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1. an abnormally high body temperature; called also pyrexia. adj., adj fe´brile, fe´verish.
2. any disease characterized by marked increase of body temperature. For specific diseases, see the eponymic or descriptive name, such as rocky mountain spotted fever or typhoid fever. Other conditions involving elevated body temperature include heat exhaustion and heat stroke.

Normal body temperature when the body is at rest is 37°C (98.6°F). This is an average or mean body temperature that varies from person to person and from hour to hour in an individual. The route by which a body temperature is measured affects the reading. The normal oral temperature ranges from 36° to 37.5°C (96.8° to 99.5°F). If the temperature is measured rectally, the norm would be 0.5°C (1°F) higher. An axillary temperature would be 0.5°C (1°F) lower. Because of these differences, the number should always be followed by the route by which the temperature was taken when the reading is recorded.

Factors that can cause a temporary elevation in body temperature include age, physical activity, emotional stress, and ovulation. If a person has a consistently elevated temperature, fever is said to exist. A low-grade fever is marked by temperatures between 37.5° and 38.2°C (99.5° and 101°F) when taken orally. A high-grade fever is present when the oral temperature is above 38.2°C (101°F).

Types of fever include continued or continuous fever, one lasting more than 24 hours without significant variation or any return to normal body temperature; intermittent fever, in which at least once during a 24-hour period the fever spikes are separated by a return to normal body temperature; remittent fever, in which elevated body temperature shows fluctuations each day but never returns to normal; and recurrent (or relapsing) fever, in which periods of fever and normal body temperature alternate and last about 5 to 7 days each.

The regulation of body temperature is under the control of the hypothalamus. Thermolysis, or dissipation of body heat, is regulated by the anterior hypothalamus in conjunction with the parasympathetic nervous system. The overall effect of heat loss is accomplished by vasodilation of the peripheral blood vessels, increased sweating, and decreased metabolic and muscular activities. The production and conservation of body heat, or thermogenesis, is regulated by the posterior hypothalamus in conjunction with the sympathetic nervous system. The mechanisms by which body heat is produced and conserved are in opposition to those that increase heat loss; that is, by constriction of cutaneous blood vessels, decreased sweat gland activity, and increased metabolic and muscular activities.

Fever develops when there is some disturbance in the homeostatic mechanisms by which the hypothalamus maintains a balance between heat production and peripheral heat loss. Although dehydration, cerebral hemorrhage, heat stroke, thyroxine, and certain other drugs can cause an elevated body temperature or hyperthermia, fever, in the precise sense of the term, occurs as a result of inflammation or infection, or both. During the infectious and inflammatory processes certain substances called pyrogens are produced within the body. These endogenous pyrogens are the result of inflammatory reactions, such as those that occur in tissue damage, cell necrosis, rejection of transplanted tissues, malignancy, and antigen-antibody reactions. Exogenous pyrogens are introduced into the body when it is invaded by bacteria, viruses, fungi, and other kinds of infectious organisms.

Endogenous pyrogens act directly on the hypothalamus, affecting its thermostatic functions by “resetting” it to a higher temperature. When this happens, all of the physiologic activities concerned with heat production and conservation operate to maintain body temperature at a higher setpoint. The symptoms of chill and shivering are the result of increased muscular activity, which is an attempt by the body to raise its temperature to the higher setting. This increased muscular activity is accompanied by an elevation of the metabolic rate, which in turn increases the demand for nutrients and oxygen. Outward signs of these internal activities include a higher pulse rate, increased respirations, and thirst caused by the loss of extracellular water via the lungs. The pulse rate increases at the rate of about eight to ten beats per minute for each degree of temperature rise.

Once the body temperature reaches the setpoint of the hypothalamic thermostat, the mechanisms of heat production and heat loss keep it at a fairly constant level and the fever persists. This is sometimes called the second stage of fever. If it continues, fluid and electrolyte losses become more severe and there is evidence of cellular dehydration. During this stage delirium in older persons and convulsions in infants and children can occur. Febrile convulsions in children are believed to be closely related to cerebral damage that becomes evident as afebrile convulsions later in life.

Prolonged fever eventually brings about tissue destruction owing to the catabolism of body proteins. Because of this the patient experiences muscle aches and weakness, malaise, and the excretion of albumin in the urine. Anorexia also is present. If the body does not receive a sufficient supply of energy from dietary intake to meet its metabolic needs, it catabolizes its own fat and protein. The patient then rapidly loses weight and can develop ketosis and metabolic acidosis.

The period during which a fever abates is called the period of defervescence. It may occur rapidly and dramatically, as the temperature falls from peak to normal in a matter of hours. This is called the crisis, that is, the critical point at which the fever is broken. A more gradual resetting of the thermostat and slow decline of the fever is called resolution of the fever by lysis.
Treatment. It is not always necessary to reduce fever and in many cases it may be best not to treat it, at least until its cause is determined. The fever pattern can provide diagnostic information and is not necessarily harmful unless it is extremely high or the patient has cardiac or respiratory disease and cannot tolerate the additional tachycardia and dyspnea that may accompany fever. An elevated body temperature can inhibit bacterial replication and the action of viruses, spirochetes, and other pathogenic microorganisms.

