The canine body operates best in a very narrow temperature range – usually from 99 to 102 degrees. Above 109, and body systems will become severely damaged.
The dog’s temperature is tightly controlled by the hypothalamus, an area in the brain. Elevations in temperature can be caused either by endogenous factors (inside the body) or exogenous (outside).
Fever occurs when the hypothalamus resets the normal body temperature higher as a result of infection or inflammation. It is initially a beneficial response, as it enhances the ability of the immune system to destroy viruses and bacteria. With fever, trying to cool a dog down will not help, as the brain is controlling the body temperature. It will only lead to shivering as the body tries to rewarm to the new set point. This is uncomfortable and expends energy. Cooling is not recommended for fevers.
Heat stroke (hyperthermia) is caused by external factors such as a hot environment or overexertion in the heat. The brain set temperature is normal, but a dog is unable to cool effectively, and so body temperature rises.
Dogs cool by two mechanisms: evaporation and conduction. Evaporation of heat occurs with heavy panting. Conduction occurs when a hot dog lies on a cool surface and heat is transferred. Dogs sweat very minimally and only through their footpads, so this is not a significant means of cooling.
Heat stroke progresses through three stages. It begins with heat stress. Initially, a dog will pant heavily, seek a cool surface, and drink water to bring body temperature down.
If a dog cannot do these things or cannot do them effectively (such as when trapped in a hot car), heat stress develops into heat exhaustion. The panting becomes much faster, heart rate elevates, the gums become red and tacky, and body temperature is likely greater than 106 degrees.
If this goes unaddressed, heat stroke develops. Body temperature exceeds 109 degrees. A dog will vomit, have profuse diarrhea, begin to seizure, and collapse.
Most Susceptible Dogs
All dogs can have heat strokes, but some are more prone to developing problems. Brachycephalic breeds like Bulldogs, Boston Terriers, and Boxers are notorious for overheating. Due to breed-related airway abnormalities such as small nostrils, long floppy palates and tonsils, and narrow, weak windpipes, brachycephalic dogs can overheat very quickly.
However, Labradors and other breeds vulnerable to laryngeal paralysis, another obstructive upper airway disease, are also at significantly increased risk. Dogs with cardiac disease can be predisposed, as well.
Another consideration is whether a dog is conditioned enough to be in the heat. Dogs who are not accustomed to exercise and exert themselves in hot weather can suffer heat stroke very rapidly. This is especially true if they are overweight or elderly.
Most commonly, any dog left in an unventilated car in temperatures above 65 degrees can suffer from heat stroke. This is most often seen in the spring and summer.
Symptoms
Heat stroke is not immediate. Symptoms begin with heavy panting and restlessness. This progresses to weakness and collapse, followed by profuse vomiting and diarrhea (often bloody). As heat stroke continues, a dog will become extremely dull to non-responsive. Seizures can develop in the late stages. Red to purple spots and patches may become visible on the skin.
Once the heat stroke goes untreated, every body system becomes involved. Shock develops. This means decreased oxygen delivery to tissue, which eventually leads to multiple organ dysfunction syndrome (MODS).
In the kidneys, the renal tubules (responsible for conserving water and electrolytes) are damaged. Water, glucose, and electrolytes are lost instead of conserved. Potassium, instead of being excreted, is retained. High levels of potassium causes cardiac arrhythmias.