Normal heat dissipating mechanisms can not compensate for excessive heat load. Excessive heat load can be caused by a whole range of things. When body temperature rises above a critical level, generalized cellular necrosis begins to occur and normal thermoregulatory mechanisms stop operating. The critical temperature for organ failure is 42.7C. In our hot and often humid Townsville summer time, it is quite easy to reach this point of no return.
All body systems can be affected by heat stress – examlples include:
- Nerve injury and brain pressure
- Dehydration and heart failure
- Toxic thermal liver damage
- Tubular damage and acute kidney failure
- Systemic circulatory breakdown and bleeding from all over
- Muscle breakdown
We tend not to see cases of heat stress in pet cats, though it is possible and might be more likely to occur with Persian (brachycephalic) types and the “locked in the hot room” situation. The key thing to understand about heat stress in dogs, is that dogs do not cool by sweating as do many other animals. Animals that do not sweat over the surface of the body can not benefit from that very effective process of controlled evaporative skin cooling that other animals like humans and horses do.
Dogs can and obviously do keep themselves cool by two methods:
- Evaporative cooling from lungs and mouth (panting)
- Conduction cooling from skin contact with cool surfaces/substances and damp shady breezy places
So long as they have plenty of fresh air and cool water, most reasonably fit/healthy dogs can manage hot weather without too much trouble at all. What they can’t cope with is a combination of excessive heat + excessive humidity. This is because humidity compromises the efficiency of their radiator system. On a humid day panting simply doesn’t work at all well. On a very humid day it doesn’t even have to be very hot and they can be in a lot of trouble if exercised or otherwise “heat stressed” in some other way. With continuing heat stress while the weather is hot-humid, a dog’s core body temperature just keeps going up and up until the unfortunate creature starts falling over and melting down.
- The notion that thick coated dogs are more likely to be affected by heat stress is probably not valid. This is because (as was pointed out above) dogs do NOT cool by sweating through the skin. Immersion in cooling water can obtain a conduction/evaporative cooling effect and coat thickness may have some bearing on how efficiently this works, but immersion is not their main inbuilt thermoregulatory mechanism.
- Brachycephalic (squash faced) breeds are more affected by heat stress because they can’t breathe properly and hence can’t pant properly. Overtyped brachycephalic dogs can quite easily die from heat stress on a hot humid day even without doing anything strenuous at all. It is not enough to keep these kinds of heatstress prone dogs from getting too hot, you have to actively ensure they stay cool.
It is a fatal mistake to leave a dog in a locked up car. Why?
- A locked up car in the sun can get as hot as an oven
- The moisture loss from the lungs of a desperate, panting, heat stressed dog will advance humidity levels in an enclosed airspace to levels approaching 100% in no time at all.
What have you got? An extremely hot environment and very high humidity ie heat + humidity. Is that bad? Yes indeed! – That’s a lethal combination! – Make the “dog in the closed car” mistake and you will have a dead dog happening… and pretty darned quickly too!
Early stage heat stress signs include the obvious symptoms of:
- Panting hard
- Actively seeking shade/cool
- Excessive salivation and enlarging tongue
- Congesting (more red) mucous membranes
- Increasing heart rate
- Starting to show anxiety/distress
Up to a point, such signs are quite normal for a hot dog. These are the signals that a hot dog is thermoregulating (working to reduce its temperature) in a completely normal way. At this stage, you can help the dog thermoregulate by doing a number of simple and pretty obvious things:
- Finding/providing shade
- Maintaining a breeze eg fanning
- Providing fresh cool water to drink
- Rinsing/dipping/wetting down.
Signs of a progressing/worsening/getting critical/potentially terminal stage heat stress (depending on the severity and duration of the hyperthermia) can include the following:
- Very rapid heart rate
- Failing circulation/tacky dry pale gums
- Trembling/fitting/falling down
- Respiratory distress
- Hemorrhagic vomiting
- Diarrhoea with blood
Such cases need intensive medical treatment. Even with such care, they may still die. Main thing is to be aware of the lethal potential of heat stress and to recognise the risk both for your pets and indeed for yourself.
Don’t forget how hot pavements and roads get in summer
Try this – Take off your shoes and then get down on your hands, elbows, knees and toes touching the ground. Hold your head about a foot above the ground. Now crawl along. This is what your dog experiences on scorching hot pavements.
Sometimes I see people getting cross with their dog because it won’t sit before crossing the road. I feel like telling them to park their bare bum and bits on the tarmac or pavement and see how long they last!
We regularly see dogs with burnt footpads and it takes weeks for these to recover. In the summer walk early morning or late evening when cooler and test the pavement temperature first. Don’t forget the beach sand will get as hot as pavers!