fever

fever a term derived from the Latin febris, refers to an elevation of body temperature due to disease or injury. Man is a homeotherm, meaning that body temperature is kept within narrow limits by complex control mechanisms. This is distinct from poikilotherms, such as reptiles and amphibians, whose body temperature is just above the ambient temperature, and varies with it. Measured in the mouth, body temperature is close to 37°C (98.6°F), and does not usually vary by more than 1.5°C, although a rise up to 40°C can occur transiently in strenuous muscular exercise. In excessive heat exposure, normal temperature regulation is defeated, resulting in heat stroke. In other circumstances any deviation from the norm in an upward direction constitutes fever. Fever, or pyrexia, refers to a rise of up to 40.5°C and hyperpyrexia to a greater rise. Above about 41.5°C a person loses orientation and may become unconscious.

In some diseases the word ‘fever’ is incorporated into the common names, such as in puerperal, scarlet, typhoid, and yellow fevers, indicating that a rise in termperature is associated with the condition. Before the advent of mass vaccination programmes those with infectious fevers were taken in to isolation hospitals, often called fever hospitals. The word ‘fever’ is sometimes used in other contexts, such as ‘fever pitch’, when an individual, group, or crowd (such as a football crowd) becomes over-excited or agitated.

The body has a complex mechanism for controlling temperature that balances heat production against heat loss. Heat is continually produced by metabolism of all body cells, to an extent that varies with the activity of glands and organs, and of the muscles (shivering is an effective way to increase heat production). Heat is lost by radiation and convection, particularly from exposed parts such as the face and hands, and by evaporation of sweat. The control of the balance between heat production and heat loss is centred in the brain, in the hypothalamus, which acts basically as a thermostat. Input signals from heat- and cold-sensitive receptors in the skin relay information to the hypothalamus (these receptors are extremely sensitive: the heat receptors are able to detect a rapid rise in temperature of 0.007°C, while the cold receptors can detect a rapid fall of 0.012°C) and the ‘thermostat’ also senses the temperature of the blood passing through. While the discomfort of the experience of hot and cold environments resides in the skin, it is the body's ‘core’ temperature that matters, as many processes within the body are disrupted if the core temperature changes. Thus if the blood temperature rises, then the output signals from the thermostat lead to vasodilation of blood vessels in the skin and to sweating, thus increasing heat loss. Or, if body temperature falls, then heat loss is curtailed by vasoconstriction of surface blood vessels, and heat production may be increased by shivering.

In fever the hypothalamic thermostat becomes set at a higher temperature. The normal blood temperature is therefore sensed as being too low, and temperature-raising mechanisms come into action, accounting for initial pallor and shivering. Conversely, when fever is abating the set temperature is lowered, and the warm skin and sweating represent heat loss mechanisms.

The commonest cause of fever is infection by viruses, bacteria, yeasts, or parasites. Substances are released by these organisms which are collectively called pyrogens (substances causing a rise in temperature — pyrexia). The pyrogens act upon white blood cells to produce further, endogenous, pyrogens; these latter can also be released from tumours, from the brain after injury or stroke, from blood clots, or in autoimmune disease. The endogenous pyrogens interact in the brain with prostaglandin synthetase, the enzyme necessary for synthesis of prostaglandins, which in turn are the main agents that alter the setting of the ‘thermostat’. This explains why taking aspirin can abolish fever, since it inhibits prostaglandin synthetase; it also explains why, in the absence of fever, aspirin has no effect on body temperature.

In general, it is the practice to use drugs to reduce fever, but this may reduce the effectiveness of macrophages (white cells) to engulf and destroy bacteria. Experimental evidence indicates that prevention of pyrexia is detrimental to survival in infected animals. It is equivocal whether or not fever can be universally regarded as a body defence mechanism, particularly as its usefulness or otherwise where there is no infection is obscure.

Alan W. Cuthbert

See also heat exposure; injury; prostaglandins; temperature regulation.