Fever

A fever is any body temperature elevation over 100°F (37.8°C).

A healthy person's body temperature fluctuates between 97°F (36.1°C) and 100°F (37.8°C), with the average being 98.6°F (37°C).  The body maintains stability within this range by balancing the heat produced by the metabolism with the heat lost to the environment.  The "thermostat" that controls this process is located in a small structure located deep within the brain.  The nervous system constantly relays information about the body's temperature to the thermostat, which in turn activates different physical responses designed to cool or warm the body, depending on the circumstances.  These responses include: decreasing or increasing the flow of blood from the body's core, where it is warmed, to the surface, where it is cooled; slowing down or speeding up the rate at which the body turns food into energy (metabolic rate); inducing shivering, which generates heat through muscle contraction; and inducing sweating, which cools the body through evaporation.

A fever occurs when the thermostat resets at a higher temperature, primarily in response to an infection.  To reach the higher temperature, the body moves blood to the warmer interior, increases the metabolic rate, and induces shivering.  The "chills" that often accompany a fever are caused by the movement of blood to the body's core, leaving the surface and extremities cold.  Once the higher temperature is achieved, the shivering and chills stop.   When the infection has been overcome or drugs such as aspirin or acetaminophen (Tylenol) have been taken, the thermostat resets to normal and the body's cooling mechanisms switch on: the blood moves to the surface and sweating occurs.

Fever is an important component of the immune response, though its role is not completely understood.  Physicians believe that an elevated body temperature has several effects.  The immune system chemicals that react with the fever-inducing agent and trigger the resetting of the thermostat also increase the production of cells that fight off the invading bacteria or viruses.  Higher temperatures also inhibit the growth of some bacteria, while at the same time speeding up the chemical reactions that help the body's cells repair themselves.  In addition, the increased heart rate that may accompany the changes in blood circulation also speeds the arrival of white blood cells to the sites of infection.

Causes & symptoms

Fevers are primarily caused by viral or bacterial infections, such as pneumonia or influenza.  However, other conditions can induce a fever, including allergic reactions; trauma, such as breaking a bone; cancer; excessive exposure to the sun; intense exercise; hormonal imbalances; certain drugs.  When an infection occurs, fever-inducing agents called pyrogens are released, either by the body's immune system or by the invading cells themselves, that trigger the resetting of the thermostat.  In other circumstances, the immune system may overreact (allergic reactions) or become damaged (autoimmune diseases), causing the uncontrolled release of pyrogens.  A stroke or tumor can cause the body's thermostat to malfunction.  Excessive exposure to the sun or intensely exercising in hot weather can result in heat stroke, a condition in which the body's cooling mechanisms fail. 

How long a fever lasts and how high it may go depends on several factors, including its cause, the age of the patient, and his or her overall health.  Most fevers caused by infections are acute, appearing suddenly and then dissipating as the immune system defeats the infectious agent.  An infectious fever may also rise and fall throughout the day, reaching its peek in the late afternoon or early evening. 

Diagnosis

A fever is usually diagnosed using a thermometer.  A variety of different thermometers are available, including traditional glass and mercury ones used for oral or rectal temperature readings and more sophisticated electronic ones that can be inserted in the ear to quickly register the body's temperature.  For adults and older children, temperature readings are usually taken orally.  Younger children who cannot or will not hold a thermometer in their mouths can have their temperature taken by placing an oral thermometer under their armpit.  Infants generally have their temperature taken rectally using a rectal thermometer.

As important as registering a patient's temperature is determining the underlying cause of the fever.  The presence or absence of accompanying symptoms, a patient's medical history, and information about what he or she may have ingested, any recent trips taken, or possible exposures to illness help the physician make a diagnosis.  Blood tests can aid in identifying an infectious agent by detecting the presence of antibodies against it or providing samples for growth of the organism in a culture.  Blood tests can also provide the doctor with white blood cell counts.  Ultrasound tests, magnetic resonance imaging (MRI) tests, or computed tomography (CT) scans may be ordered if the doctor cannot readily determine the cause of a fever.

Treatment

Physicians agree that the most effective treatment for a fever is to address its underlying cause, such as through the administration of antibiotics.  Also, because a fever helps the immune system fight infection, it usually should be allowed to run its course.  Drugs to lower fever can be given if a patient (particularly a child) is uncomfortable.  These include aspirin, acetaminophen (Tylenol), and ibuprofin (Advil).  Aspirin, however, should not be given to a child or adolescent with a fever since this drug has been linked to an increased risk of Reye's syndrome.  Bathing a patient in cool water can also help alleviate a high fever.

A fever requires emergency treatment under the following circumstances:

• Newborn (three months or younger) with a fever over 100.5°F (38°C)
• Infant or child with a fever over 103°F (39.4°C)
• Fever accompanied by severe headache, neck stiffness, mental confusion, or severe swelling of the throat.

A very high fever in a small child can trigger seizures (febrile seizures) and therefore should be treated immediately.  A fever accompanied by the above symptoms can indicate the presence of a serious infection, such as meningitis, and should be brought to the immediate attention of a physician.

Prognosis

Most fevers caused by infection end as soon as the immune system rids the body of the pathogen and do not produce any lasting effects.  The prognosis for fevers associated with more chronic conditions, such as autoimmune disease, depends upon the overall outcome of the disorder.