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. [1][2]

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.

Horses living in hot and humid climates are most likely to be affected by anhidrosis. [1]

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. [4]

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. [4]

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Prevalence of Anhidrosis

Horses of any breed, age, sex, and color can develop anhidrosis. Epidemiologic studies estimate the condition affects 2-6% of horses. [5][6]

A large proportion of anhidrosis cases are reported along the Gulf Coast of the United States. [7]

In a study of 4,620 horses on 500 farms in Florida, 2% of horses were found to have anhidrosis. [5] 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. [5]

A study of 834 Thoroughbred horses on four central Florida farms found that 6.12% had the condition. [6]

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. [3]

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. [3]

Equine Sweat Glands

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.