the reduction in exercise. 
Medical treatments investigated for anhidrosis are generally unsuccessful. Treatments that have been found to be unsuccessful include administering ACTH, alpha-2 agonists, prostaglandins, antihistamines, Clenbuterol, and Methyldopa. 
The currently accepted treatment protocol for anhidrosis primarily focuses on providing symptomatic relief when the weather is hot and humid.
To slow the progression of anhidrosis in your horse, consider the following management interventions:
- Relocate the Horse: If possible, affected horses should be relocated to a cool environment for 10 to 30 days. Horses with mild to moderate anhidrosis typically start sweating again once moved to a cooler environment. 
- Cool the Stable/Stall: If transporting the horse to a cooler locale is not possible, use cooling devices including fans, misting machines, and air conditioners to lower the temperature in their living environment.
- Provide Shade: Ensure that horses living outdoors have access to shelter or trees that provide shade.
- Feeding Electrolytes: Administering sodium, potassium, and chloride helps to promote adequate water intake and electrolyte balance.
Some horses affected by anhidrosis resume sweating during the cooler winter months or upon relocation to a cooler climate for a period of 10 days or more.
However, in severe cases horses may not improve despite appropriate environmental management.
It may not be possible to completely prevent anhidrosis if the horse is in a hot, humid environment. However, there are steps you can take to mitigate the clinical signs of this condition and reduce the risk of recurrence.
Anecdotal reports suggest that horses that have previously experienced anhidrosis can avoid recurrence if they are housed in cool environments (ie. air-conditioned stall) even while living in hot and humid conditions. 
The following management practices may help lessen signs of anhidrosis in horses:
- Avoid Exercising in the Heat: Do not exercises horses with anhidrosis during the hottest parts of the day and avoid training during hot weather. Restrict physical activity to cool mornings or evenings.
- Maintain Fitness Level: Excess body condition and strenuous exercise may contribute to the development of anhidrosis. Keep your horse at a healthy body weight and match their training program to their conditioning level.
- Feed Electrolytes: Increasing intake of electrolyte minerals can support general health and encourage hydration.
- Provide Fresh Water: Horses need access to cool, clean drinking water at all times to maintain adequate hydration. Water is crucial to help horses maintain their body temperature (thermoregulation).
- Restrict Exposure to Heat: Keep horses that are prone to anhidrosis stalled or in a shaded paddock during the hottest times of the day.
- Turnout in Areas with Natural Water Sources: Some horses will cool themselves in ponds or streams located in their pastures.
Unfortunately, the causes of anhidrosis are not well understood and treatment options are limited. There is no way to cure the condition, but symptoms can be alleviated by keeping the horse in a cool environment and avoiding activities that contribute to overheating.
Management practices that help the horse dissipate heat are the best way to prevent heatstroke and other complications.
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