After taking my own dog for an off leash walk recently, that ended up literally in a fox chase, I was reminded of how quickly we and our pets can overheat. When the weather is humid and hot some of our more ambitious or high performance dogs can overdo it in 5 minutes or less. Older, heavier pets especially black labs, are susceptible too.
Signs of heat exertion included rapid respiration rate approaching 200 breaths (pants?) per minute, a tongue that has curled up edges instead of flat, weakness, trembling and laying down. Some susceptible individuals have exercise induced collapsing syndrome, especially Labradors. Heart problems and blood sugar problems must also be differentiated by a veterinarian.
The best treatment for a hot dog is to get the animal immersed in water 65-70 F (around 20C) I immediately took my dog, now on leash to the lake, walked her in up to her chest, splashed it all over her. Minutes later she voluntarily lay down with just her neck and head sticking out. A child's wading pool would have worked well too. Just having one available if you have a busy dog, would be great. Some of them don't know when to quit. If you ever see severe breathing difficulty or bluish mucous membranes, that is an EMERGENCY and seek Veterinary help while cooling with wet towels and a fan. Fortunately we don't see this in cats - too smart? Maybe the water thing is what they are avoiding!