AskDefine | Define pyrexia

Dictionary Definition

pyrexia n : a rise in the temperature of the body; frequently a symptom of infection [syn: fever, febrility, febricity, feverishness]

User Contributed Dictionary



  1. fever

Extensive Definition

This is about the medical condition. For other uses, see Fever (disambiguation).
Fever (also known as pyrexia, from the Greek pyretos meaning fire, or a febrile response, from the Latin word febris, meaning fever, and archaically known as ague) is a frequent medical symptom that describes an increase in internal body temperature to levels above normal. Fever is most accurately characterized as a temporary elevation in the body's thermoregulatory set-point, usually by about 1–2°C.
Fever differs from hyperthermia; hyperthermia is an increase in body temperature over the body's thermoregulatory set-point (due to excessive heat production or insufficient thermoregulation, or both). Carl Wunderlich discovered that fever is not a disease but a symptom of disease.
The elevation in thermoregulatory set-point means that the previous "normal body temperature" is considered hypothermic, and effector mechanisms kick in. The person who is developing the fever has a cold sensation, and an increase in heart rate, muscle tone and shivering attempt to counteract the perceived hypothermia, thereby reaching the new thermoregulatory set-point. A fever is one of the body's mechanisms to try to neutralize the perceived threat inside the body, be it bacteria or a virus.

Measurement and normal variation

When a patient has or is suspected of having a fever, that person's body temperature is measured using a thermometer.
At a first glance, fever is present if:
  • temperature in the anus (rectum/rectal) or in the ear (otic) is at or over 38.0°C (100.4°F)
  • temperature in the mouth (oral) is at or over 37.5°C (99.5°F)
  • temperature under the arm (axillary) is at or over 37.2°C (99.0°F)
The common oral measurement of normal human body temperature is 36.8±0.7 °C (98.2±1.3 °F). This means that any oral temperature between 35.9 and 37.5 °C (96.9 and 99.5 °F) is likely to be normal.
However, there are many variations in normal body temperature, and this needs to be considered when measuring for fever. The values given are for an otherwise healthy, non-fasting adult, dressed comfortably, indoors, in a room that is kept at a normal room temperature (73° to 76°F) , during the morning, but not shortly after arising from sleep. Furthermore, for oral temperatures, the subject must not have eaten, drunk, or smoked anything in at least the previous fifteen to twenty minutes.
Body temperature normally fluctuates over the day, with the lowest levels around 4 a.m. and the highest around 6 p.m. Therefore, an oral temperature of 37.2°C (99.0°F) would strictly be a fever in the morning, but not in the afternoon. An oral body temperature reading up to 37.5 °C (99.5 °F) in the early/late afternoon or early/late evening also wouldn't be a fever. Normal body temperature may differ as much as 1.0 °F between individuals or from day to day. In women, temperature differs at various points in the menstrual cycle, and this can be used for family planning (although temperature is only one of the variables). Temperature is increased after meals and eating, and psychological factors also influence body temperature.
There are different locations where temperature can be measured, and these differ in temperature variability. Tympanic membrane thermometers measure radiant heat energy from the tympanic membrane (infrared). These may be very convenient, but may also show more variability.
Children develop higher temperatures with activities like playing, but this is not fever because their set-point is normal. Elderly patients may have a decreased ability to generate body heat during a fever, so even a low-grade fever can have serious underlying causes in geriatrics.


Temperature is regulated in the hypothalamus, in response to PGE2. PGE2 release, in turn, comes from a trigger, a pyrogen. The hypothalamus generates a response back to the rest of the body, making it increase the temperature set-point.


A pyrogen is a substance that induces fever. These can be either internal (endogenous) or external (exogenous). The bacterial substance lipopolysaccharide (LPS) is an example of an exogenous pyrogen.


The cytokines (such as interleukin 1) are a part of the innate immune system, produced by phagocytic cells, and cause the increase in the thermoregulatory set-point in the hypothalamus. Other examples of endogenous pyrogens are interleukin 6 (IL-6), and tumor necrosis factor-alpha.
These cytokine factors are released into general circulation where they migrate to the circumventricular organs of the brain, where the blood-brain barrier is reduced. The cytokine factors bind with endothelial receptors on vessel walls, or interact with local microglial cells. When these cytokine factors bind, they activate the arachidonic acid pathway.


One model for the mechanism of fever caused by exogenous pyrogens includes LPS, which is a cell wall component of gram-negative bacteria. An immunological protein called lipopolysaccharide-binding protein (LBP) binds to LPS. The LBP–LPS complex then binds to the CD14 receptor of a nearby macrophage. This binding results in the synthesis and release of various endogenous cytokine factors, such as interleukin 1 (IL-1), interleukin 6 (IL-6), and the tumor necrosis factor-alpha. In other words, exogenous factors cause release of endogenous factors, which, in turn, activate the arachidonic acid pathway.

PGE2 release

PGE2 release comes from the arachidonic acid pathway. This pathway (as it relates to fever), is mediated by the enzymes phospholipase A2 (PLA2), cyclooxygenase-2 (COX-2), and prostaglandin E2 synthase. These enzymes ultimately mediate the synthesis and release of PGE2.
PGE2 is the ultimate mediator of the febrile response. The set-point temperature of the body will remain elevated until PGE2 is no longer present. PGE2 acts on neurons in the preoptic area (POA) through the prostaglandin E receptor 3 (EP3). EP3-expressing neurons in the POA innervate the dorsomedial hypothalamus (DMH), the rostral raphe pallidus nucleus in the medulla oblongata (rRPa) and the paraventricular nucleus of the hypothalamus (PVN). Fever signals sent to the DMH and rRPa lead to stimulation of the sympathetic output system, which evokes non-shivering thermogenesis to produce body heat and skin vasoconstriction to decrease heat loss from the body surface. It is presumed that the innervation from the POA to the PVN mediates the neuroendocrine effects of fever through the pathway involving pituitary gland and various endocrine organs.

