Anhidrosis in Horses

Updated December 15, 2023 | Reviewed By: Joan Maree Hinken DVM, CVA, CVSMT

Anhidrosis is the decreased ability or complete loss of ability to sweat. Horses suffering from anhidrosis are often called “non-sweaters” or referred to as having “dry coat.”

Horses rely on sweating to thermoregulate and maintain a consistent internal temperature. They lose about 65 to 70% of their body heat through sweating and the evaporation of sweat. Not being able to produce an adequate amount of sweat can put your horse at risk of overheating (hyperthermia) or heat stroke.

Causes of Anhidrosis

The underlying cause of anhidrosis remains unclear. Experts believe either something is wrong on the stimulation end (such as with the neurotransmitter adrenaline) or something is wrong on the receptor end (such as decreased numbers of receptors or decreased sensitivity of these receptors).

Some theories suggest that anhidrosis could be caused by an overstimulation of the sweat glands, which can be put into overdrive during the hottest times of the year and may essentially start to shut down. Any horse can develop anhidrosis, no matter their age, gender, color, or breed. However, horses that live in hot, humid environments are more likely to be affected. Being born or raised in a warm climate does not give a horse any advantage over not developing the condition than one brought up in a colder climate.

Areas with high humidity can further reduce the amount of sweat that will evaporate, hence creating a more serious challenge for cooling your horse down. Oftentimes, a big change in environment (like horses traveling to Florida for the winter) can suddenly trigger non-sweating. Anhidrosis can also develop gradually over time and in varying degrees of severity.

Signs and Symptoms of Anhidrosis in Horses

• A lack of sweat during or after exercise, or in warm weather. Some horses may sweat very lightly or just in patches. For example, at times when your horse should be lathered, such as during a summer afternoon ride when all other horses are sweating, they may only be a little damp under their tack.
• Labored breathing with shallow, rapid breaths and flaring nostrils. The horse will be trying as hard as he can to cool himself down by releasing heat. They will have an elevated respiratory rate for an extended period after exercise, usually for longer than 30 minutes. Their internal body temperature could reach as high as 104° Fahrenheit (the normal range is 99.5 to 100.5° F).
• A poor-quality coat with dry or flaky patches of skin and loose or thinning hair (especially on the forehead).
• Poor performance or a decline in their performance. They may be lethargic and become quickly and easily exhausted during training, which could limit their potential career.
• Not drinking as much water as usual and/or a loss of appetite.

Diagnosing Anhidrosis

If you suspect your horse isn’t sweating properly, call your veterinarian so they can examine your horse and rule out any other conditions with similar signs. Your vet will perform a physical exam and run diagnostic tests through injections that stimulate the sweat glands (with epinephrine or terbutaline). They will then measure the amount of sweat produced. Blood may also be taken so your veterinarian can measure your horse’s electrolyte levels.

Treatment and Management of Anhidrosis in Horses

While it may never completely resolve, there are many options that may help manage the symptoms of anhidrosis. A few options that veterinarians, horse owners, and barn managers have seen success with include:

• vitamin, electrolyte, and salt supplements to support healthy sweating
• herbal remedies
• acupuncture treatment programs
• thyroid hormones
• giving your horse a dark beer

There is no one-and-done treatment for all affected horses - some options may be successful for one horse but ineffective on his barn mate. Some horses with anhidrosis regain the ability to sweat when moved from the hot, humid climate to a cooler, drier one, yet others don't.

Supplements That May Support Healthy Sweating

• Electrolytes such as chloride, sodium, potassium, calcium, magnesium, and other minerals are components of normal sweat and play an important role in nerve and muscle function. Even though an anhidrotic horse may not be losing minerals through sweat, they may still have abnormalities in their electrolyte levels (found through blood tests).
• Salt found in electrolytes will also encourage your horse to drink and help to maintain proper hydration levels. Adding an electrolyte supplement to your horse’s daily diet may help support their ability to sweat normally and promote overall well-being.
• Many sweat-promoting supplements are formulated to include l-tyrosine, an amino acid that is a building block of brain chemicals (neurotransmitters) and supports the proper function of the thyroid, adrenal, and pituitary glands.
• Vitamins E and C (known as tocopherol and ascorbic acid respectively) are antioxidants that support and protect the body from the oxidative stress of exercise, travel, and illness.
• Vitamin B3 (niacin) is also a key component in many formulas for its cell-protecting effects and role in energy production.
• Feeding a horse with anhidrosis a low sugar diet may be a good option if a veterinarian determines the root of the issue to be stress-related.

Each supplement has its own recommendations for how and when to start your horse on the product. This may involve giving the supplement before the hot season begins and reducing their workload for a few weeks when getting started. Remember to consult with your veterinarian before starting a product to ensure it’s the right one for your unique horse and the specific issue being addressed.

Tips to Help Keep Anhidrotic Horses Cool

• When bringing your horse to a hot, humid climate, allow him to acclimate with 10-14 days of turnout and light work before returning to regular training and showing.
• Get your horse "legged up" or conditioned with interval work or distance riding before the hot months so you don't have to work on basic fitness then.
• Work your horse during the cooler parts of the day, usually in the morning or evening.
• Avoid turning your horse out during the hotter parts of the day when the sun is strong, and make sure they always have a shaded area and plenty of water. If night turnout is available to your horse, it may be a good option to consider.
• Observe your horse closely during exercise for signs of overheating, such as rapid breathing or heart rate, and fatigue.
• Cool your horse off with water as cold as he will tolerate and quickly scrape it off. Keep reapplying and scraping until his temperature has returned to normal.
• Provide cool air with good barn ventilation and fans or misters.

Key Takeaways

Anhidrosis is a serious condition that requires changes in your horse’s daily management. Your veterinarian is your best resource for diagnosing anhidrosis and should be consulted before beginning any treatment plan. You may need to adjust your expectations of your horse’s ability to perform and their potential career. With early attention to the signs of non-sweating and careful management, you can keep your horse comfortable and prevent overheating.