Signs, Causes, and Treatment of Dehydration in Horses

Dehydration in horses happens when fluid losses outstrip fluid intake. A 500 kg horse can lose 10–15 litres of sweat per hour during hard work in hot weather. If those losses are not replaced — through drinking, electrolytes and feed water — performance falls, recovery slows, and the horse becomes vulnerable to colic, heat stress, exertional rhabdomyolysis ("tying up") and kidney strain. Severe dehydration can be life-threatening.

This guide covers how much horses should drink, the early and late signs of dehydration, the at-home checks you can do in two minutes, electrolyte management, correct cooling technique, and the treatment thresholds that warrant veterinary intervention. It is written for British and South African owners — two climates that present the problem in very different ways.

How much water does a horse need?

Baseline intake for a 500 kg horse

A 500 kg adult horse drinks roughly 20 to 55 litres of water a day under normal management. The lower end of that range applies to a stabled horse at rest on grass-rich forage; the upper end applies to a horse in work or on a dry hay-only diet. In genuine hot weather the BHS puts it higher still, at 50 to 70 litres — four to six buckets — for a 500 kg horse. Ponies scale down proportionally; broodmares nursing a foal scale up sharply, and the Merck Veterinary Manual puts a lactating mare’s requirement at 12–14 litres per 100 kg bodyweight, or 60–70 litres a day for a 500 kg mare.

Those are averages, not targets. The number that matters is your horse’s normal. A horse that reliably empties two buckets a day and suddenly leaves one full has told you something useful; a horse you have never measured has not.

How exercise multiplies the need

Hard work in hot, humid weather can drive sweat losses of 10–15 litres per hour. A horse competing over 80 km on a 28 °C day can lose 25–40 litres of fluid before crossing the finish line, and may need a further 30–50 litres over the following 12 hours to fully rehydrate. This is why hydration management dominates the culture of the endurance ride, and why endurance horses — overwhelmingly Arabians and Arabian crosses — are vetted on hydration parameters at every hold.

Diet drives baseline water intake

Fresh grass is roughly 70–80% water by weight. Hay is typically 10–15% water. A horse moved from spring grass to a hay-only winter diet can double or triple its drinking-water requirement almost overnight, because the water it used to eat now has to be drunk. Add a cold trough with ice on the surface and intake falls further at exactly the moment the requirement rises. That combination is behind a great many UK winter impaction cases.

Causes of dehydration in horses

Sweat losses in hot or humid work

Horse sweat is hypertonic — it contains more dissolved salts than the horse’s blood. That is unusual among mammals, and it is the single most important fact in this article. Every litre lost takes with it roughly 3.1 g sodium, 5.3 g chloride and 1.6 g potassium, plus traces of calcium and magnesium. This is why a horse can drink freely and still test dehydrated: it replaces the volume but not the salts, blood sodium falls, thirst switches off, and the kidneys dump the water straight back out.

Anhidrosis — when a horse can’t sweat

Some horses lose the ability to sweat properly — a condition called anhidrosis. It is most often reported in hot, humid climates: the southeastern United States, and in South Africa the humid coastal belt of KwaZulu-Natal and the Lowveld. The primary risk is heatstroke rather than dehydration as such, but the two travel together. Affected horses show patchy or absent sweat under saddle, laboured breathing as their fallback cooling mechanism, and poor heat tolerance. Any horse that stops sweating in hot weather needs veterinary assessment, not just a cold hose.

Travel, transport and competition

Horses routinely drink less away from home. The water tastes different, the routine is broken, and stress suppresses thirst. A long journey in summer can cost a horse eight to fifteen litres of fluid before it has done a stroke of work — horses lose fluid continuously through respiration and sweating while balancing in a moving box, and many will not drink at a service station.

