A: As temperatures rise, nonsweating, or anhidrosis, becomes a concern for some horses. Heat loss through sweat evaporation is the main method by which horses cool themselves; therefore, a horse that doesn’t sweat adequately is at risk for overheating and heat stress.
Horses can suffer from this condition to varying degrees, meaning that some horses only experience some decrease in sweating—or sweating over some areas of the body but not others—while other horses completely cease sweating. This puts the horse at risk of severe hyperthermia (overheating).
In many cases, an owner will notice that their horse isn’t sweating as heavily as expected for work being done or that their horse sweats less than others in the barn doing similar amounts of work. In most cases, the condition is performance-limiting, because horses that can’t adequately cool themselves develop elevated heart rates and respiratory rates that ultimately reduce their athletic potential. Although its most commonly diagnosed in performance horses, anhidrosis does occur in nonperformance horses, too, and interestingly appears to occur more frequently in dark colored horses. Studies estimate that about 20% of racehorses in training in southern Florida suffer from anhidrosis.
So, what should you do if you think your horse has developed anhidrosis?
First, it’s important to get a diagnosis of nonsweating from your veterinarian. Other conditions can lead to increased respiratory and heart rates during exercise, so a full physical exam is warranted. Your veterinarian can also run a diagnostic test for anhidrosis by injecting dilutions of terbutaline or epinephrine that should stimulate the sweat glands to release sweat. The amount of sweat produced is measured, and affected horses don’t sweat as expected.
The condition’s cause isn’t fully understood, but veterinarians generally consider it linked to sweat gland overstimulation. Epinephrine (also known as adrenaline) is a hormone and neurotransmitter that stimulates the beta-2 receptors in sweat glands to release sweat. It’s thought that if these receptors become overstimulated by epinephrine, they’ll start to shut down and reduce sweat production.
Because the cause isn’t fully understood, pinpointing a treatment is hard. Your veterinarian will likely take a blood sample to test for electrolyte levels, because balanced electrolytes are vital and one of the simplest treatments is electrolyte supplementation based on identified abnormalities.
Horse owners and managers use many anecdotal treatments, such as feeding dark beer and administering supplements containing vitamin C or L-tyrosine. However, when tested in controlled research settings none of these approaches have proven effective. Similarly, while one study suggested a possible connection to thyroid function due to possible iodine deficiency, the researchers cautioned that—if supplementing iodine or thyroid hormone appears to result in a renewed ability to sweat—this shouldn’t be taken as proof that hypothyroidism was the cause. In consultation with your veterinarian these anecdotal treatments might be worth trying.
Your horse will need careful management until a successful treatment plan is identified. If working the horse, do so early in the morning or late in the evening. Make sure your horse has a stall or shade available. Misters and careful application of barn-safe fans (some fans not designed for use in agricultural or barn settings can pose a fire risk; also take care to keep cords away from horses and to not overload circuits) can benefit horses with anhidrosis.
It’s not uncommon for anhidrosis to spontaneously resolve. This makes identifying a successful treatment even more complicated, because owners often believe it was the chosen treatment that worked when in reality the condition might have spontaneously resolved even without treatment. For other horses the only successful treatment is to move them to a cooler and often less humid location.
With careful management, you can keep your horse comfortable, but you should be prepared to make changes in your performance expectations and have a plan in place for what you’ll do should your horse become overheated.