Horses That Can’t Sweat (Anhidrosis)

What can you tell me about anhidrosis? I understand it has something to do with a horse not sweating properly, but I don’t understand why horses develop this problem. Also, I heard there’s a treatment for it but you have to start it BEFORE the horse actually stops sweating, which is different than most other treatments.

Anhidrosis is a frustrating condition because scientists still haven’t figured out exactly why horses gradually lose the ability to sweat. Based on numerous studies, experts think 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 receptors). Most recently, a team of researchers found evidence that sweat glands of horses with anhidrosis secrete chloride ions differently than other horses. Chloride is a component of normal sweat along with sodium, potassium, calcium, magnesium and other minerals (electrolytes).

The manufacturer of the most popular supplement for anhidrosis, One AC, offers another explanation (from the website):

“One AC is based on the theory of imbalance of dopamine to the noradrenaline/adrenaline complex. Dopamine is first used by the brain, then by the cardiovascular system and last by the sweating system. A reduction in dopamine below a certain level allows the well-known vasoconstrictive properties of the noradrenaline/adrenaline complex to predominate and reduce the blood carrying capacity of the peripheral vascular system to a minimum, thereby reducing the ability of the sweat glands to function properly.”

Because there are no prescription medications to prevent or treat anhidrosis, One AC is probably the supplement you are referring to, the one that should be started before the horse has a problem. According to the manufacturer, horses that have been diagnosed with anhidrosis should begin receiving One AC before the beginning of the hot season. In fact, some owners keep their nonsweating horses on One AC year-round and simply lower the dose during the cooler months. The manufacturer also suggests reducing strenuous training or work for a minimum of three weeks after first putting the horse on One AC for best results.

Because sweating is a horse’s primary means of cooling itself, horses that don’t sweat can become overheated and perform poorly or even collapse. Here are some additional tips to help keep horses cool:

• If you’re bringing a horse used to a cool, dry climate into a hot, humid one, 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 before the hot months. This way you won’t have to do distance riding or interval work in addition to regular schooling when it’s warmest.
• Work your horse during the coolest parts of the day--usually morning or evening
• Observe your horse closely during exercise for signs of overheating, such as rapid breathing or panting, rapid heart rate and fatigue.
• Cool your horse off with as water as cold as he will tolerate! Upper level event horses are routinely sponged off with ice water until the water scraped off is the same temperature as the water going on.
• Provide cool air with good barn ventilation, fans (especially misting fans), and even air conditioning.
• Stimulate drinking and replenish the minerals lost in sweat with an electrolyte scientifically formulated specifically for horses.
• Look for and treat any underlying diseases, especially respiratory conditions, and reduce other sources of stress.

Horses that live in hot, humid climates are the ones that tend to develop anhidrosis, and for years it was thought that it was more common in horses that moved to these warmer regions from cool, dry climates. However, studies have shown that just as many horses that were born and raised where it’s hot and muggy lose the ability to sweat as those who move there. And no breed, sex, or gender is safe, as studies have shown any adult horse is just as likely as another to develop anhidrosis. The bottom line: all horses should be closely supervised and managed appropriately when heat and humidity are high. Horses with anhidrosis just need a bit more TLC.