Illness — diarrhoea, colitis and colic

Diarrhoea and colitis pull fluid out of the body through the gut faster than any other route. A horse with severe colitis can lose tens of litres in 24 hours and become critically dehydrated while standing in a stable with a full water bucket. Dehydration is also both cause and consequence of colic: impaction and sand colic are more likely in an under-drunk horse, and the pain of any colic stops a horse drinking. Understanding how the equine digestive system handles fluid makes the link obvious — the large colon acts as the horse’s internal water reservoir, and it only works if it is filled.

Frozen, dirty, or restricted water

A frozen trough, a blocked automatic drinker, a stagnant green bucket, or a dominant horse guarding the water point are all common and badly under-diagnosed causes. Water being present is not the same as water being drunk. Good paddock management means watching who actually drinks, not just checking the trough is full.

Signs of dehydration in horses (early to severe)

Early signs (before the deficit shows on examination)

A fluid deficit below about 5% of bodyweight is undetectable on physical examination (Merck Veterinary Manual), so at this stage you are looking for changes in routine rather than anything you can measure at the horse:

• Reduced or absent interest in feed
• "Flat" attitude; quieter than normal
• Slightly tacky gums; thicker, stickier saliva
• Darker, more concentrated urine passed less often
• Mildly reduced performance and a longer recovery after work
• Slight increase in resting heart rate

Moderate signs (roughly 6–8% body weight loss)

• Sunken-eye appearance
• Dry mouth rather than merely tacky; thick, sticky saliva
• Capillary refill time over 3 seconds
• Heart rate persistently elevated at rest (50+ bpm)
• Reduced manure output; harder, drier droppings
• Visible loss of condition over the flank

Severe signs (over 8% body weight loss — emergency)

• Deeply sunken eyes
• Very dry mouth; complete loss of skin elasticity
• Capillary refill 5+ seconds; gums pale, or dark and muddy red
• Heart rate over 60–80 bpm at rest
• Cold extremities and signs of shock (weak, thready pulse)
• Colic signs, marked depression, reluctance to stand

Severe dehydration is a veterinary emergency. Intravenous fluid therapy is the only practical way to restore circulating volume quickly enough to prevent cardiovascular collapse. A 500 kg horse at 8% dehydration is short roughly 40 litres — no horse can drink that in the time available.

At-home checks you can do in two minutes

1. Skin pinch (skin tent) test — and why it cannot rule dehydration out

Pinch a fold of skin on the point of the shoulder or the side of the neck and release. In a well-hydrated horse it flattens promptly. A fold that stands up as a visible tent is worth acting on.

A normal pinch, though, proves nothing — and an earlier version of this page implied that it did. Pritchard, Burn, Barr and Whay timed a standardised skin tent at six body sites in 50 horses and checked it against plasma osmolality, the laboratory measure of dehydration (Equine Veterinary Journal, 2008, 40(6):558-64, PMID 18356129). There was no significant association between the two. Tent duration varied with which side of the horse was tested, where on the body, coat moisture and the animal’s age, and the authors concluded that neither the skin tent nor mucous membrane dryness was a valid or repeatable indicator of dehydration. The British Horse Society now lists relying on the skin pinch, or on gum dryness, among the common mistakes owners make, and Horse & Hound’s vet library says the same.

So do not put a stopwatch on it, and never read a normal result as the all-clear. The most reliable field measure in that study was the horse’s drinking — how much water it took, and how many and how long its drinking bouts were. Watch the bucket, not the clock.

2. Capillary refill time (CRT)

Press your thumb firmly against the upper gum just above an incisor for two seconds, then release. The blanched white patch should return to normal pink within one to two seconds. Over three seconds suggests significant dehydration or circulatory compromise.

3. Gum colour and moisture

Healthy gums are pale pink and slick — wet enough that a finger slides. Tacky or dry gums are abnormal, as is any colour that is pale, dark muddy red, blue-tinged or grey. Pale gums with a slow refill often indicate shock; muddy red or purple gums suggest endotoxaemia and are frequently seen alongside severe colitis or a strangulating colic. Either warrants an immediate call.

