Why Is My Horse Not Sweating? Anhidrosis in Horses

By Dr Duncan Houston

A horse that finishes work in hot weather with a dry coat should make you pay attention.

Horses rely heavily on sweating to cool themselves. When a horse cannot sweat properly, body temperature can rise quickly, especially during exercise, transport, humid weather, or poor ventilation. This condition is called anhidrosis, and it can become dangerous if it is missed or brushed off as “just poor fitness.”

The tricky part is that anhidrosis is not always obvious. Some horses stop sweating completely. Others sweat only in patches, breathe harder than expected, tire early, or simply do not recover normally after work.

This article explains what anhidrosis is, how worried you should be, what to do immediately, and when a horse needs urgent veterinary help.

Quick Answer

Anhidrosis means a horse has a reduced or absent ability to sweat in response to heat or exercise. It is most common in hot, humid climates and is a serious concern because horses lose a large proportion of body heat through sweat evaporation. If your horse is hot, breathing hard, dull, weak, uncoordinated, or not cooling down after exercise, stop work, start active cooling, and call your vet. (Large Animal Hospital)

What Is Anhidrosis in Horses?

Anhidrosis is the partial or complete failure of sweating.

In a normal horse, heat from exercise and the environment is managed largely through sweating. As sweat evaporates from the skin, it removes heat from the body. In humid conditions, that evaporation becomes less efficient, which is why anhidrosis is especially risky in hot, damp climates. (Large Animal Hospital)

A horse with anhidrosis may:

• Sweat less than expected

• Sweat only in patches

• Stay dry after work in hot weather

• Breathe rapidly to compensate

• Take too long to cool down

• Become dull, weak, or exercise intolerant

The important point is this: a dry horse in hot weather is not always a comfortable horse. Sometimes the dry coat is the warning sign.

Why Does Sweating Matter So Much for Horses?

Horses are large athletes. They generate a lot of heat during work, and they need an efficient way to get rid of it.

Sweating is one of their most important cooling systems. When that system fails, the horse has to rely more on breathing, skin blood flow, shade-seeking, drinking, and reduced activity. Those backup systems are not always enough, especially during exercise or high humidity.

This is why anhidrosis can lead to:

• Poor performance

• Heat stress

• Delayed recovery after work

• Dehydration risk

• Collapse in severe cases

• Life-threatening heat illness

In practice, what worries vets most is not just the lack of sweat. It is the combination of poor sweating, rising body temperature, rapid breathing, and failure to recover.

What Causes Anhidrosis?

The exact cause of equine anhidrosis is still not fully understood.

Current thinking involves abnormal sweat gland function and altered gland responsiveness after repeated heat and stress stimulation. In chronic cases, sweat glands may eventually become atrophied. Research has also explored chloride movement across sweat gland cells and possible genetic influence, but there is no single proven cause that explains every case. (Large Animal Hospital)

Possible contributing factors include:

• Long-term exposure to hot, humid conditions

• Heavy work in heat

• Individual genetic susceptibility

• Sweat gland receptor dysfunction

• Chronic overstimulation of sweat glands

• Possible electrolyte and chloride handling changes

• Certain medications in specific situations

One important edge case is foals treated with macrolide antibiotics, such as erythromycin, azithromycin, or clarithromycin. These drugs can cause drug-associated anhidrosis and severe hyperthermia in foals, although this is a different situation from the typical adult “non-sweater.” (Merck Veterinary Manual)

Which Horses Are Most at Risk?

Anhidrosis is most often seen in horses living in hot, humid climates. It can affect performance horses, pleasure horses, locally bred horses, and imported horses.

It is a myth that only horses moved from cooler climates are at risk. Horses born and raised in hot climates can also develop anhidrosis. University of Florida notes that studies have not identified a clear age, sex, or colour predisposition, and both locally bred and imported horses can be affected. (Large Animal Hospital)

Risk may be higher when a horse is:

• Worked in hot, humid weather

• Stabled in poorly ventilated conditions

• Transported in heat

• Overweight or poorly conditioned

• Recovering poorly after exercise

• Exposed to repeated heat stress

• Already showing reduced sweating

The practical takeaway is simple: do not assume your horse is safe because they are young, fit, local, or used to the climate.

