Chelsea Luedke DVM, MS
For many horse owners, the word anhidrosis immediately brings to mind horses living in Florida, Texas, or other persistently hot and humid climates. Historically, that's where we've seen - and studied - the condition most frequently.
But after increasingly long, hot summers across much of the country, I think anhidrosis deserves attention well outside of the Southeast.
As an equine veterinarian, I've seen horses that previously seemed to sweat normally begin struggling during extended periods of summer heat. Sometimes they still sweat under their mane, between their hind legs, or underneath tack but noticeably less over the rest of their body. Others become almost completely dry despite exercising in conditions where every other horse in the barn is dripping with sweat.
Owners will often describe it as though their horse's sweat glands have simply become "exhausted."
That's actually not a terrible way to visualize what we're seeing clinically - but scientifically, it's more complicated than that.
What Is Anhidrosis?
Anhidrosis is the partial or complete inability of a horse to sweat appropriately in response to heat or exercise.
Sweating is incredibly important to horses because evaporation of sweat is their primary method of dissipating excess body heat. When that cooling mechanism fails, horses compensate by breathing faster in an attempt to lose heat through the respiratory tract.
This creates one of the classic presentations of an anhidrotic horse: a horse that is unusually dry but breathing very rapidly after exercise, or even while simply standing in hot weather.
Other signs can include:
- Decreased or patchy sweating
- Rapid respiratory rate or "panting"
- Elevated body temperature
- Exercise intolerance
- Lethargy during hot weather
- Decreased appetite or water intake
- A dry or dull hair coat
- Hair loss in some chronically affected horses
In severe cases, anhidrosis can result in dangerous hyperthermia, collapse, neurologic signs, and potentially death.
Why Does a Horse Suddenly Stop Sweating?
Unfortunately, this is where our answers become much less satisfying.
Despite decades of research, we still don't completely understand why some horses develop anhidrosis and others don't.
Hot and humid environments clearly play a major role. Horses living in these conditions are continuously stimulated to sweat, sometimes essentially around the clock for weeks or months.
We often casually describe this as the sweat glands becoming "exhausted." While that terminology isn't technically proven, there is evidence that chronically affected horses develop both functional and structural abnormalities within their sweat glands.
One proposed mechanism involves dysfunction or desensitization of the signaling pathways responsible for stimulating sweat production. Over time, chronically affected sweat glands can also develop visible microscopic abnormalities, including thinning of glandular cells, thickening of the basement membrane, and fibrosis around the glands.
Exactly what starts that process - and why one horse develops it while the horse in the next stall does not - is still unknown.
What we can say is that anhidrosis isn't exclusively a Florida problem.
Acute episodes can occur in horses living in temperate climates during prolonged summer heat waves or following travel into a substantially hotter environment. This may become increasingly relevant as areas that historically experienced short periods of extreme heat instead experience weeks or months of sustained high temperatures.
Diagnosing Anhidrosis Isn't Always Straightforward
Sometimes the diagnosis seems obvious.
If I have a horse exercising beside several other horses and everyone else is dripping wet while one horse is essentially dry, anhidrosis moves very high on my list.
Partial anhidrosis can be much harder to recognize.
Owners may initially notice that their horse isn't sweating as much as usual or only sweats underneath the saddle, between the hind legs, or around the neck. Exercise intolerance and an unusually high respiratory rate may actually be noticed before the lack of sweating.
We also have to remember that anhidrosis isn't the only reason a horse can have difficulty regulating body temperature.
A veterinary examination may include bloodwork to look for dehydration, electrolyte abnormalities, systemic illness, or other conditions that could contribute to exercise intolerance or abnormal thermoregulation.
When confirmation is needed, the most objective diagnostic tool available is a quantitative intradermal terbutaline sweat test (QITST). Small concentrations of terbutaline are injected into the skin to stimulate local sweat production, allowing us to evaluate the responsiveness of the sweat glands.
The test can be extremely helpful, but it isn't something most ambulatory veterinarians perform routinely, and interpretation can be more nuanced in horses with partial disease.
That means anhidrosis often remains a combination of history, clinical observation, exclusion of other problems, and - in selected cases - formal sweat testing.
Can We Treat Anhidrosis?
This is probably the most frustrating part of the disease.
We have a lot of treatments people try. We have far fewer treatments that have consistently worked in controlled scientific studies.
There is currently no FDA-approved medication that reliably restores sweating in an anhidrotic horse.
You'll hear about everything from electrolyte formulations and commercial "non-sweater" supplements to vitamins, minerals, thyroid supplementation, acupuncture, herbal medicine—and, famously, Guinness beer.
And yes, I've had owners who swear that a Guinness a day helped their horse start sweating again!
The problem is separating a true treatment response from the natural fluctuation of the disease, changes in temperature and humidity, altered workload, and placebo effect.
What About Supplements?
Various nutraceutical products marketed toward non-sweaters contain combinations of electrolytes, amino acids, vitamins, minerals, herbs, or other ingredients.
Anecdotally, I've seen horses appear to improve on some of these products, and I don't necessarily discourage owners from trying a reputable supplement if the ingredients and amounts are safe.
However, it's important to set realistic expectations: we currently don't have strong controlled evidence demonstrating that a particular nutraceutical reliably restores sweat-gland function.
Electrolyte supplementation deserves a slightly different discussion. Horses sweating heavily can lose substantial amounts of sodium, chloride, potassium, and other electrolytes, so maintaining appropriate electrolyte intake and hydration is important during hot weather.
But correcting an electrolyte deficiency and treating true chronic anhidrosis are not necessarily the same thing. Simply giving more electrolytes doesn't reliably turn the sweat glands back on.