4. Resting heart rate

Use a stethoscope behind the left elbow, or feel the facial artery running under the jawbone. A healthy adult horse at rest sits at 28–44 beats per minute. A rate that stays above 50 bpm at rest, alongside any other sign on this page, is a genuine flag — and a rate that will not settle after work is one of the earliest objective signs of a horse in fluid deficit.

Normal vital signs and what changes with dehydration

Take these readings on a healthy day, write them on the stable door, and you will spot a deviation months later in seconds. The temperature range above is the British Horse Society’s 37.5–38.5 °C, which it publishes alongside a pulse of 28–44 bpm in its About the Horse coach guidance, and the range Horse & Hound gave in June 2026. Published ranges genuinely differ at the bottom end — Horse & Hound’s own vet library, with Karen Coumbe MRCVS, gives 37–38 °C — which is precisely why the reading that counts is your own horse’s, not a textbook range. Know normal before you need to.

Electrolytes and what horses actually lose in sweat

Why water alone is not enough

Because equine sweat is saltier than blood, replacing only the water dilutes blood sodium and chloride. That dilution suppresses the thirst response and signals the kidneys to excrete more fluid. The result is the classic presentation of a horse that drinks all afternoon and still shows a skin tent at evening stables. The fix is to replace water and salts, in proportion.

Per-litre composition of horse sweat

Figures from the Merck Veterinary Manual, adapted from Harris, Coenen, Frape, Jeffcott and Meyer in The Equine Manual (2nd edn). An earlier version of this page gave chloride as 5.5–6.5 g per litre, a range that excludes the published figure.

Doing the arithmetic properly

This is where a lot of well-meant yard advice goes wrong, so it is worth being precise. A horse losing 20 litres of sweat over a hard competition day loses roughly 62 g sodium, 106 g chloride and 32 g potassium.

Plain table salt is sodium chloride — about 39% sodium and 61% chloride by weight. So a 60 g scoop of salt supplies only about 24 g of sodium and 36 g of chloride. That is a sensible daily ration for a horse in light to moderate work, but it replaces well under half the sodium lost on a heavy sweating day; matching 62 g of sodium from salt alone would take roughly 160 g. Salt also contains no potassium, calcium or magnesium at all.

The practical conclusion: plain salt is fine for hacking, schooling and general maintenance, and it is cheap. A properly balanced commercial electrolyte — products such as NAF Electro Salts or Equine America Apple-Lytes in the UK — earns its place when the horse is doing hard, sweaty or prolonged work, because it supplies the potassium and the trace minerals that salt cannot. Check the label for the actual sodium, chloride and potassium content per dose; a great many "electrolyte" products are mostly sugar.

Timing: forget multi-day "loading"

A persistent myth holds that you should load a horse with electrolytes for two or three days before a competition. Horses have very limited capacity to store excess electrolytes — surplus sodium and potassium are simply excreted in urine within hours, so dosing on Thursday does nothing for a Sunday cross-country round.

What the evidence does support is giving electrolytes close to the effort: a dose in the hours before travelling or working, with water freely available, and further doses during and after prolonged work as losses accumulate. A 2022 study in Animals, Tracing Acid-Base Variables in Exercising Horses, found that a substantial hypotonic electrolyte dose given an hour before work blunted the acid-base disturbance of moderate exercise — the gut acting, in the authors’ phrase, as a temporary reservoir to be drawn on during the effort that follows.

Two safety rules are absolute. Never give concentrated electrolyte paste to a horse that is not drinking — it draws fluid into the gut and makes dehydration worse. And always offer plain, untreated water alongside any electrolyte-treated bucket, because many horses refuse flavoured water and will simply drink nothing.

How to cool a hot, dehydrated horse

Heat and fluid loss are the same emergency viewed from two angles, and the correct cooling technique has changed since many of us learned it. The old instruction — hose briefly, scrape the water off, walk, repeat — is wrong, and it is worth unlearning deliberately.