How Common Is Anhidrosis?

Anhidrosis is common enough in hot, humid regions that horse owners should know the signs, but prevalence varies by location, climate, and study design.

University of Florida reports epidemiologic estimates of about 2 to 6 percent of horses, with one Florida farm study finding 1.8 percent of horses affected and 11.2 percent of farms reporting at least one case. (Large Animal Hospital)

So rather than treating anhidrosis as rare or inevitable, it is better to think of it as a serious regional risk that needs early recognition.

Signs Your Horse May Have Anhidrosis

The classic sign is a horse that should be sweating but is not.

This may happen after work, during hot turnout, after transport, or while other horses nearby are visibly sweating.

Common signs include:

• Little or no sweat during hot weather

• Patchy sweating, such as sweating only under the saddle or between the hind legs

• Rapid breathing after mild work

• Longer recovery time after exercise

• Increased rectal temperature

• Poor performance

• Early fatigue

• Heat intolerance

• Dry, flaky skin, especially on the face or forehead

• Dullness or reduced appetite in chronic cases

Some horses still sweat in small areas, which can make the condition easy to miss. A horse does not need to be completely dry to have a sweating problem. (Large Animal Hospital)

How Worried Should You Be?

A horse that sweats less than usual deserves attention. A horse that cannot cool down deserves urgent action.

The most important decision point is recovery. If your horse does not cool down promptly after stopping exercise and starting cooling, do not wait.

Is It Anhidrosis or Something Else?

Not every tired or hot horse has anhidrosis. Your vet will consider other possible causes, especially if the signs are sudden, severe, or unusual.

Possible differentials include:

• Poor fitness

• Overwork in hot weather

• Dehydration

• Electrolyte imbalance

• Pain or lameness

• Respiratory disease

• Cardiac disease

• Fever or infection

• Exertional rhabdomyolysis, also called tying-up

• Heat stress without true anhidrosis

• Medication-related heat intolerance

• Poor ventilation or transport stress

This is where clinical context matters. A horse that is sweating normally but overheated after hard work is different from a horse that stays dry while every other horse on the property is wet with sweat.

How Do Vets Diagnose Anhidrosis?

A vet may suspect anhidrosis based on the history, examination, climate, exercise response, temperature pattern, and sweating pattern.

Useful information includes:

• When the horse stopped sweating normally

• Whether sweating is absent or patchy

• Rectal temperature before and after work

• Breathing rate after exercise

• Recovery time compared with previous normal

• Whether other horses on the property are sweating normally

• Recent medications

• Workload, transport, stabling, and turnout conditions

If the diagnosis is unclear, your vet may perform an intradermal sweat test using agents such as epinephrine or terbutaline. These tests assess whether the sweat glands respond locally. (Large Animal Hospital)

Do not rely only on whether the coat looks dry. The better question is: is this horse cooling normally for the conditions and workload?

When Is This an Emergency?

Anhidrosis can become an emergency when a horse is overheating or showing signs of heat illness.

Call your vet urgently if your horse has:

• No sweat in hot conditions after exercise

• Persistent high rectal temperature

• Rapid breathing that does not settle with rest

• Weakness or reluctance to move

• Collapse

• Staggering or poor coordination

• Disorientation or abnormal behaviour

• Seizures

• Dark or congested gums

• Signs of dehydration

• Severe depression

• Refusal to drink

• Worsening signs despite cooling

Merck Veterinary Manual notes that exertional heat illness can progress to serious neurologic signs, collapse, seizures, coma, and death if heat dissipation fails. Rapid cooling is a major part of treatment. (Merck Veterinary Manual)

A horse that is hot, dry, distressed, and not recovering should be treated as urgent.