And What About Guinness?
Dark beer is probably the most famous home remedy for anhidrosis.
There are various theories about why it might work - from B vitamins and yeast products to other components of the brewing process; but there is no good scientific evidence demonstrating that Guinness or another dark beer restores sweating in horses.
That doesn't mean owners haven't reported improvement. They absolutely have.
It simply means that, from a veterinary standpoint, I would categorize Guinness as an anecdotal therapy rather than an evidence-based treatment.
If a horse begins sweating after starting Guinness, I'm happy to see the improvement - but I'm also going to pay attention to whether the weather changed, workload decreased, or another management change occurred at the same time.
Acupuncture and Herbal Therapy: Some Interesting Evidence
One treatment approach actually has undergone a randomized, placebo-controlled study.
Researchers evaluated 44 anhidrotic horses treated with a combination of acupuncture and Chinese herbal medicine versus sham acupuncture and placebo.
The results weren't dramatic, but they were interesting.
Approximately 26% of treated horses demonstrated a treatment response compared with 4% of control horses at selected testing concentrations. Unfortunately, the improvement was relatively modest and wasn't sustained four weeks after treatment ended.
So I wouldn't describe acupuncture and herbal therapy as a cure—but there may be a subset of horses that responds, and it remains an area worth studying further.
Management Is Still Our Most Reliable "Treatment"
Until we understand the underlying disease better, management remains incredibly important.
For mildly affected horses, that might simply mean changing the exercise schedule.
Ride early in the morning or later in the evening. Shorten workouts. Incorporate more walking breaks. Monitor respiratory rate and temperature closely. Provide shade, excellent airflow, fans, cool drinking water, and aggressive cooling after exercise.
For a significantly affected horse, continuing the same workload in hot weather simply isn't safe.
Remember: an anhidrotic horse has lost its primary cooling mechanism.
Cold-water hosing is extremely effective for cooling a hot horse. Current recommendations support repeatedly applying cold water over as much of the body as possible while maintaining airflow. You do not need to spend time scraping every application of water off before applying more.
Sometimes Geography Really Is the Treatment
For horses with severe or persistent anhidrosis, the most consistently successful intervention remains moving them to a cooler climate.
And I've seen this work.
Some horses that struggle tremendously through hot, humid summers begin sweating normally again after moving to a cooler environment. For horses that only become anhidrotic during prolonged periods of summer heat, even a temporary geographic change may provide enough environmental relief to improve their ability to thermoregulate.
Unfortunately, not every horse recovers completely.
Chronic anhidrosis can become persistent, presumably because long-standing disease produces more permanent structural changes within the sweat glands.
This is why I take early changes in sweating seriously rather than waiting until a horse completely stops sweating.
What Does the New Research Tell Us?
There have been some interesting developments.
Genetics has been one major area of investigation. A previous genome-wide association study suggested that a variant involving the KCNE4 potassium-channel gene might be associated with equine anhidrosis, and genetic testing based on that finding became commercially available.
More recent research has challenged that association.
Studies using horses whose sweating ability was objectively evaluated with intradermal terbutaline testing found that horses carrying the proposed risk variant could sweat normally. Additional work published in 2026 again failed to find an association between the proposed KCNE4 variant and anhidrosis.
So at this point, I would not use a KCNE4 genetic test to diagnose anhidrosis or predict whether an individual horse will develop it.
The bigger unanswered question remains why chronic environmental stimulation eventually results in sweat-gland dysfunction in certain horses.
A new 2026 review in Veterinary Clinics of North America: Equine Practice again highlights how incomplete our understanding remains. Research continues to focus on the cellular mechanisms controlling equine sweat secretion and the pathways that may become dysfunctional in anhidrotic horses.
Understanding those pathways is probably what will eventually give us a truly effective treatment.
The Take-Home Message
Anhidrosis isn't just a Florida or Texas problem anymore.
If your normally sweaty horse suddenly isn't sweating appropriately during a prolonged stretch of hot weather, pay attention—especially if they're breathing harder than expected, recovering slowly after exercise, or becoming increasingly exercise intolerant.
Early management can make a tremendous difference.
Reduce the workload, exercise during cooler portions of the day, provide aggressive external cooling, maintain hydration and appropriate electrolyte intake, and involve your veterinarian if the change persists.
Supplements, acupuncture, herbal treatments—and even that daily Guinness—may help individual horses, but none should give us false confidence that a horse can safely continue exercising when they're unable to regulate their body temperature.
Ultimately, the horse's ability to stay cool matters more than whether we can make them sweat.
And sometimes the best prescription really is a cooler summer.
References
Palmer AT. Disorders of Water and Tissue Perfusion (Diabetes Insipidus, Renin-Angiotensin-Aldosterone Axis, and Anhidrosis). Veterinary Clinics of North America: Equine Practice. 2026. doi:10.1016/j.cveq.2026.07.011.
MacKay RJ. A review of anhidrosis in horses. Equine Veterinary Education. 2015. doi:10.1111/eve.12220.
Mallicote MF, et al. Efficacy of treatment of equine anhidrosis with acupuncture and Chinese herbs is low but higher in treated horses compared with placebo. Journal of the American Veterinary Medical Association. 2024.
van der Graaf K, et al. A missense mutation in the KCNE4 gene is not predictive of equine anhidrosis. 2025.
Additional Evidence Fails to Associate Variation in KCNE4 With Equine Anhidrosis. 2026.
University of Florida IFAS Extension. Anhidrosis in the Horse (Non-Sweaters)—What Do We Know?
University of Florida Large Animal Hospital. Anhidrosis: Help—My Horse Doesn't Sweat! Updated 2025.