Research conducted for the FEI ahead of the 1996 Atlanta Olympics, and extended since by Dr David Marlin and others, found that the fastest cooling method is the continuous application of copious cold water. Scraping is unnecessary, and thermal imaging shows that pausing to scrape actually allows surface temperature to rise. The fear that leaving water on a horse "traps heat" is a myth: water is a far better conductor of heat than air, and cold water on a hot horse moves heat out of the horse, not into it. The water warming up on the coat is the process working.

In practice, for a horse that is hot and distressed:

1. Move it into shade, and into moving air if there is any.
2. Apply cold water continuously over the whole body — including the large muscle masses of the quarters, not just the neck and legs.
3. Keep going. Do not stop to scrape; do not ration the water.
4. Walk only if it is safe and does not interrupt the water.
5. Take a rectal temperature every few minutes and stop active cooling once it is heading towards normal.
6. Call a vet if temperature stays above 39 °C, if the horse is wobbly or disoriented, or if it has stopped sweating.

Cooling and rehydration are separate jobs and both need doing. See our glossary entries on cooling out and cooling down for how this fits into the wider post-work routine.

Treatment: when to call the vet

Mild dehydration (manage at home)

If the horse is bright, eating, drinking as much as it normally does, and has normal vital signs: offer free-choice water plus a second bucket of correctly diluted electrolyte solution; add salt to the feed; soak the hard feed or offer a sloppy mash; turn out if you can, since grazing horses drink more readily than stabled ones. Recheck vital signs every two to four hours.

Moderate dehydration (call your vet)

If the horse is drinking less than usual for it, the resting heart rate is up, the gums are tacky or dry, capillary refill is slow, or the horse is dull and off its feed — call. You do not need to collect several signs first: the BHS position is that dehydration can progress quickly and is not always easy to recognise, so it is safest to take veterinary advice whenever you are unsure. Your vet will assess hydration objectively from blood work — packed cell volume and total plasma protein — rather than from the outside of the horse. Treatment may involve intravenous or nasogastric fluids, electrolyte correction, and dealing with whatever caused it.

Severe dehydration (emergency)

Sunken eyes, a persistent skin tent, very slow capillary refill, a heart rate over 60 bpm at rest, or any sign of shock or collapse needs immediate veterinary attention and intravenous fluid replacement. Recovery generally requires hospitalisation and 24–72 hours of IV fluids with electrolytes corrected in the drip. Do not attempt to manage this with buckets.

Prevention in UK and SA conditions

UK summer (May–September)

British summers now routinely produce genuine heat events — the national record of 40.3 °C was set at Coningsby, Lincolnshire in July 2022, and horses in the UK are not acclimatised to it. Practical steps:

• Two clean water sources per field, sited in shade where possible.
• Free-choice salt lick in every paddock and every stable.
• Soak hard feeds, chaff, beet pulp and mashes to add fluid through the feed.
• Avoid hard schooling between 11 a.m. and 4 p.m. on hot days.
• Have a hose, buckets and a thermometer ready before you need them.
• Give electrolytes around the effort, not days ahead of it.

UK winter (December–February)

Cold-water suppression is the most preventable cause of winter impaction colic in Britain. Horses genuinely dislike near-freezing water: in Kristula and McDonnell’s 1994 study at the University of Pennsylvania, ponies offered warmed water in cold weather drank a mean of 40% more than those offered water at near-freezing ambient temperature. Adding hot water from a kettle to take the chill off is one of the cheapest interventions in horse care. Break ice twice daily, keep salt available year-round, and remember that the switch from grass to dry forage has already raised the requirement.

South African summer (October–April)

The Highveld combines altitude, fierce UV and afternoon thunderstorms; the coastal belt adds humidity that blunts evaporative cooling. Practical steps:

• Schedule work for early morning or after 5 p.m.
• Provide real shade — tree cover or an open-sided shelter.
• Check troughs for blue-green algae, which blooms fast in warm standing water and is toxic.
• Watch for anhidrosis in horses kept in humid coastal regions.
• Treat electrolyte management as a year-round job for working horses, not a summer-only one.

