We talk about a fever when the body temperature increases above normal values, i.e. above 37.2-37.5°C.
Causes, when to worry and what to do.
This alteration is caused by a shift to pathological values of the hypothalamic regulatory system, due to the action of endogenous chemical mediators: the pyrogenic cytokines. These endogenous pyrogens are released in response to the action of infectious agents and their products (exogenous pyrogens) or to inflammatory and necrotic foci of a non-infectious nature. Fever is, therefore, an acute phase immune response to various causes capable of inducing infection and/or disease and represents a clinical sign common to many medical conditions.
Body temperature: normal values and alterations
For our organism, it is very important to maintain a relatively constant internal temperature, since in this way the various metabolic processes can occur at an optimal speed.
Fever represents a reaction of the organism to phenomena capable of influencing the body temperature value, such as an illness or an infection, but it is not in itself an illness. This manifestation is to be considered, in fact, a symptom.
Normally, in a healthy individual the body temperature is between 36.4 and 37.2 degrees centigrade (°C). However, during the day the internal thermal environment undergoes variations linked to the cyclic activity of the metabolism and of some biological substances secreted with cardiac rhythm.
Did you know that…
The fluctuation in body temperature during the day means that around 6 pm the athlete reaches his maximum performance level. A temperature slightly higher than normal in fact improves the speed of propagation of nervous stimuli and increases metabolism, facilitating energy production.
How is fever detected?
The temperature varies slightly in various parts of the human body. In clinical practice, the parameter is measured at one of the following points: oral cavity, axilla or rectum.
Did you know that…
The body temperature is one of the vital parameters useful for monitoring a patient’s condition: measuring body temperature can be useful for checking whether a person is ill or whether the therapeutic treatment is working adequately.
The value detected in the rectum is closer to the real internal temperature with greater precision (approximately 37°C ± 0.5°C). The axillary temperature, however, is lower than the central one (36.6°C ± 0.5°C), as is the sublingual temperature which is approximately 0.2-0.5°C lower.
The most reliable temperature would, in any case, be the lower esophagus.
At what values are we talking about fever?
Body temperature is in the range of 36.4-37.2°C.
Fever is defined when in an adult the body temperature exceeds 37.2-37.5°C.
A child has a fever when the temperature is at or above one of these levels:
38°C measured rectally;
37.5°C measured orally;
37.2°C measured under the arm (axillary).
Body temperature is regulated by the balance between thermogenesis and heat dissipation, meaning the production and subsequent release of heat by the body.
Our body constantly produces heat (thermogenesis) as a by-product of the chemical reactions (metabolism) that continuously occur in all cells. If the heat produced were not released, the average human body temperature would increase by approximately 1.5°C per hour, even at rest and under normal environmental conditions.
The body maintains a dynamic equilibrium between heat input (thermogenesis and absorption) and output (heat dissipation), primarily through water evaporation from the mucous membranes (enhanced by breathing) and sweating (about 30 ml of sweat per hour through perspiratio insensibilis). This balance keeps the core body temperature around 37°C, since the body eliminates as much heat as it absorbs or produces.
Fever should not be confused with heat stroke, a particularly dangerous condition in which the increase in body temperature is not spontaneous, but triggered by an external factor, such as entering a car left in the blazing sun for hours.
The thermoregulatory center of the body is located in the hypothalamus. This physiological “thermostat” not only receives signals from peripheral receptors (hot–cold), but is also directly sensitive to the temperature of the blood that flows through it. The hypothalamus, in turn, sends efferent signals that regulate both heat production and heat loss, through the sympathetic nervous system and the somatomotor system.
Fever itself is not a disease, but rather a symptom that arises in response to an underlying illness.
Fever is caused by a modification of the set point value regulated by the hypothalamus, which forces the thermoregulatory system to raise the body’s temperature in a controlled manner to higher-than-normal levels.
In most cases, fever is a response to bacterial or viral infections that stimulate certain immune system cells (white blood cells) to multiply and release various chemical substances, including cytokines. Some cytokines act on the brain’s thermoregulatory centers by raising the set point value and therefore behave as pyrogens (fever inducers). Since the temperature increase caused by endogenous pyrogens (e.g., IL-1 and TNF-α) boosts the speed of many immune responses against invading microorganisms, fever is considered beneficial, as it improves the body’s ability to defend itself.
Moreover, most pathogens are killed at temperatures close to 40°C, making fever a valuable defense mechanism.
Cytokines also have pain-inducing effects and can lead to loss of appetite, nausea, and general discomfort. This explains the common association of fever with symptoms such as headache and muscle aches.
The hypothalamus is able to increase body temperature by controlling the function of various organs and tissues. Among the body’s many tools to generate more heat while reducing heat loss, the most important include:
Vasoconstriction (reduction of blood flow to the skin)
Increased muscle tone (shivering), increased heart rate (tachycardia), and a general increase in metabolic activity (higher basal metabolic rate)
This temperature rise leads to both inhibition of pathogen proliferation and a marked enhancement of immune cell activity. In fact, even a half-degree increase over normal temperature can significantly boost the responsiveness of B and T lymphocytes against invading pathogens.
Fever Classification
Based on recorded temperature, fever can be classified as follows:
Low-grade fever (febbricola): when temperature does not exceed 38°C (100.4°F)
Mild fever: between 38°C and 38.5°C (100.4–101.3°F)
Moderate fever: between 38.5°C and 39°C (101.3–102.2°F)
High fever: between 39°C and 39.5°C (102.2–103.1°F)
Hyperpyrexia: between 39.5°C and 41°C (103.1–105.8°F)
Fever Patterns
Depending on how it progresses, fever can be described as:
Continuous fever: daily temperature fluctuations are less than 1°C, and the fever remains persistent over time.
Remittent fever: daily fluctuations (at least two) exceed 1°C, but the temperature never returns to baseline; this is typical of most febrile illnesses.
Intermittent fever: temperature rises and falls during the day, returning to normal by morning. If the variations are extreme, it is called septic fever. Intermittent fever is characteristic of certain malignant neoplastic diseases.
Recurrent fever: febrile episodes are separated by periods during which body temperature returns to normal. This pattern is typical of illnesses such as malaria, brucellosis, and Borrelia infections.
Symptoms Associated with Fever
The onset of fever may be accompanied by other symptoms, depending on the underlying cause. These can include:
Paleness (due to peripheral vasoconstriction)
Goosebumps (piloerection)
Chills and cold sensation
Fatigue or irritability
General malaise
Sweating
Headache
Muscle pain
Other Warning Signs to Report to a Doctor
Additional alarm symptoms that may occur with fever and should be reported promptly to your primary care physician include:
Sore throat
Skin rash
Nausea and vomiting
Swollen lymph nodes
Shortness of breath (dyspnea)
Chest tightness or pressure
Loss of consciousness, hallucinations, or seizures
As we have seen, the feverish response is nothing more than a powerful and effective means of defense. The increase in temperature can therefore be considered as a real medicine that our body possesses to defend itself from viral and bacterial infections.
Depriving the body of such important support by taking antipyretic drugs (capable of lowering body temperature) could in many cases have the opposite effect than hoped for.
For example, the use of antibiotics can lower the immune system by eliminating not only pathogenic bacteria, but also useful ones. Furthermore, if the disease is caused by viruses, this class of drugs proves not only to be completely ineffective but even harmful because, for the reasons mentioned above, it lengthens recovery times and favors the recurrence of new infectious episodes.
Therefore, if the fever is not particularly high, the most effective therapy is rest combined with the classic recommendations which include the consumption of easily digestible meals, abundant water intake and abstention from smoking or alcohol.
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