Other Names
Dry coat, non-sweating syndrome, puff disease (historical), equine anhidrosis.
What is Equine Anhidrosis?
Equine anhidrosis is a condition in which a horse is unable to sweat normally in response to heat or exercise. Sweating is the horse’s primary thermoregulatory mechanism — horses generate enormous amounts of metabolic heat during exercise, and evaporative cooling from sweat is the most effective way to dissipate that heat. When the sweating mechanism fails, even moderate exercise or warm environmental conditions can cause dangerous overheating (hyperthermia).
The condition predominantly affects horses in hot, humid climates — it is particularly common in the southern USA, the Caribbean, South East Asia, and Australia. In the UK, it is less common but does occur, particularly in horses imported from tropical or subtropical climates or in susceptible individuals during warm British summers. Horses that have recently been relocated from cool to hot climates are at higher risk during the adaptation period.
What Causes Equine Anhidrosis?
The exact mechanism of anhidrosis is not fully understood. Current evidence suggests that chronic overstimulation of sweat glands in response to sustained high temperatures leads to down-regulation of sweat gland receptors and gland exhaustion. Beta-adrenergic receptors in the sweat glands appear to become desensitised following prolonged adrenergic stimulation, reducing or eliminating the sweat response to adrenaline (the primary stimulant of equine sweat glands).
Predisposing factors include:
- High ambient temperature and humidity — particularly prolonged exposure
- Horses moved from cool to hot climates without adequate acclimatisation time
- Individual genetic susceptibility — some horses and bloodlines appear more prone
- Dark-coloured horses may be at slightly higher risk due to greater solar heat absorption
Symptoms and Signs
Signs range from partial (oligohidrosis — reduced sweating) to complete (anhidrosis — total absence of sweating):
- Absence of sweat or dramatically reduced sweating during exercise or in warm conditions — a horse that should be sweating but has a dry coat is a classic presentation
- Hyperthermia (high body temperature) after exercise or in warm environments — rectal temperature may reach 40–42°C
- Rapid, laboured breathing (tachypnoea) as the horse attempts to dissipate heat via respiratory evaporation — panting
- Flared nostrils and distressed appearance after mild exercise
- Exercise intolerance — the horse tires rapidly and may be reluctant to continue working
- Dull, dry, rough-looking coat
- Patchy sweating — some horses sweat only around the ears, face, or under the mane but nowhere else on the body
- Weight loss and poor body condition in chronic cases
Diagnosis
Diagnosis is based on clinical signs and confirmed by the intradermal terbutaline sweat test. A small amount of terbutaline (a beta-adrenergic agonist) is injected intradermally at a clipped site (typically the neck or shoulder). In a normal horse, a sweat response is visible at the injection site within 30 minutes. In an anhidrotic horse, the sweat response is absent or greatly reduced. The test can also be used to assess the degree of anhidrosis by comparing sweat response between treated and untreated injection sites.
This is a useful, inexpensive, and practical field test that any equine vet can perform. Routine blood tests are typically unremarkable in anhidrosis but help rule out other causes of heat intolerance or poor performance.
See our entry on equine lameness for comparison with other causes of poor performance and exercise intolerance.
Treatment
There is no consistently effective cure for equine anhidrosis, but several management strategies and treatments have been reported to help in some horses:
Environmental management (most important):
- Move the horse to a cooler climate — affected horses often recover normal sweating function when relocated to temperate environments. This is the most reliably effective intervention.
- Work the horse only in the coolest parts of the day (very early morning or late evening)
- Provide shade, fans, and cool water dousing after exercise to assist thermoregulation
- Reduce exercise intensity and duration during hot, humid periods
Dietary supplements: Electrolyte supplementation (particularly sodium chloride) is commonly recommended. There is anecdotal and some limited evidence for supplementation with:
- One AC (One Animal Care) — a US commercial product containing brewers’ yeast used as a supplement for anhidrosis in horses; some horses show improved sweating with this product
- Salt (sodium chloride) added to feed
- Vitamin E and other antioxidants
Clenbuterol: Some vets have used clenbuterol (a bronchodilator with adrenergic activity) in an attempt to restimulate sweat gland function. Results are variable.
Beer/dark beer: A traditional remedy with some anecdotal support — some horse owners report improvement with supplemental feeding of dark beer, possibly due to its yeast content. The evidence base is not strong but it is harmless.
For horses that cannot be relocated to a cooler climate, careful exercise management combined with active cooling (water hosing, fans, electrolytes) and exercise restricted to the coolest times of day is the most practical approach.
Prevention
- Allow adequate acclimatisation time (3–6 months) when moving horses from cool to hot climates before subjecting them to significant work demands
- Avoid overworking horses during the hottest part of the day in warm climates
- Ensure adequate salt and electrolyte intake year-round
- Monitor for early signs — patchy sweating or reduced sweating compared to herd mates should trigger investigation
When to Call the Vet
- A horse shows signs of heat stress — high temperature, distressed breathing, disorientation — after exercise in warm weather
- You notice your horse is not sweating normally during or after exercise
- A horse from a warm climate begins showing signs of overheating during a British summer
- Confirm the diagnosis with a terbutaline sweat test before the summer season if your horse has had episodes of reduced sweating previously
Prognosis
The prognosis for affected horses in hot, humid climates that cannot be relocated is guarded for full athletic function — affected horses are fundamentally limited in their ability to exercise safely in warm conditions. For horses relocated to cooler climates, recovery of normal sweating function occurs in many cases over several months, and the long-term prognosis for return to normal function is good. UK-based horses affected by anhidrosis during warm summers often improve spontaneously as temperatures drop in autumn.
Frequently Asked Questions
Is anhidrosis dangerous for horses?
Yes, potentially life-threatening. A horse that cannot sweat cannot effectively regulate its body temperature during exercise or in hot weather. Without thermoregulation, core body temperature rises rapidly (hyperthermia), which can cause heat stroke, multi-organ failure, and death. Any horse showing signs of inability to cool down after exercise must be treated as an emergency — immediately move to shade, apply cold water to the body, and call your vet.
Can anhidrosis be cured permanently?
There is no guaranteed cure. Many horses do recover normal sweating function when moved to a cooler climate or after a cool winter season provides the sweat glands time to recover. Others remain permanently anhidrotic as long as they are in a hot environment. Management focuses on preventing dangerous overheating rather than curing the underlying gland dysfunction.
Does the One AC supplement really work?
One AC (brewers’ yeast-based supplement) has a loyal following among horse owners in hot climates, with many anecdotal reports of improvement. Formal clinical trials are limited and results are mixed. It is safe to trial and some horses do appear to benefit — typically showing improvement in sweating within 4–8 weeks. It is most useful as part of a broader management approach alongside environmental cooling and exercise modification rather than as a standalone cure.
My horse sweats only around the ears — is this anhidrosis?
Patchy sweating — sweating only in certain areas such as the ears, face, or under the mane while the rest of the body remains dry during exercise — is a classic presentation of oligohidrosis (partial anhidrosis). The areas where sweating persists often correspond to regions with particularly high sweat gland density or where skin is thinner. This pattern should prompt veterinary assessment and a terbutaline sweat test to confirm the diagnosis and assess the degree of dysfunction.