What To Do Right Now if Your Horse Is Not Sweating

If your horse is hot and not sweating properly, act quickly.

1. Stop exercise immediately.
   Do not “walk it off” for too long if the horse is already overheating.

2. Move the horse to shade or a cooler area.
   Use fans if available.

3. Start active cooling.
   Cold hose the horse, especially over the large muscle groups and neck. Continue cooling while monitoring response.

4. Offer cool, clean water.
   Do not force water into the horse.

5. Check rectal temperature if safe.
   Temperature helps, but do not delay cooling just to get a reading.

6. Call your vet if signs are moderate, severe, or not improving.
   A horse that does not cool down needs veterinary help.

For moderate to severe heat illness, Merck describes rapid external cooling with cool water, fans, shade, and ongoing monitoring for rebound hyperthermia. (Merck Veterinary Manual)

How To Manage a Horse With Anhidrosis

There is no simple guaranteed cure for anhidrosis.

Management focuses on preventing overheating, reducing heat exposure, adjusting exercise, supporting hydration, and involving your vet early.

Practical management steps include:

• Exercise only in the coolest part of the day

• Avoid work during high humidity

• Use fans in stables

• Provide deep shade

• Use misters or sprinklers where suitable

• Cold hose after work

• Monitor rectal temperature after exercise

• Reduce workload during hot months

• Keep transport cool and well ventilated

• Provide constant access to clean water

• Consider relocation to a cooler climate in severe cases

University of Florida notes that moving the horse to a cooler climate is the only proven therapy, although careful management can help reduce overheating risk in affected horses. (Large Animal Hospital)

That is not always practical, of course. But it matters because owners are often sold false hope in a tub.

What About Electrolytes and Supplements?

Electrolytes can be useful for horses losing sweat, working in heat, or needing support with hydration planning. They do not reliably “cure” anhidrosis.

For a horse with anhidrosis, electrolyte support should be discussed with your vet, especially if the horse is not drinking well, is dehydrated, has kidney concerns, or is already receiving other treatments.

Be careful with concentrated salt or electrolyte pastes in dehydrated horses. Merck notes that forced oral administration of concentrated salt pastes to dehydrated horses can cause abdominal discomfort and may irritate gastric ulcer disease if not given with food or diluted in water. (Merck Veterinary Manual)

Supplements marketed for anhidrosis are common. Some owners report improvement, but University of Florida notes that many supplement approaches remain anecdotal or have limited research support. Some attempted medical treatments may also be ineffective or potentially dangerous when used unnecessarily. (Large Animal Hospital)

The sensible position is:

• Correct hydration and electrolyte management matter

• Do not assume a supplement solves the problem

• Do not keep training a horse hard just because you started a product

• Use temperature, sweating, breathing, and recovery as your real measures

Can a Horse With Anhidrosis Still Compete?

Some mildly affected horses can continue light or modified work with careful management. Others cannot safely train or compete in hot months.

The decision depends on:

• Severity of sweating failure

• Climate and humidity

• Work intensity

• Recovery time

• Rectal temperature after exercise

• Breathing rate

• Competition schedule

• Transport conditions

• Whether signs are getting worse

A horse with mild partial anhidrosis may manage reduced work at dawn with aggressive cooling. A horse that stays dry, overheats, and takes a long time to recover should not be pushed through competition.

In practice, the question is not “can this horse compete?”
The better question is: can this horse cool safely before, during, and after the work being asked of them?

If the answer is no, the workload needs to change.

What Should You Monitor?

For suspected non-sweaters, track patterns rather than relying on memory.

Monitor:

• Weather conditions

• Humidity

• Work duration and intensity

• Sweating pattern

• Rectal temperature before and after exercise

• Time taken to return toward normal

• Breathing rate after work

• Appetite

• Drinking

• Attitude

• Any episodes of weakness or poor recovery

This is especially useful because anhidrosis can be partial, intermittent, or progressive.

