Heatstroke in dogs and cats: the signs, and why a parked car is the killer
Heatstroke is not 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 is one of the few warm-weather emergencies that kills otherwise healthy animals, and almost all of it is preventable.
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 cannot.
What heatstroke looks like
Early, it is heavy relentless panting, a dog that cannot 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 does not 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.
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 is worth taking seriously — an overweight dog overheats sooner and recovers slower than a lean one of the same breed.
Preventing it
Walk in the cool of early morning or late evening on hot days, and skip the walk entirely when it is 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 cannot hold it there for seven seconds it is 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.
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 does not 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.
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.
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.