Fever in the Adult Patient

Epidemiology

Morbidity and mortality rates from febrile illnesses vary dramatically with age. Younger adults with fever usually have benign self-limited disease with low mortality. The challenge in this group is to identify rare causes of fever such as meningitis or sepsis when confronted with a predominance of self-limited viral and focal bacterial diseases. Patients older than 65 years, or those with chronic disease who have fever, represent a group at higher risk for serious disease with higher rates of morbidity and mortality. For example, the incidence of community-acquired pneumonia is 2.6 times greater in adults 65 to 79 compared with younger adults, and in those over 79 years, it is seven times greater. The relative mortality and morbidity rates for any given infection are much higher in the geriatric population; more than 50% of sepsis cases occur in patients older than 65 years, with a resultant mortality up to 26%. Even viral illnesses that are generally not fatal, such as influenza or COVID-19, can be lethal in older adults. Three body systems—the respiratory tract, urinary tract, and skin and soft tissue—are the target for more than 80% of these infections.

Pathophysiology

Body temperature is normally controlled within a narrow range by the preoptic area of the hypothalamus. In the anterior hypothalamus, neurons directly sense the blood temperature. Temperature is subsequently controlled by a combination of vasomotor changes, shivering, changes in metabolic heat production, and behavioral changes. Normal temperature range is usually 36.0°C to 37.8°C (96.8°F to 100.0°F). There is a circadian rhythm within this range, with lower temperatures in the morning and higher temperatures in the late afternoon. Fever occurs when this normal range is reset to a higher value.

There is no consensus on the threshold core temperature that defines fever. The Centers for Disease Control and Prevention define fever as a core temperature greater than 38.0°C (100.4°F) in the absence of fever-reducing medication. However, most authorities agree that a core body temperature of 38.3°C (100.9°F) represents a significant fever. Fever is distinct from hyperthermia. Hyperthermia is an elevation of the temperature related to the inability of the body to dissipate heat; fever is the elevation of body temperature caused by thermoregulatory pathways in response to infections and certain other medical circumstances ( Box 8.1 ). Most cases of temperatures higher than 41.0°C (105.8°F) are a result of hyperthermia, but febrile illness may also be considered.