Competition riders in South Africa should also note that electrolyte and supplement use at affiliated events falls under the South African Equestrian Federation’s rules and veterinary regulations, which follow FEI anti-doping and controlled medication principles. Plain electrolytes are not prohibited substances, but combination products sometimes contain ingredients that are — read the label before a competition, not after.

Travel — UK and SA

For any long journey: give electrolytes in the hours before loading rather than days before, offer water at every stop, take water from home if your horse is fussy, and offer a sloppy mash on arrival. South African long-haul transport covers serious distances — Johannesburg to Cape Town is roughly 1,400 km — so agree a hydration and rest plan with the transporter in advance rather than assuming one exists.

Frequently asked questions

How long can a horse go without water?

Survival time is the wrong question, and the "three to six days" figure this page used to carry could not be traced to any veterinary source. Horses go badly wrong long before survival is in doubt. Deprived of water while transported in hot, humid conditions, horses had serum electrolyte concentrations above the reference range by 19 hours, and after 24 hours three of the thirty in that study were judged unable to continue (Friend, Martin, Householder and Bushong, JAVMA 1998, PMID 9530423). Even partial restriction bites: mares held on reduced water ate progressively less hay and lost 48.5 kg over three weeks (Houpt, Eggleston, Kunkle and Houpt, Equine Veterinary Journal 2000, PMID 10952384). Horses can, meanwhile, survive 72 hours of deprivation and roughly 12% of their body mass in water (Sneddon, van der Walt and Mitchell, Journal of Applied Physiology 1991, PMID 1917731) — which is exactly why survival time tells you nothing about when to act. A horse that has stopped drinking is a reason to ring your vet, not something to watch for another day. An earlier version of this page said to wait 48 hours. That was our error, and it was too long.

Can dehydration cause colic in horses?

Yes, and it is one of the better-established links in equine medicine. Reduced fluid intake dries the gut contents and slows transit, which makes impaction colic considerably more likely — the classic UK presentation is a stabled horse in a cold snap, off grass, on dry hay, drinking from an icy bucket. The relationship also runs the other way: a horse in pain from colic of any cause will stop drinking, deepening the fluid deficit.

Will adding electrolytes encourage a horse to drink more?

Indirectly, yes. Sodium is the trigger for the thirst response, so adding salt or a balanced electrolyte to the feed raises blood sodium slightly and prompts drinking. But this only works if plain water is available — always offer an untreated bucket alongside any treated one, because a horse that dislikes the taste will drink nothing at all and you will have made the problem worse.

Can I just give plain table salt instead of a commercial electrolyte?

For most horses in light to moderate work, 30–60 g of plain salt daily in the feed is adequate and sensible. For heavy work, endurance, or hot-weather competition it is not, because salt supplies only sodium and chloride — a horse sweating 20 litres also loses around 32 g of potassium plus calcium and magnesium, none of which are in salt. That is the point at which a balanced commercial product is worth the money.

My horse won’t drink at competitions — what should I do?

Take water from home and offer it in his usual bucket on arrival. If he is a persistent offender, flavour his water at home for a fortnight beforehand with a small quantity of apple juice, then add the same flavour to away water so the taste is familiar. Soaked feeds and a sloppy mash on arrival contribute a surprising volume of fluid, and a horse that will not drink will often eat a wet mash.

Is dark urine a reliable sign of dehydration?

It is suggestive, not diagnostic. Concentrated urine follows a genuine fluid deficit, but horses also pass darker urine after beet pulp, on certain medications, and in some muscle conditions — exertional rhabdomyolysis can produce coffee-coloured urine and is a separate emergency requiring its own veterinary response. Read dark urine alongside drinking behaviour, capillary refill, gum moisture and heart rate rather than on its own.

This article is general guidance for horse owners and is not a substitute for veterinary advice. If you suspect your horse is dehydrated, ill or in pain, contact your veterinary surgeon.