Heatstroke in dogs and cats: the signs, and why a parked car is the killer
Heatstroke isn't the same as being hot, and by the time a dog looks in trouble it often already is. The signs are worth knowing before the day you need them — and the one rule about parked cars has no exceptions.
A dog cools itself almost entirely by panting, which works until the air is too warm to help.
When it stops working the temperature climbs fast, and heatstroke is what happens next: a body running hot enough to damage the gut, the kidneys and the clotting system, sometimes within minutes. It's one of the few warm-weather emergencies that kills otherwise healthy animals, and almost all of it is preventable.
Why panting runs out
It's worth understanding the mechanism, because it explains every piece of advice that follows.
Dogs have sweat glands only in their paws, which contribute almost nothing. Cooling happens in the mouth and airway: air moves over wet surfaces, water evaporates, and evaporation carries heat away. That works beautifully in dry moving air and badly in still humid air, which is why a muggy twenty-eight degrees is more dangerous than a dry thirty-two, and why a dog in a sealed car has no way out of the problem — it's breathing air it has already warmed and moistened itself.
It also explains why a short muzzle is such a handicap, why a thick coat slows heat loss, and why a dog that's already panting hard when you set off has no reserve left. Panting is a cooling system with a ceiling, and everything about heatstroke is what happens above that ceiling.
The rule about cars
Never leave an animal in a parked car in warm weather, not for five minutes, not with the windows cracked, not in the shade.
A car in mild sunshine becomes an oven because the glass traps heat faster than the small gap of an open window can shed it, and a cracked window changes the timeline by a few minutes and not the outcome. This is the single most common way healthy dogs die of heat, and the only safe version is to leave the dog at home.
If you find a dog shut in a hot car in distress, call the police rather than waiting for the owner. In several countries the law protects a bystander who acts to save an animal in immediate danger, but the phone call comes first because the people who arrive can get into the car and you generally can't.
What heatstroke looks like
Early, it's heavy relentless panting, a dog that can't settle, and thick sticky drool.
Then the gums turn brick-red or oddly pale, the dog wobbles or seems confused, and it may vomit or have diarrhoea. Collapse and seizures are the late signs, and by then the damage is already being done. A dog doesn't have to look dramatic to be in trouble: one that has simply gone quiet and dull after exertion in the heat is a dog to take seriously.
Cats hide it better and get it less, because they seek out cool spots and rarely over-exert, but the warning signs are the same — open-mouth panting in a cat is never normal and is a reason to act, not to wait.
What to do, in order
Move the animal into shade or indoors, and start cooling it now — this is one of the rare emergencies where first aid before the car journey genuinely changes the outcome.
Pour or spray cool (not ice-cold) water over the body, especially the belly, armpits and groin, keep the air moving with a fan or an open window, and offer small amounts of water to drink if it will. Then phone the vet and drive, cooling as you go. The old advice to use ice or freezing water is out of date: very cold water makes the surface vessels clamp shut and can slow the cooling, so cool water and airflow are what you want.
A few things not to do, because they're common and they hurt. Don't throw a wet towel over the dog and leave it there — a soaked towel that stays on traps heat instead of shedding it, so wet the coat and keep the air moving instead. Don't force water down the throat of an animal that's dazed or struggling to swallow. And don't wait to see whether it improves before ringing, because the phone call and the cooling happen at the same time.
Ring ahead so the practice is ready, and go even if the dog seems to recover, because the dangerous part of heatstroke is the organ damage that shows up over the following hours. A dog that looks fine again can still be in a serious way inside.
Who overheats first
Flat-faced breeds — pugs, bulldogs, French bulldogs, Persians — are in a category of their own, because the same short muzzle that defines them leaves far less airway to pant through, and they get into trouble on a walk that a longer-nosed dog would shrug off.
Add the very old, the very young, the overweight, the thick-coated, and any animal with a heart or airway condition, and you have the list of who to watch hardest. On a hot day, none of them should be exercised at all until it cools down.
Weight is its own risk factor here, which is one more reason it's worth taking seriously — an overweight dog overheats sooner and recovers slower than a lean one of the same breed.
One more group belongs on the list: any dog in the first warm week of the year. Animals acclimatise to heat over a couple of weeks, so the first twenty-five-degree day in spring hurts far more than the same temperature in August, and a great many cases arrive on exactly that day. Treat an early heatwave as hotter than the number says.
Preventing it
Walk in the cool of early morning or late evening on hot days, and skip the walk entirely when it's genuinely hot — a missed walk never hurt a dog and a heatstroke can kill one.
Test the pavement with the back of your hand: if you can't hold it there for seven seconds it's too hot for paws. Always carry water on a warm-weather outing, provide shade and never leave a dog tied out in the sun, and remember that a conservatory, a caravan or a summer-hot room is the same trap as a car. On long car journeys keep the air conditioning on the animal, not just the front seats.
At home on a hot day, make the cool places available. A tiled floor, a shaded room with the curtains drawn, a fan at floor level, a damp towel to lie on if the dog chooses it, and water in more than one place — a cat in particular will drink more from a bowl away from its food. A paddling pool in the shade entertains some dogs and cools them properly; a frozen stuffed toy or ice cubes in the water bowl do no harm and pass an afternoon.
And replace the exercise rather than simply removing it. A hot day is a good day for scent games indoors, a puzzle feeder, ten minutes of training in a cool room, or a chew — all of which tire a dog without adding a degree of body heat. A dog that's under-exercised for one day is fine; one that was walked at noon in August may not be.
What are the first signs of heatstroke in a dog?
Heavy, frantic panting, an inability to settle, and thick ropey drool come first, then brick-red or pale gums, wobbliness or confusion, and vomiting. Collapse and seizures are late signs. A dog that has gone quiet and dull after exertion in the heat is already a concern.
How long can a dog be left in a car on a warm day?
It should never be left at all. A parked car turns into an oven within minutes even in mild sunshine, and a cracked window doesn't change the outcome. Leave the dog at home instead.
What should I do if my dog has heatstroke?
Move it into shade, pour cool — not ice-cold — water over the body, keep air moving, offer small drinks, and phone the vet while you drive, cooling as you go. Go even if it seems to recover, because organ damage can show up hours later.
Should I use ice-cold water to cool an overheated dog?
No. Very cold water makes the surface blood vessels clamp shut and can slow cooling. Cool water and moving air are what bring the temperature down safely — and don't leave a soaked towel lying on the dog, which traps heat rather than shedding it.
Why is humid weather more dangerous than dry heat?
Because dogs cool by evaporating water from the mouth and airway, and evaporation barely works in still, humid air. A muggy twenty-eight degrees can be harder on a dog than a dry thirty-two, which is why the thermometer alone is a poor guide.
Which dogs are most at risk of overheating?
Flat-faced breeds like pugs, bulldogs and French bulldogs are most at risk because a short muzzle leaves little airway to pant through. The very old, very young, overweight, thick-coated, and any dog with a heart or airway problem are also high-risk and should not be exercised in the heat.
Why do so many cases happen on the first hot day?
Because animals take a couple of weeks to acclimatise to heat, so an early heatwave hits far harder than the same temperature later in the summer. Treat the first warm days of the year as hotter than the number suggests.
How do I exercise my dog in a heatwave?
Mostly indoors and mentally. Scent games, a puzzle feeder, short training sessions in a cool room and something to chew all tire a dog without raising its temperature. Walk at dawn or after dark if at all, and skip it entirely when it's genuinely hot.
Can cats get heatstroke?
Yes, though less often, because they seek shade and rarely over-exert. Open-mouth panting in a cat is never normal and is a reason to cool it and call the vet rather than wait.