You finish your morning ride, hose your horse down, and realize something feels off — but you can’t quite name it. He seems dull. His skin is hot and dry even though he was working hard. There’s no sweat, or barely any, and what’s there is concentrated in a few odd patches — behind the ears, maybe under the girth. The rest of him is dry as a bone.
That’s not normal. That’s a horse whose thermoregulation is failing.
Anhidrosis — the partial or complete inability to sweat — is one of the most underdiagnosed summer health problems in the South, and Aiken is squarely in the high-risk zone. It doesn’t show up dramatically. It creeps in over weeks. By the time most riders realize what they’re dealing with, they’ve already been working a horse who can’t cool himself.
Why Anhidrosis Happens
The honest answer is that we don’t have a tidy explanation. What we know: prolonged exposure to high heat and humidity desensitizes the sweat glands’ response to epinephrine — the hormone that triggers sweating. The glands become down-regulated, and over time, they stop responding normally.
Horses that are new to the South are at higher risk, especially if they’re coming from northern climates and are doing hard work during their first or second Southern summer. But horses that have lived here for years are not immune. I’ve seen horses develop anhidrosis after five or six Aiken summers with no prior history. Stress, illness, hormonal changes, and heavy workload in peak heat all appear to be contributing factors.
Certain breeds seem more prone — Thoroughbreds and horses with a lot of Thoroughbred blood come up frequently — but any horse can be affected.
What It Actually Looks Like
This is where riders get into trouble: the signs are easy to rationalize away, especially in the early stages.
Partial anhidrosis — which is far more common than total non-sweating — looks like this:
- Sweating is patchy. You’ll see moisture behind the ears, at the flanks, sometimes between the hind legs, while the neck, back, and barrel remain dry during and after work.
- Sweat is thicker than normal — almost foamy and concentrated where it does appear.
- The horse recovers slowly after work. Heart rate and respiratory rate stay elevated longer than they should.
- Rectal temperature after moderate work is higher than expected. Above 103°F after a moderate ride in the morning is a flag.
- He seems flat. Not lame, not acutely sick — just dim. Less interested in his surroundings, less engaged under saddle, less eager at feeding time.
- He may start breathing harder during work than his fitness level should require — sometimes with a distinctive “flank heaving” pattern.
Total anhidrosis is more obvious but also more dangerous, because horses with zero sweating capacity can go into heat stroke quickly. A completely dry horse with a rectal temperature above 104°F after any physical exertion is an emergency.
What It Does Not Look Like
Anhidrosis does not necessarily look like a horse in visible distress. That’s the trap.
A partially anhidrotic horse will often still work reasonably well in the morning, especially in the first twenty minutes before core temperature climbs. He may not show obvious resistance or pain. He’ll just be a little less than himself — and in the summer, when everyone and everything is a little less than themselves, it’s easy to miss.
This is why I push hard on monitoring temperature, not just watching behavior. Temperature tells you what behavior often hides.
How to Check
Get a digital rectal thermometer and use it. That’s the whole instruction. Normal resting temperature is 99–101.5°F. After a moderate workout on a hot morning, a horse with functional thermoregulation should be returning to near-normal within 30–45 minutes. A horse sitting at 103°F or above an hour after a moderate ride needs your attention.
You can also do a simple skin pinch test for hydration and check capillary refill time as part of your regular assessment — both are affected as heat stress accumulates. But temperature is your most direct data point.
Get in the habit of taking temperature at least a few times a week in July and August, not just when you suspect a problem.
What to Do If You Think Your Horse Isn’t Sweating
First, reduce workload immediately. Do not try to work through this. A horse who can’t thermoregulate is not a horse who needs a longer conditioning ride to toughen up.
Second, call your vet. Anhidrosis needs a clinical diagnosis — there are other causes of reduced sweating and exercise intolerance, and you want to rule them out.
Third, look at environmental management. Can he be stalled during the hottest hours with fans and good airflow? Can you shift work to 6 or 7 a.m.? Can he get turnout only in the evening? These changes are not optional add-ons — they’re essential to giving an anhidrotic horse a functional life.
There are a few supportive options that have some anecdotal backing and are commonly used in the South:
- One AC (a beer yeast supplement) has the most widespread use and some positive anecdotal reports, though clinical evidence is mixed. Many horses in Aiken are on it through summer as a precautionary measure.
- Electrolytes — done right — matter here. If you haven’t read the post on what horses actually need for summer hydration, that’s worth your time before you start throwing products at the problem.
- Some horses do well being relocated to a cooler climate for a period, which can allow the sweat glands to recover.
Recovery is possible but not guaranteed, and some horses do not fully recover their normal sweating response within a single season.
Prevention Is the Better Game
If you’re in the South and your horse is doing any meaningful work through July and August, you should be thinking about anhidrosis before you see signs of it.
That means scheduling work early — and I mean early, not 8 a.m. early, but 5:45 a.m. in the ring before full sun early. It means keeping work volume appropriate to the conditions, not just your competition schedule. It means cooling out thoroughly every single ride: scraping, cold water, fans, shade. It means not asking a horse to stand tied in the sun after work because you’re running behind.
I’ve talked about this more broadly in the context of building a summer competition calendar that doesn’t destroy your horse by August — the principle there applies directly here. Accumulated heat stress across weeks is what tips horses into anhidrosis, and small decisions made every day either add to or reduce that load.
If you want to go deeper on the nervous system and thermoregulation side of summer horse management, we covered some of the physiology behind it on The Elevated Equestrian podcast — it’s worth listening to before the real heat sets in.
Monitoring Gear Worth Having
A reliable digital rectal thermometer is non-negotiable. So is a good cooling kit: a sweat scraper, large sponge, and access to cold water. If you’re working horses through peak summer, a portable fan for your cool-out area is a serious quality-of-life improvement — for the horse and for you.
I’ve reviewed a few barn essentials and cooling recovery products over at /blog if you want my take on specific options. Keep it simple and keep it functional.
The Bigger Point
Anhidrosis is not rare. In the South, in the summer, in working horses — it is a genuine occupational hazard. The riders who catch it early are the ones who are paying attention to baseline: what does this horse normally look like after a Tuesday morning hack? How does his recovery normally feel? What’s his normal summer energy level?
When something shifts from that baseline — even subtly — trust it. The dry skin, the flat affect, the temperature that lingers a few degrees high. These are not nothing.
Your horse cannot tell you he’s struggling to stay cool. But he’s showing you, if you know what to look for.