Hypothalamus response

The brain ultimately orchestrates heat effector mechanisms via the autonomic nervous system. These may be The autonomic nervous system may also activate brown adipose tissue to produce heat (non-exercise-associated thermogenesis, also known as non-shivering thermogenesis), but this seems mostly important for babies. Increased heart rate and vasoconstriction contribute to increased blood pressure in fever.


According to one common rule of thumb, pyrexia (fever) is generally classified for convenience as:
The last is a medical emergency because it approaches the upper limit compatible with human life.
Most of the time, fever types can not be used to find the underlying cause. However, there are specific fever patterns that may occasionally hint the diagnosis:
  • Pel-Ebstein fever: A specific kind of fever associated with Hodgkin's lymphoma, being high for one week and low for the next week and so on. However, there is some debate as to whether this pattern truly exists.
  • Continuous fever: Temperature remains above normal throughout the day and does not fluctuate more than 1°C in 24 hours, e.g. lobar pneumonia, typhoid, urinary tract infection, brucellosis, or typhus. Typhoid fever may show a specific fever pattern, with a slow stepwise increase and a high plateau.
  • Intermittent fever: Elevated temperature is present only for some hours of the day and becomes normal for remaining hours, e.g. malaria, kala-azar, pyaemia, or septicemia. In malaria, there may be a fever with a periodicity of 24 hours (quotidian), 48 hours (tertian fever), or 72 hours (quartan fever, indicating Plasmodium malariae). These patterns may be less clear in travelers.
  • Remittent fever: Temperature remains above normal throughout the day and fluctuates more than 1°C in 24 hours, e.g. infective endocarditis.
A neutropenic fever, also called febrile neutropenia, is a fever in the absence of normal immune system function. Because of the lack of infection-fighting neutrophils, a bacterial infection can spread rapidly and this fever is therefore usually considered a medical emergency. This kind of fever is more commonly seen in people receiving immune-suppressing chemotherapy than in apparently healthy people.
Febricula is a mild fever of short duration, of indefinite origin, and without any distinctive pathology.


Fever is a common symptom of many medical conditions:
Persistent fever which cannot be explained after repeated routine clinical inquiries, is called fever of unknown origin.

Usefulness of fever

"Give me a fever, and I can cure any illness." -- Hippocrates (ca. 400 BC)
There are arguments for and against the usefulness of fever, and the issue is controversial. There are studies using warm-blooded vertebrates and humans in vivo, with some suggesting that they recover more rapidly from infections or critical illness due to fever.
Theoretically, fever can aid in host defense. The overall conclusion seems to be that both aggressive treatment of fever has demonstrated that fever has several important functions in the healing process:
Treatment of fever is normally done by lowering the set-point, but facilitating heat loss may also be effective. The former is accomplished with antipyretics such as ibuprofen or acetominophen (aspirin can be given to adults, but can cause Reye's Syndrome in children). Heat removal is generally by wet cloth or pads, usually applied to the forehead. Heat loss may also be accomplished by heat conduction, convection, radiation, or evaporation (sweating, perspiration), or a combination of these. This is particularly important for babies, where drugs should be avoided. However, using water that is too cold can induce vasoconstriction, and reduce effective heat loss.


Further reading

  • Rhoades, R. and Pflanzer, R. Human physiology, third edition, chapter 27 Regulation of body temperature, p. 820 Clinical focus: pathogenesis of fever. ISBN 0-03-005159-2
  • Kasper, D.L.; Braunwald, E.; Fauci, A.S.; Hauser, S.L.; Longo, D.L.; Jameson, J.L. Harrison's Principles of Internal Medicine. New York: McGraw-Hill, 2005. ISBN 0-07-139140-1.
pyrexia in Arabic: حمى
pyrexia in Bengali: জ্বর
pyrexia in Bulgarian: Треска (болест)
pyrexia in Catalan: Febre
pyrexia in Czech: Horečka
pyrexia in Danish: Feber
pyrexia in German: Fieber
pyrexia in Modern Greek (1453-): Πυρετός
pyrexia in Spanish: Fiebre
pyrexia in Esperanto: Febro
pyrexia in Basque: Sukar
pyrexia in French: Fièvre
pyrexia in Hindi: ज्वर
pyrexia in Ido: Febro
pyrexia in Indonesian: Demam
pyrexia in Italian: Febbre
pyrexia in Hebrew: חום (תסמין)
pyrexia in Latin: Febris
pyrexia in Lingala: Fɛ́fɛlɛ
pyrexia in Hungarian: Láz
pyrexia in Malayalam: പനി
pyrexia in Malay (macrolanguage): Demam
pyrexia in Dutch: Koorts
pyrexia in Newari: ज्वर
pyrexia in Japanese: 発熱
pyrexia in Norwegian: Feber
pyrexia in Norwegian Nynorsk: Feber
pyrexia in Pushto: تبه
pyrexia in Polish: Gorączka
pyrexia in Portuguese: Febre
pyrexia in Quechua: Ruphariy
pyrexia in Russian: Лихорадка
pyrexia in Sicilian: Frevi
pyrexia in Sinhala: උණ
pyrexia in Simple English: Fever
pyrexia in Slovak: Horúčka
pyrexia in Slovenian: Vročina
pyrexia in Finnish: Kuume
pyrexia in Swedish: Feber
pyrexia in Tamil: காய்ச்சல்
pyrexia in Telugu: జ్వరం
pyrexia in Ukrainian: Лихоманка
pyrexia in Yiddish: פיווער
pyrexia in Chinese: 发热
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