Hyperthermia is the term that describes elevation in body temperature above normal ranges in response to a trigger such as inflammation in the body, excessive internal heat production, hot external environment or failure of the body to disseminate heat properly. While normal body temperatures vary slightly among different dogs, generally accepted body temperature of dogs are below 103° F (39° C). Temperatures above this are termed as hyperthermia.
Hyperthermia is categorized into fever and non-fever hyperthermia. When hyperthermia is due to inflammation in the body (such as bacterial infection), it is termed as fever hyperthermia. Non-fever hyperthermia results from all other causes of elevated body temperature such as hot external environment, excessive internal heat production and failure of body to disseminate heat properly. Other causes of non-fever hyperthermia include excessive exercise, hyperthyroidism, and lesions in hypothalamus — part of brain that regulates body temperature. Heat stroke is a form of non-fever hyperthermia that occurs due to inability of heat dissipating mechanism of the body to accomodate external heat. That is when heat gain exceeds body's ability to dissipate it. A temperature of 106° F (41° C) or higher without signs of inflammtion is associated with heat stroke and can lead to multiple organ dysfunction.
Malignant hyperthermia is another type of hyperthermia. This is a hereditary condition in which certain anesthetics cause high body temperatures and muscle rigidity.
During onset of hyperthermia, the dog's body temperature begins to ascend uncontrollably, due to failed thermoregulation. Thermoregulation is the ability of a mammal to maintain its body temperature within certain margins, despite fluctuations in the proximate temperature.
Although all breeds are susceptible to non-fever hyperthermia, brachycephalic dog breeds (dog breeds with wide heads and short stature) are generally more affected with non-fever hyperthermia. The condition tends to affect younger dogs more than older dogs.
There are various symptoms of non-fever hyperthermia and heat stroke including
Excessive drooling (ptyalism)
Increased body temperature – above 103° F (39° C)
Reddened gums and moist tissues of the body
Production of only small amounts of urine or no urine
Sudden (acute) kidney failure
Rapid heart rate
Irregular heart beats
Stoppage of the heart and breathing (cardiopulmonary arrest)
Fluid build-up in the lungs; sudden breathing distress (tachypnea)
Vomiting blood (hematemesis)
Passage of blood in the bowel movement or stool
Black, tarry stools
Small, pinpoint areas of bleeding
Generalized (systemic) inflammatory response syndrome
Disease characterized by the breakdown of red-muscle tissue
Death of liver cells
Changes in mental status
Wobbly, incoordinated or drunken gait or movement (ataxia)
Unconsciousness in which the dog cannot be stimulated to be awakened