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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.

Lydia F. Gray, DVM, MA SmartPak Staff Veterinarian and Medical Director Dr. Lydia Gray has earned a Bachelor of Science in agriculture, a Doctor of Veterinary Medicine (DVM), and a Master of Arts focusing on interpersonal and organizational communication. After “retiring” from private practice, she put her experience and education to work as the American Association of Equine Practitioner’s first-ever Director of Owner Education. Dr. Gray continues to provide health and nutrition information to horse owners through her position at SmartPak, through publication in more than a dozen general and trade publications, and through presentations around the country. She is the very proud owner of a Trakehner named Newman that she actively competes with in dressage and combined driving. In addition to memberships in the USDF and USEF, Dr. Gray is also a member of the Illinois Dressage and Combined Training Association (IDCTA). She is a USDF “L” Program Graduate and is currently working on her Bronze Medal. Find Dr. Gray on Google+

Posted in Diseases and Conditions, Seasonal Horse Care

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38 comments on “Horses That Can’t Sweat (Anhidrosis)
  1. Jim says:

    I am leasing a horse and I live in Houston, TX. Hot, Humid miserable weather in the summer, especially this year. My vet came to do a wellness check on this horse, a 14 year old gelding. When told that he had anhidrosis, she recommended not purchasing. Said I was in for lots of extra care, very limited output from the horse in this climate and since I didn’t have a barn only shade trees I didn’t have a good way to cool him.. I’m curious what your thoughts are.

  2. kelsi says:

    hello i have a horse named apollo and i live in florida and as we know is hot and muggy but he does not sweat at all but when im outside with him he acts just like a normal horse and when i ride he sweats alot…. any advice ???????

  3. Erin says:

    I would also like Susan’s contact information. I have a 2 year old filly that is starting to show signs – she has not completely stopped sweating yet. I gave her a tube of electrolytes and put a mineral block in her feed bin yesterday, and I noticed she sweat more and cooled down easier after I rode her this morning. Any info is much appreciated….

    • Alyson Scotti says:

      Try VITAMIN E. It worked for my horse after trying everything from AC One to Guinness beer. I am located in South Florida.

  4. Eileen says:

    I have an 11 year old paint mare that just stopped sweating te last couple of days. I would like to get in touch with Susan. We also have another horse in our barn that hasn’t been sweating for about 6 weeks. He has been on beer and 1AC and they aren’t helping.


    • Alyson Scotti says:

      Try VITAMIN E. It worked for my horse after trying everything from AC One to Guinness beer. I am located in South Florida.

  5. Susie says:

    Hello Susan, I, like the others, have a 10 yr old gelding who is having great difficulty with this ungodly Texas heat this year. What dietary changes should I be making with my guy. He is a pleasure horse though I have quit riding him until I get him under control. Is it a low protein diet? No alfalfa hay, low protein grain ration? Any info you can give me will be greatly appreciated.

  6. Kimberly says:

    I own a 5 year old QH Gelding who colicked and went toxic on me right before Memorial Day this year. He spent a week in the equine hospital and the vet said he should not have lived through it. The vet has no idea what caused him to colic, but what I noticed is he wasn’t sweating and I live in SC. I put him on One AC as well as Quench and Probios and he now sweats like he should and I haven’t had any problems with him colicking again. We have had heat indexes of 110 to 115 this summer. I know the One AC is working because he is sweating normally now.

  7. Linda Lilly says:

    I have been asked to assume responsibility for a 20 year old solid black miniature mare, who has been diagnosed with anhydrosis and can not sweat when the tempertaure gets above 90 degrees, I am disabled and on a fixed income and am wondering how to best accomplish this. I have several acres and a pond, but no barn or stalls. Does it take a lot of money to care for an animal like this? What is the easiest way to help her? I have no expierence with horses.

  8. kathleen williams says:

    Dear Susan,

    I just got some One AC today, but would also like to try to find food that has the right Chelates of iron, copper and whatever else my horse needs. Can I just get that food and try it, or do I need to do the hair testing? How much does the testing cost? Where can I get the food? Thanks.

  9. chris says:

    Horse not sweating, what can i give him

    • SmartPak SmartPak says:

      Hi Chris – Our best advice is to work with your veterinarian and read this blog from our staff veterinarian, Dr. Lydia Gray. This blog offers both management suggestions as well as supplements that may lend support for horses with Anhidrosis. Please let us know if you have any questions!

    • Alyson Scotti says:

      Try VITAMIN E.
      It worked for my horse after trying everything from One AC to Guinness beer. I am located in South Florida.

  10. SmartPak SmartPak says:

    If you are looking to reach Susan Cook, please contact her directly at

  11. Sonny says:

    If you have a horse that will not sweat, forget the One AC and all the other wife’s tale treatments they probably will do no good. Just provide a pasture with shade trees and wait for cold weather to work them. You’ll be doing them as well as your self a favor!!

    • cathy says:

      I agree I have tried everything, nothing has worked.

    • Lj says:

      I’ve tried 1-AC on my Clydesdale/Frisian mare. It has not helped at all. Live in Central East Coast of FL. Going to try the change in her diet to less fiber very carefully! I just hope when it cools off in mid-October she will start sweating again. Good luck everyone, maybe one day there will be an answer and treatment for our horses!

      • Alyson Scotti says:

        Try VITAMIN E.
        It worked for my horse after trying everything from One AC to Guinness beer. I am located in South Florida.

  12. Tara says:

    I got on here to search for info that I have learned from trial and error. I cannot find anyone mentioning this, but it works for the five anhydrosis horses that live on my property. Also every person I have suggested it to…it helped their horses too. I know this information was available in the 90s, but noone seems to remember! High fiber diets can contribute to anhydrosis in the horse. You find info on how fiber can keep a horse warm in the winter, but no one mentions that it also keeps them warm in the summer. Too warm! I have horses that I feed free choice hay to year round. But these particular anhidrosis horses, I have to pull them off the round bales or the dense grassy pasture during the summer. These horses have plenty of shade from the trees, but the heat produced from digesting all that fiber pushes their system over the edge. I restrict them to 1-2 flakes of hay twice a day. My feed already is 14 percent protein, low in carbs and has decent fiber content. Some of these horses are related and tend to have Tb in their bloodlines. I have managed a few barns where they owners wanted their horses to have hay three times a day, plus beet pulp, and their feed was already a nice low carb, high fiber, high quality feed. This is very popular in dressage barns. The horses could barely stand the heat with a misting fan, and night turnout. They were heaving with every breath. I finally convince them to cut the fiber back to what the rest of the boarders were fed. And the horses started getting more comfortable within a week! I am also a big fan of One AC, but after putting them on this diet, I havent needed it. I think the combination of diet and One AC and electrolytes could really help those of you that are desperate to help your horse. It has allowed me to use these horses year round. Please try this!

    • Jan says:

      Thanks Tara,

      I have often thought of this, fiber thing keeping horses warm in the winter. My mare is 24 years young and has just started showing symptoms of not sweating and heavy breathing, overheating. She is going blind and I only turn her out on pasture when I am around the barn working,which is around 1 to 4 hours a day. I also give her alfafa cubes soaked in water and regular grass hay when she is in her stall. Since the weather is getting hotter I will limit her pasture time and cut out the alfalfa and see how this goes. I Would like to know what the AC supplement consist of.

    • Tina says:

      Very interested in trying your diet suggestions…what kind of hay are you feeding, and what part of the country are you located ? I live in central Fl and my mare has been on One AC for 2 months, with little to no change…
      Thanks, Tina

  13. Pam Hameka says:

    Alyson, how much vitamin E did you give your horse?

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