The desert iguana (Dipsosaurus dorsalis) is found only in the Sonoran and Mojave Deserts of the south-western United States and northern Mexico and on a few islands in the Gulf of California. They are well adapted to life in a warm and dry environment. They can tolerate body temperatures up to 46 °C. Courtesy of Tanya Dewey from Encyclopedia of Life under Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
Fever is an increase in body temperature usually due to infection. Since it is commonly believed to be a detrimental condition, people with fevers take drugs and other measures to decrease their temperatures as quickly as possible. Maybe the drug companies, in an attempt to market their fever-reducing products, have successfully portrayed fevers as harmful situations. Despite fever's negative publicity, would you believe that a fever, when not too high, is usually a beneficial condition?
How do we know that this apparently far-fetched idea is legitimate? Ask the lizards!
They know from experience. This idea was confirmed through an experiment on them. After a group of desert iguanas (see picture above) were infected with bacteria, some of the lizards were allowed to get a fever. Many lizards regulate their body temperature behaviourally, for instance by seeking shadow when too hot, and sunshine when too cool. By such behaviour, these infected lizards "chose" to increase their body temperature, i.e. to get themselves a fever. The rest of the lizards were prevented from getting a fever. The group of lizards who got a fever had a much higher rate of survival than the group who were not allowed to get a fever. From this and other experiments, it can be concluded that fever seems to be a protective condition. The type of fever found in the lizards is called a behavioural fever. Behavioural fever has also been found in fish and, possibly, in insects.
To clarify the protective role of a fever, the mechanism responsible for the temperature elevation must be understood. When humans are infected with a virus or a bacterium, your white blood cells send chemicals to the brain telling it to increase the set point of your body temperature, i.e. to reset the body thermostat to a higher temperature. Body temperature is then raised to the new set point. This is achieved through increased heat production and improved heat conservation. Heat production is increased by local contractions in the skeletal muscles, i.e. shivering. Heat conservation is achieved by the constriction of peripheral blood vessels, leading to a diminished heat transport to the skin and thus to an increased insulation of the body. Before you have increased your body temperature to the new set point, you feel cold and your skin turns white.
The function of increasing body temperature is very likely to enable your immune system to respond more efficiently or to inhibit the growth and reproduction of bacteria or viruses. The exact mechanisms are still unknown. But it has been shown that several immunologically active mediators, so called cytokines, are released from immune cells, such as macrophages, during fever in humans. These mediators reset the body thermostat, which is situated in the hypothalamus of the brain. They act by increasing the concentrations of substances called prostaglandins. Several fever-reducing drugs, for instance aspirin, cause their effects by inhibiting prostaglandin synthesis.
Once you have defeated the infective agent, the temperature set point is returned to normal. You then restore your body temperature to the normal set point by increasing your heat loss. Blood vessels in the skin are dilated, leading to an increased skin blood flow and thus to an increased skin temperature and an increased heat loss from the skin surface. Heat loss is also increased by sweating. Heat is consumed when the water in the sweat evaporates. Before body temperature has been lowered to the new set point, your skin turns red and you feel hot.
So, rather than being the cause of infection, fever is almost certainly beneficial because it is helps the
body to fight and get rid of the infection. However, the pharmaceutical companies
are not completely misleading. Fevers that get too high can be dangerous, and in such cases fever-reducing drugs must be used.
Read more about fever on another page (in Swedish).
The text has been updated and enlarged in 2013 by Dr Anders Lundquist (senior lecturer at the Department of Biology, Lund University, Sweden).
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R.W. Hill, G.A. Wyse, and M. Anderson: Animal Physiology (3rd ed, Sinauer, 2012).
W.W. Reynolds and M.E. Casterlin: Behavioural fever in teleost fishes (Nature 11:222-2222, 1976).
L.K. Vaughn, H.A. Bernheim, and M.J. Kluger (1974). Fever in the lizard Dipsosaurus
dorsalis. (Nature 252:473-474, 1974).
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