True Fever Versus Non-Febrile Hyperthermia

Body temperature is one of the most common parameters obtained during a physical examination. All too often, an elevation in temperature is assumed to be a true fever caused by an infectious agent or that a normal temperature indicates an absence of disease. The anterior hypothalamus controls thermoregulation by balancing heat loss and heat production maintaining a narrow temperature range for most dogs and cats of 100.0° – 102.0° (normal set point). Heat loss is prevented by vasoconstriction, piloerection, shivering, postural changes and other heat-seeking behaviors. Heat gain is inhibited by vasodilation of superficial vessels, panting, postural changes, cool seeking behaviors and grooming (cats).

Hyperthermia, which indicates an elevation of core body temperature, can be sorted into two categories: true fever and non-febrile hyperthermia. Non-febrile hyperthermia occurs when heat gained exceeds heat lost through conditions such as inadequate heat dissipation (heat stroke, hyperpyrexic syndrome), exercise hyperthermia (normal exercise, hypocalcemia, seizure disorders) or pathologic/pharmacologic origin (lesions in or around the anterior hypothalamus, malignant hyperthermia, hypermetabolic disorders). Treatment includes carefully cooling the patient with tepid water and a fan (NO ICE) until the temperature reaches 103.0° along with intravenous fluids and oxygen. Over zealous cooling can lead shivering (more heat production) and to vasoconstriction which will slow heat loss and decrease cutaneous blood flow. This causes capillary sludging which can contribute to the development of DIC (disseminated intravascular coagulation). Steroids and NSAIDs (non-steroidal anti-inflammatory drugs) are contraindicated in this condition due to the fact non-febrile hyperthermia is not an inflammatory process.

True fever is caused by the anterior hypothalamus resetting the body’s normal temperature set point after being exposed to exogenous pyrogens which in turn will trigger the release of endogenous pyrogens. Exogenous pyrogens include infectious agents (bacteria, bacterial products, fungi, viruses, rickettsia, protozoa), immune complexes, tissue inflammation/necrosis and several pharmacological agents. Endogenous pyrogens are cytokines that include interleukins, interferons, and tumor necrosis factors. Some fever-producing cytokines can be released directly from neoplastic cells as well. The cytokines trigger the arachidonic acid cascade releasing prostaglandins that cause the set point to be increased. The body will engage heat gain mechanisms to maintain the new higher body temperature.

Should a true fever be treated? The detrimental side effects of fever include increased oxygen consumption, increased carbon dioxide production, increased cardiac output and energy use. A febrile patient may then have a decreased appetite and less mobility which can further lead to a cascade of SIRS (systemic inflammatory response syndrome) and multiple organ failure. Fever in a head trauma patient should be addressed immediately as it increases the risk of mortality. There are, however, host benefits to having a fever. It may decrease the duration of morbidity from infectious diseases and may block replication of bacteria and viruses. A fever can also be used to monitor progress of treatment or indicate a new concern such as a nosocomial infection or additional inflammation such as phlebitis, post-transfusion reaction, aspiration pneumonia, or pancreatitis to name a few possibilities. True fevers of a duration more than 2-3 days or >106° need to be treated specifically with NSAIDS or steroids and possibly external cooling. Using total body cooling with water and/or fans will certainly bring the temperature down but the body will then use compensatory mechanisms (vasoconstriction, shivering) to raise the temperature back to the new set point which will counter-productively increase the metabolic rate and oxygen consumption. Unless the patient is critically ill with a temperature of >106°, a logical approach to assessing hyperthermia would include a thorough physical examination and thorough history. Ask questions about travel, vaccinations, flea/tick exposure, previous injuries or infections, exposure to other animals, signs of disease in other pets in the household as well as previous or current drug therapy. This will more clearly distinguish true fever from non-febrile hyperthermia and aid in the appropriate diagnosis and treatment of the underlying cause.

References available on request