True Fever vs. Other Causes of Elevated Body Temperature (Hyperthermia)

By November 20, 2012Blog

Hyperthermia is a broad term indicating elevated body temperature from a variety of causes.  A true fever is a term reserved for those hyperthermic cases where there is an underlying infectious, inflammatory, or cancerous process causing release of pyrogens that act upon the thermoregulatory control center “set-point” to result in an elevated resting body temperature.

In general, the resting body temperature of a mammal is a balance between the heat produced by the body as a result of metabolic processes and muscular activity, and the heat dissipated by panting, through the skin and foot pads, etc.  An imbalance between the heat produced and heat loss can result in hyperthermia in the absence of a true fever.

Heat stroke is a common example of insufficient ability to dissipate heat, either as a result of exposure to high ambient temperature (ie being kept inside a parked car on a hot sunny day), due to excessive exertion/exercise, or even mild activity if the pet is not accustomed to it or acclimated to the climate.  Once the pet is removed from the source of external/ambient heat, or the excessive activity stopped, and the pet is either actively cooled as described below or simply placed in air-conditioned environment, the body can start to restore the balance between heat production and heat loss.

Generalized seizure activity or severe muscular tremors can be considered an example of increased heat production, especially if prolonged.  Once the seizure activity or muscle tremors are treated by a veterinarian, the body can again start to restore the balance between heat production and heat loss.

Factors that can decrease the ability of a pet to adequately dissipate heat include obesity, brachycephalic breeds (like bull dogs or pugs), or laryngeal dysfunction or paralysis.  Maintaining a lean body weight is a general recommendation for all pets.  Minimizing exercise, excitement (especially in the hottest parts of the day) and keeping brachycephalic pets and pets with laryngeal dysfunction cool and in air-conditioning is important.

In a true fever, part of the acute phase response is to adjust the “set-point” of the body due to pyrogen release as a result of infectious, inflammatory or immune, or cancerous processes.  Active cooling in a patient with a true fever can be counter-productive by causing shivering, causing a secondary hyperthermia.  Identifying and treating the underlying cause of the fever is the most important, and over time with the appropriate treatment, the “set-point” will be re-adjusted to the accepted normal range.

With heat stroke, excessive seizure activity or severe muscular tremors, active cooling should be initiated at rectal temperatures of 106 F or above, as this is the temperature at which proteins and body tissues can begin breaking down.  Cooling the entire body with luke-warm or tepid water on a wet-sink and room-temperature intravenous (IV) fluid therapy is most appropriate.  Icepacks or ice baths can result in peripheral vasoconstriction that will not allow heat to dissipate from the skin, may shunt heat back to the body core, and may result in shivering and more heat production are methods that are less appropriate.  In extreme cases, surgical intra-abdominal lavage, gastric lavage or urinary bladder lavage with cool sterile saline can be attempted but are usually not necessary.  Active cooling efforts should cease after treatment of generalized seizures/muscle tremors, or when the body temperature reaches 103 F, as there is often a rebound hypothermia after active cooling.  The pet should be thoroughly dried with clean towels.  Glucocorticoids are not generally recommended unless a cancerous process is identified, there is no evidence to suggest an infectious process, and is part of a chemotherapy protocol.

If your dog or cat is suspected to have heat stroke due exposure to high ambient temperatures, has excessive muscle activity due to generalized seizures or muscle tremors, has exertional heat stroke, please call your primary care veterinarian or emergency veterinarian immediately.

Please discuss maintenance of a lean body weight at your pet’s next wellness visit.

Dr. Jessica Diaz