Anhidrosis

I like few things better than lounging on top of one of the vet trucks, supervising the work that goes on at my clinic. As a Florida cat in the summer, it’s very apparent to me that you humans and your horses sure do come into the clinic sweaty this time of year. This is just one of the many reasons I’m glad to be a cat. I keep my cool without having to leak water out of my skin and get stinky! We cats just “glisten” from our paws, and maybe pant a bit if we get hot, which is obviously superior. But my docs say it’s a pretty big problem if horses don’t sweat. They actually want them to get gross and sweaty to stay healthy! There’s a fancy name for when they don’t do it well enough – Anhidrosis.

Normal sweating

It’s really important for a horse to sweat so it can keep his internal organs at a normal temperature even when he’s working hard or its hot outside. Horses have a lot of sweat glands in their skin, and a lot of them are a different type than the ones you humans have. Have you noticed that when your horse sweats, it’s often white and frothy looking? That’s because they have something called apocrine sweat glands that make a sweat rich in proteins and lipids that causes them to look “lathered up”. Sweat also contains a lot of water and electrolytes like sodium, potassium, and chloride. As the water evaporates off your horse’s body, it cools him down, transferring the heat into the air.

What’s the deal with non-sweaters?

When your horse has a decreased ability to sweat like he should in response to an increased body temperature, it’s called anhidrosis. Because sweating is the horse’s primary way of cooling himself, if he doesn’t sweat appropriately it can cause him to get overheated, not perform well, or even develop dangerous heat stroke. Horses lose 65-70% of body heat through their sweat, so if they’re unable to do that, it’s a pretty big deal. Anhidrosis can have a significant effect on your horse’s ability to be ridden and perform or even just function as a pasture pet.

During hot weather or hard exercise, a horse with anhidrosis won’t sweat as much as he should, and might not sweat at all. They can have patchy sweat (like only under the saddle pad), a little sweat, or no sweat. He might appear to be working much harder than he should be or breathing heavily to try to cool off, even after he’s done working or just while standing around.

Anhidrosis is a problem mainly in hot and humid, tropical kind of places, like my dear home state of Florida and other states around it. This is especially a problem when the temperatures stay high for long periods of time, and don’t drop down much at night, like the perma-summer we seem to have here in Florida. Lucky us! It’s not as common in cooler, more temperate climates, but it can happen there too, especially in heat waves. Making matters worse, when the humidity is high, the sweat is even less able to evaporate into the already water-saturated air, so it decreases how effectively sweat can cool the horse.

It’s not known exactly why or how anhidrosis happens, but it’s thought that the sweat glands get over-stimulated and so they start to work less well. The onset can be sudden or more gradual. It can happen to horses that are born in a hot climate as well as horses that are new to it. So being from Florida doesn’t protect them, unfortunately. Between 2-6% of horses are estimated to have anhidrosis. No links to specific breeds, ages, colors, or sex have been identified – the risk is equal for all horses.

Chronic anhidrosis has been linked to atrophy (degeneration) of the sweat glands, leading to a permanent loss of the glands’ ability to produce sweat. Researchers are looking into associations with decreased excretion of chloride through the urinary system and in sweat glands, but this is still being investigated.

Recognizing and diagnosing anhidrosis

Because some horses with anhidrosis still sweat a bit, it can be tricky to recognize if your horse is affected. Some areas of the body can continue to sweat, but not enough to cool the horse sufficiently. For example, a horse with anhidrosis might continue to sweat under his mane or saddle pad.  You may notice first that your horse doesn’t perform well in hot weather. He may take longer than normal to recover after exercise or breathe heavily even when standing in the shade. He may have a higher than normal pulse or temperature.

Here are some things to watch for:

• Lack of sweat, or a small amount of sweat, in situations when other horses are sweating a lot
• His coat is dry and clean to the touch
• Fast or labored breathing
• Flared nostrils
• Low energy or fatigue
• Slow ability to cool down after exercise
• Increased body temperature, especially higher than 104 degrees (normal is 98.5-100.5 degrees)
• Seeking and remaining in the shade (especially if food is elsewhere)
• Breathing hard and looking distressed even when standing around not working
• Sometimes, eating or drinking less than normal
• In chronic cases – dry, itchy or flaky skin. Sometimes there is hair loss, especially on the forehead

My docs can often diagnose a horse based on their physical exam and the signs your horse is showing. If the diagnosis isn’t certain, there is a test they can do to check the amount of sweat your horse is able to produce. For this test, my doc injects a small amount of a medication called terbutaline into the horse’s skin to elicit sweating in that spot. That tests the amount of sweat your horse can produce compared to a normal horse.

Treatment

So, if your horse is a non-sweater, what to do? Well sure, if you move your horse to a cooler climate where hot, humid weather is less likely, they will certainly be more comfortable. Many horses will start sweating again when they are in a cooler environment, and that is the only sure-fire approach. But, it may not be a realistic option for many people and horses.

There have been several supplements and remedies that have been tried over the years, with variable success. These include electrolyte supplements, One AC supplement, and even dark beer such as Guinness. Most of these are fairly safe to try, though there is not a lot of evidence that they will work. At Springhill Equine, we often use a combination of acupuncture and Chinese herbal therapy and find that can be quite successful in some cases.

Here are some other ways to keep a non-sweater more comfortable:

• Keep him in a stall on shaded paddock during the day. A severely affected horse may not be able to tolerate turnout on hot days.
• Provide fans, misters, or a sprinkler. Some horses will cool off in a pond if they have access to one.
• Make sure he always has cool water to drink
• Keep work to a minimum during the hot times of the year and ride early in the morning before it gets too hot

It’s better to prevent your horse from getting overheated than to have to treat him if he does, but if you get in a bind here are some steps to follow:

• Move him somewhere shady
• Hose him down with a continuous flow of cool water or sponge him repeatedly with water from a bucket filled with water and ice
• Use portable fans
• Offer him cold water to drink
• Take his temperature, heart rate, and respiratory rate if you can
• Call my docs!

So, if your horse is all stinky and sweaty after a ride, and you have to bathe him and wash your saddle pads yet again, don’t grumble about it, be happy about that sweat!

Until next week,

Tony

P.S. The humans just released a podcast about sweaty (and not sweaty) horses. It’s got a lot of interesting insights, such as how to estimate how much water your horse is losing through sweat. You know, useful information. You can listen to it right from my Podcast Page, or subscribe to Straight from the Horse Doctor’s Mouth wherever you get your podcasts.