Heat stroke is a form of nonpyrogenic hyperthermia, which essentially means a high temperature not caused by a fever. It occurs when dogs are no longer able to self-regulate and keep their temperature at a comfortable level.
We've all seen dogs locked in cars on hot days. No dog owner would deliberately subject their pet to the stress and potentially lethal danger of overheating but, just as people occasionally drive with no lights in poor light, maybe they just forgot to take account of the very real danger. Whatever the reason, dogs die in hot cars!
The following passage is from the emergency veterinary site operated by Vets Now, the largest national provider of emergency veterinary care in the UK;
Dogs don't tolerate high temperatures as well as humans do. Unlike us, dogs typically rely on panting to keep themselves cool and panting is the most important way in which a dog regulates its temperature. However, because they only have sweat glands in their feet and around their nose, they are less efficient than we are at cooling themselves down.
There are some increased risk factors, and dogs who are overweight or suffer from the respiratory abnormalities typically affecting flat-faced breeds have a higher risk of experiencing heat stroke. However, it's all too easy to be drawn into a false sense of security if you believe that, because your dog is fit and healthy, there's little or no risk. We need to remember that any dog can easily overheat if exposed to hot temperatures and a lack of drinking water and ventilation. This can happen very quickly as in a closed car on a summer's day, the temperature can reach more than 150 F. In these conditions it is not surprising that even a fit and healthy dog can die within fifteen minutes.
It can take dogs up to a couple of months to acclimatise to appreciable changes in temperature. We've seen significant variation in our own UK weather in recent years and this really should be a serious consideration if you've acquired a pet passport and are planning to take your dog abroad with you on holiday. There are two types of heat stroke in dogs which should concern us: exertional and non-exertional.
Exertional heatstroke occurs during exercise and is much more common on hot sunny days when dogs haven't had a chance to acclimatise to the sudden rise in heat.
Non - exertional heatstroke is when a dog is subjected to a significant rise in temperature but doesn't have access to ventilation, or drinking water, to keep themselves cool. This type of heatstroke typically occurs in a parked car, a garden with no shade, or a very hot room.