Animals can suffer fatal heatstroke within minutes. Never leave your dog (or any pet) alone in a car- even with the windows open and avoid exercising them in the midday summer sunshine.
When its a little warm outside dogs tend to regulate their internal temperature by panting. This has a cooling effect, however if their surroundings are too hot, this will not be effective. If your pet's temperature raises significantly this can make them ill and result in damage to their internal organs. Hot/humid conditions and a lack of adequate shade can also be significant risk factors for heat stroke.
The mains signs to watch out for are- rapid breathing, drooling, unsteadiness, vomiting, diarrhoea and collapse. Large dogs, those with heavy hair coats and also dogs with short noses are more sensitive to heat stroke. Animals kept in cars even when its cool outside are at risk.
If you are concerned about your pet exhibiting any of the symptoms please call your vet immediately and place tap water soaked towels over them and pour cool water over their ears and paws. Do not use ice as this is too cold. They must be taken to the vet asap.
Heat stroke is a dangerous condition and can even be fatal. Prompt treatment increases the chance of them making a good recovery.