Virtual Medical Centre

Symptoms

Fever

• What is a Fever? 
• The Purpose of a Fever 
• What Causes a Fever?
• What Tests are done for a Fever?

What is a Fever?

A fever is a raising of the body temperature above the normal level and can be caused by many different things. While there are no strict rules, a fever is generally considered to be a temperature above 37.2^oC in the morning or 37.8^oC at other times of the day. When someone is suffering from a fever they are sometimes said to be 'febrile'.

Although fever can be a sign of illness, it is recognised to play an important role in fighting infections, and for this reason a slight fever does not necessarily need treatment. Most bacteria and viruses that cause infections in people thrive best at 37^oC and raising the body's temperature above this helps fight them off. Most fevers are caused by infections, usually viruses.

Parents often fear that having a fever will cause brain damage in their child. However this rarely occurs unless the fever is dangerously high. Sometimes a fever can trigger a seizure in a child, in what is known as a febrile convulsion (febrile seizure). While this is not dangerous by itself, and does not result in any brain damage, it is important to bring the child to see a doctor every time one occurs so they can try and work out the underlying cause of the fever.

When there is ongoing fever of over 38^oC, lasting for 3 weeks or more, without an obvious cause, this is called a fever of unknown origin, and this is dealt with separately.

The Purpose of a Fever

For the body to keep functioning, it must be kept within a fairly small range of temperatures. This is controlled by a region of the brain known as the hypothalamus. The hypothalamus collects information from all over the body about temperature, and can adjust the heat being produced and being lost so that it stays remarkably stable throughout the day. Sometimes though, the level that the hypothalamus 'sets' this stable temperature at is raised a little higher than usual. It does this through stimulation by the bacteria and viruses themselves, as well as some chemicals called 'cytokines', released into the blood by cells of the immune system. This is most commonly done to try and fight off infections.

Infections are most commonly invasions of the body by bacteria or viruses, and if left unchecked they can do massive amounts of damage. It is up to the body's immune system to fight off these infectious agents and one of the methods that the immune system uses is a fever. Bacteria and viruses can survive within a very small range of temperatures, and so by raising the body's temperature, they cannot grow as fast and can be destroyed faster. In addition, a fever activates the body's acquired immune system to make more white blood cells, antibodies, and other infection-fighting agents.

Sometimes the body can be fooled into raising its temperature for other, non-infectious reasons either because the immune system is fighting off something it things is 'foreign' like a drug, or has become confused and is trying to attack some of the body's own systems such as in as rheumatoid arthritis or systemic lupus erythematosus (SLE).

Normal Temperatures

Normally, the body keeps the temperature (measured using an oral thermometer) between 36oC and 37.2oC. However, this varies with the time of day, being coolest in the morning and warmest in the afternoon.

A woman's menstrual cycle can elevate temperature by one degree or more. It can also be raised by physical activity, strong emotion, eating, heavy clothing, medications, high room temperature, and high humidity. This is especially true in children.

Measuring a Fever

The most accurate and convenient way to measure the body's temperature is still with the use of an oral thermometer.

Rectal measurements are sometimes made in children under four, but keep in mind that these are usually about 0.4oC warmer than if taken within the mouth. This is thought to be due to the cooling effect of breathing.

Thermometers that take the temperature from the ear drum are much more convenient, but less accurate, than mercury oral thermometers.  

What Causes a Fever?

Fever can be caused by a wide variety of diseases, including:

• Infections, such as:
  • Colds or flu-like illnesses
  • Sore throats and strep throat
  • Ear infections
  • Viral gastroenteritis or bacterial gastroenteritis
  • Glandular fever
  • Urinary tract infection
  • Upper respiratory infections (such as tonsillitis, pharyngitis or laryngitis)
  • Serious infection including pneumonia, appendicitis, tuberculosis, and meningitis.
• Medications such as:
  • Antibiotics
  • Antihistamines
  • Barbiturates
  • Blood Pressure lowing drugs
• Fever can occur in infants who are overdressed in hot weather or a hot environment
• Blood Vessel diseases and autoimmune disorders (where the immune system attacks the body) such as rheumatoid arthritis or systemic lupus erythematosus (SLE).
• AIDS and HIV infection
• Inflammatory bowel disease
• Cancers such as leukaemia or lymphoma

If a fever has been going on for a particularly long time without anyone being able to find the cause then it is described as a fever of unknown origin.  

What Tests are done for a Fever?

Infections are the most common cause of fever if it has been present for only a few days. If fever lasts longer than this, tests may be needed to find the cause. Your doctor will ask questions about the fever, and perform a physical examination, which may include a detailed examination of the skin, eyes, ears, nose, throat, neck, chest, and abdomen to look for the cause of the fever.

Depending on the findings from questions and examination, certain investigations may be needed. These may include:

• Urine analysis and culture (to grow any bacteria)
• Blood tests (to check levels of white cells, inflammatory markers, and to see if there are any infectious organisms in the blood stream)
• Chest X-ray (which can help diagnose check infections)
• Stool cultures
• Special tests for unusual infections such as tuberculosis or HIV
• Tests for autoimmune and systemic disease such as rheumatic fever
• CT scanning and ultrasound.