Body temperature in humans is regulated by an area of the brain called hypothalamus. The thermoregulatory center here keeps the core body temperature within a closely set limit. The body temperature varies by 0.5 ◦ C during a 24 hour period being lowest at about 6 A.M. and maximum at about 4 to 6 P.M.

A temperature of > 37.2◦ C in the morning at 6 and > 37.7 ◦ C in the afternoon is considered to suggest the occurrence of fever.

In most fevers, the body temperature is set at a higher level by increased level of Prostaglandin E in the blood and brain tissues in the hypothalamus. This occurs due to the production of various chemicals in the body.

In cases of infection or inflammation, various cytokines (IL -1, IL-6, TNF etc.) cause a rise in PGE. This may be mediated by production of various toxins e.g. Endotoxins, TSST etc. Once the temperature is set at a higher level, more heat is produced in the body and less is dissipated. This leads to higher core temperature.

The mechanisms for decreasing heat loss from the skin is by vasoconstriction (narrowing of blood supply to the skin). This allows less heat to be lost by the skin and is perceived as cold skin. More heat is produced by increased muscle contraction (shivering) or added heat production from the liver by increasing metabolism. These soon result in a higher body temperature. In most fevers, diurnal variations continue.

Covering the body by blankets raises the temperature faster as heat dissipation is no longer available.

Hyperthermia is a condition where body temperature is elevated without a higher “set point “ for the temperature in the brain. This can be rapidly fatal and in this condition, antipyretics do not work. This occurs either because heat is not lost from the body due to excessive insulation or environmental temperatures are very high as in heat stroke. Sometimes the cooling mechanism by sweating fails due to a skin condition. Metabolic derangements may also continuously produce heat. The use of drug “ECSTASY” and atropine may result in hyperthermia. Treatment is by rapid cooling by immersion in Ice, cooling blankets etc.

In hyperpyrexia, the temperature Is >41.5 ◦ C. However the set point is higher and treatment with antipyretics lowers the temperature. This can occur in severe infections and some diseases of the brain. Malignant hyperthermia is another condition in which temperatures are very high. Usually, there is a history of intake of drugs acting on CNS. This condition can also be very dangerous.

Treatment of fever is undertaken in common fevers if the temperature is >40 ◦ C or if headaches, body aches etc are troublesome. If temperatures are only moderately elevated, the antipyretics may not be required.

The antipyretics in common use are acetaminophen, aspirin, nimesulide and ibuprofen. These act by Cyclooxygenase inhibition which reduces PGE levels.  Quinine can also decrease temperature. Along with this, cooling by external means may be required in some cases. If antipyretics are taken, taking them in frequent doses reduces the discomfort due to “chills and rigors”.

Causes of Fever

Infections usually viral are a common cause of a fever. Most are self-limiting and last for a few days to 2 weeks. Bacterial, fungal and protozoal infections are also very common. Malaria, HIV, TB, Chickenpox, rubella, measles, picorna viruses, influenza are examples of infectious fevers.

Other causes of fever are inflammatory diseases, malignancies, drugs, metabolic conditions and various others. If the body’s defense mechanisms are lowered (due to low WBCs ), infections may become very dangerous. The rise of temperature does not correlate well with the severity of infections. In children, temperatures swing over wider ranges and settle as rapidly as they rise.