If it is decided that treatment is necessary, there are two major goals: to identify the cause and to provide symptomatic relief. Antipyretic drugs such as aspirin and acetaminophen (Tylenol) are generally safe and effective. However, acetaminophen is preferred in children and when the patient has gastrointestinal sensitivity, allergy to aspirin, or a clotting disorder or is suspected of having Reye's syndrome.

Fluids and electrolytes are replaced orally or intravenously as indicated by laboratory tests and signs of dehydration. Frequent, small feedings of high-calorie, high-protein foods are recommended to combat fatigue and debility caused by the increased metabolic rate. The selection of oral liquids and foods should be based on the patient's preferences. Vitamin supplements may be prescribed in prolonged, low-grade fevers.
Patient Care. The patient with acute hyperpyrexia or hyperthermia will require extreme measures to lower the body temperature as quickly and safely as possible in order to prevent brain damage. Victims of heat stroke should be cooled rapidly. In order to keep the temperature at a tolerable level until the thermostat is reset, a cooling blanket or hypothermia mattress may be used. Care must be taken to maintain the integrity of the skin and avoid sudden and extreme hypothermia when such a device is used. Other measures include sponging parts of the body with cool water to increase heat loss through evaporation of moisture. The part being sponged should be left exposed to the air until it is almost dry, and then should be lightly covered while another part is being sponged. A cold compress on the forehead helps to reduce the fever and relieve headache and delirium. An alternative to sponging and a cool bath is the application of ice packs to specific parts of the body, such as the abdomen, groin, axillae, and spine. Fanning can also be effective, especially if the patient's torso is covered with a sheet saturated with water.

Chills are uncomfortable and sometimes frightening to the patient. When the patient complains of feeling chilled or cold, some form of external warmth should be provided. An extra blanket is helpful as is a hot water bottle filled with warm, not hot, water. As the body temperature declines the difference between body temperature and environmental temperature will decrease and the patient will begin to feel warmer. During the second stage of fever the patient may complain of feeling hot; the skin feels warm to the touch and the face is flushed. These symptoms are the result of vasodilation of surface blood vessels, an attempt by the body to prevent further escalation of the body temperature.
fever of unknown origin (FUO) a febrile illness of at least three weeks' duration with a temperature of at least 38.3°C on at least three occasions and failure to establish a diagnosis in spite of intensive inpatient or outpatient evaluation (three outpatient visits or three days' hospitalization). The duration of febrile illness needed to establish a diagnosis of FUO varies among authorities and is sometimes given as shorter than three weeks.

Classic fever of unknown origin, as defined by the preceding criteria, is distinguished from neutropenic and nosocomial FUO, as well as that associated with human immunodeficiency virus infection. In the neutropenic form, fever is accompanied by a neutrophil level that is lower than 500/mm3 or is expected to fall below that level within one or two days. The nosocomial form is a fever that occurs on several occasions in a hospitalized patient in whom neither fever nor infection was present on admission. In HIV-associated FUO, fever occurs in a person with human immunodeficiency virus infection on several occasions over a period of four weeks of outpatient care or three days of hospitalization. In all three of these forms of FUO, the cause of the fever cannot be determined after three days of investigation, including two days of incubation of cultures.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


Abbreviation for fever of unknown origin.
Farlex Partner Medical Dictionary © Farlex 2012


Fever of unknown origin, see there.
McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc.


Abbreviation for fever of unknown origin.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012
References in periodicals archive ?
burnetii antibodies were detected in a relatively large proportion of patients with FUO. Evidence continues to emerge documenting the presence of Q fever in livestock and humans in the Kingdom of Saudi Arabia.
Aletaha, "Fever of unknown origin (FUO) revised," Wiener Klinische Wochenschrift, vol.
According to this study, marrow showing reactive changes is the most frequent finding in patients with FUO as seen in 27.5% of cases (relative frequency 0.275).
With regard to the final diagnosis, 248 patients (66.3%) were diagnosed with an infection etiology for FUO. Of these patients, 46 had cytomegalovirus infection (CMV).
Training related injuries (207), chicken pox (110) and fever of fever unspecified (FUO) (102) were the main reasons for morbidity and temporary disability in recruits.
Each primary infection (PI) and SI was classified as a microbiologically documented infection (MDI), clinically documented infection (CDI) (objective and detectable signs of infection with a lack of microbiological documentation), or fever of unknown origin (FUO).
Fuo fo erioed, a dyna pam mae cymunedau Cymraeg yn marw.
When fever is seen as prolonged beyond the expected time course (e.g., 10 days for a presumed viral respiratory tract infection or three weeks for mononucleosis), concern for fever of unknown origin (FUO) may ensue.
Fever of unknown origin (FUO) was described in 1961 by Petersdorf and Beeson with the criteria of fever above 38.3[degrees]C lasting more than three weeks without a diagnosis despite a one-week hospitalized examination (1).