A simple exercise and recovery log can help your vet decide whether the horse is improving, worsening, or unsafe to keep in work.

Common Mistakes Owners Make

Assuming a dry coat means the horse handled the heat well
In hot conditions, a dry coat after work can mean the horse failed to sweat.

Continuing exercise to “build fitness”
Poor fitness can cause fatigue, but fitness does not fix true anhidrosis.

Waiting until collapse before calling the vet
Delayed recovery, rapid breathing, and no sweat are already warning signs.

Relying on supplements alone
Supplements may help some individual horses, but they should not replace cooling, workload changes, and veterinary input.

Ignoring humidity
High humidity makes sweat evaporation less efficient, so even a horse that sweats can struggle to cool.

Not checking temperature
Rectal temperature trends can be very helpful, especially after exercise.

Using concentrated electrolytes without enough water
Electrolytes should support hydration, not worsen gut discomfort or dehydration risk.

How To Reduce the Risk of Overheating

You may not be able to prevent every case of anhidrosis, but you can reduce heat stress risk.

Useful steps include:

• Train early morning or late evening

• Avoid exercise during peak heat and humidity

• Keep horses fit gradually, not suddenly

• Provide shade in turnout areas

• Improve barn ventilation

• Use fans safely

• Give constant access to clean water

• Feed salt or electrolytes appropriately under veterinary guidance

• Cool horses aggressively after hot work

• Acclimatise slowly when weather changes

• Avoid unnecessary transport during heat

• Monitor high-risk horses closely

A 500 kg horse in minimal work may drink around 21 to 29 litres daily, but water requirements can rise substantially with heat, exercise, lactation, dry feed, and sweating losses. Merck notes that a 500 kg horse exercising for an hour in a hot environment may need more than 72 litres to replace sweat and evaporative losses. (Merck Veterinary Manual)

So, for hot-weather horses, water access is not a small management detail. It is central to safety.

FAQs

Can anhidrosis go away on its own?

Sometimes sweating improves when the weather cools or the horse is moved to a cooler climate. However, many cases persist or recur in hot, humid conditions. If your horse has stopped sweating normally, it is worth involving your vet rather than hoping it settles.

Is anhidrosis painful?

Anhidrosis itself is not usually painful in the way a wound or colic is painful, but overheating is very distressing and can become life-threatening. Affected horses may feel weak, exhausted, anxious, or unable to recover properly after work.

Can supplements cure anhidrosis?

There is no consistently proven supplement cure. Some owners report improvement with certain products, but the evidence is variable. Cooling, workload reduction, hydration support, and veterinary management are more important than relying on supplements alone.

Should I ride a horse that is not sweating?

Do not ride if the horse is hot, dry, breathing hard, dull, weak, or slow to recover. Mild cases may tolerate carefully modified work in cool conditions, but this should be guided by temperature monitoring and veterinary advice.

What temperature is dangerous for a horse?

Rectal temperature is useful, but behaviour and recovery matter too. A horse that is disoriented, weak, collapsing, breathing hard, or not cooling down is urgent even before you have a perfect temperature reading. In suspected heat illness, start cooling and call your vet.

Final Thoughts

Anhidrosis is not just a sweating problem. It is a cooling problem.

The horse that worries me most is the one that should be sweating heavily but stays dry, breathes hard, and takes too long to recover. That horse is telling you their heat-control system is not coping.

Early recognition makes a major difference. Reduce heat exposure, stop work when recovery is poor, cool aggressively when needed, monitor patterns, and involve your vet before the horse reaches crisis point.

A suspected non-sweater can often be managed, but only if the management respects the risk. In hot, humid weather, ignoring anhidrosis can turn a performance issue into a medical emergency.

If you are unsure whether your horse’s sweating pattern is normal, whether exercise is safe, or whether signs of heat stress need urgent care, ASK A VET™ can help you decide what to do next.