Today I saw a dog for an emergency that had suddenly collapsed in the yard. When it came in it had a temperature of 105.2 F. Considering that a dog's normal temperature is 101-102, that's pretty high, and is very dangerous. Thankfully we were able to get his temperature down to normal, and he ended up stabilizing and doing fine. But this brings up an important discussion.
Heat stress and heat stroke can kill a pet, and I have seen it happen several times. When the core body temperature gets above 105, cell membranes and body proteins can begin to degrade and break apart. This can lead to numerous problems such as brain damage, internal and external bleeding, diarrhea, muscle damage, and many other serious consequences.
Truthfully, this is usually a completely preventable problem. Pets who are kept outside on hot days with limited shade or water are the ones most likely to succumb. By providing cool, shady areas and plenty of fresh water, the risk can be lowered. Dark colored or black pets are at a higher risk because the dark fur absorbs the light and therefore the heat more easily. In very hot and open areas, it is usually best to bring dogs inside to prevent dangerous overheating.
If your pet collapses in the heat, or you find them laying in the sun unable to move, you need to consider this a life-or-death situation. The faster you can get them to a vet, the better the chance of recovery. But the longer they have been overheated, the worse the prognosis. Don't wait and see if they will recover, but get to the closest veterinarian or emergency clinic immediately.