Hyperthermia in dogs

by Victoria Aspinall BVSc., MRCVS



Have you ever sat in a parked car on a hot day, perhaps waiting for a friend who has just popped into a shop and have you noticed how quickly the air in the car heats up? It can easily become quite unbearable within about 10 minutes reaching temperatures of 50°C or more in full sun. So what do you do? You open a window or you get out of the car or if you can, you switch on the air conditioning. Now imagine that you are a dog sitting in a car. The dog can’t do any of the things that you can to lower the temperature and it doesn’t understand when its owner is coming back, so it starts to pant and dribble and then to move around and over a relatively short period it may collapse, have a fit and then die. Imagine the distress it has gone through and imagine the distress to you when you return to find your dog collapsed or dead.

So what has happened to this dog?  The answer is that it has suffered from a physiological abnormality known as Hyperthermia or Heat Stroke. This is obviously most likely to occur on hot summer days but dogs left in cars or shut in conservatories on moderately cool days may also be at risk. Any animal that is unable to cool off may suffer from the condition but some of the most susceptible breeds are dogs with short noses such as pugs, bulldogs and boxers. Older dogs and puppies may also be more vulnerable than healthy adults. Hyperthermia may also be a symptom of some types of poisoning such as with slug bait (metaldehyde) and it may be seen in any species – rabbits and guinea pigs kept in small cages on hot patios in the summer are particularly susceptible.

The domestic dog Canis familiaris, is a mammal and all mammals are endothermic or warm blooded. This means that their internal or core temperature remains at a steady value of around 39°C no matter what the external temperature and it is controlled by the hypothalamus deep inside the brain. If an animal is to remain healthy it is essential that this core temperature remains constant because this is the temperature at which all the chemical reactions of the body are designed to work. Surrounding the body’s core is an outer ‘shell’ the temperature of which can vary within a range of 38.3- 38.9°C. Receptors within the shell transmit information about blood temperature to the hypothalamus and this initiates a response to promote heat loss by dilation of the blood vessels, panting or sweating (although this is reduced in fur covered animals) or to conserve heat by constricting the blood vessels, shivering or by fluffing up the fur to trap warm air beneath. Hyperthermia develops when the normal physiological mechanisms of the body are unable to maintain the core temperature within a safe range. Death is likely to occur when it reaches about 42°C.

A dog suffering from hyperthermia will feel hot to touch, will pant rapidly and salivate excessively and its gums may be bright red or purple. It may be unsteady on its feet and it may be distressed or depressed. Some dogs may vomit, often with blood in it and they may have diarrhoea. If the dog’s temperature is taken using a rectal thermometer the reading will be off the scale. Eventually the dog will collapse, may begin to fit and if nothing is done, it will die.

So what should be done in a situation like this? The first thing is to remove the animal, whatever the species, from the hot area immediately. If this means that you have to break the window of the car then so be it – you cannot stand by and watch a dog die. The next thing, prior to taking the animal to the veterinary surgeon, is to lower the body temperature by wetting the coat with cool water. Be inventive and use whatever you have to hand. The obvious answer is to use buckets of water or a hose, but there may be a pond, lake or river near by – just make sure that you do not drown the dog. Just a word of warning – using very cold water may cause the core temperature to fall dangerously low and too quickly which could lead to the development of other life threatening conditions. Very small dogs may be more vulnerable to developing hypothermia (an abnormally low body temperature) so cool them down using lukewarm water – it will almost certainly be cooler than their core temperature at the time.

When you get to the surgery, the veterinary surgeon will continuously monitor the animal’s temperature and will take steps to lower it to a safe range. These may include using icepacks, cooled intravenous fluids or even the use of ice cubes administered per rectum. He/she will ensure that the dog is able to breathe and will give oxygen if the gums are blue or purple. The dog will be checked for signs of shock, heart problems, kidney failure and clotting problems as these are all potential complications of hyperthermia. Once the animal’s temperature has returned to normal it will probably be kept in the surgery overnight to ensure that all the body’s mechanisms have returned to normal function.

Click here for '10 ways to prevent hyperthermia in dogs'