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Fever

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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 37.2oC in the morning or 37.8oC 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 37oC 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 38oC, 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:

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 and ultrasound.

What is the Treatment for a Fever?

Treatment depends on the duration and cause of the fever, and on other accompanying symptoms. For instance if and infection is the cause of the fever, this will be treated with antibiotics. If the fever is mild and no other problems are present, no medical treatment may be required.

If the fever is making you feel too uncomfortable, then Paracetamol (2 tablets, every 6 hours for adults) can be taken as needed. This will not treat the cause of the fever, but can help lower the body’s temperature and make you feel more comfortable.

General advice for people with fever include:

  • Drink plenty of fluids
  • Remove excess clothing or blankets. The environment should be comfortably cool.
  • A lukewarm bath or sponge bath may help cool someone with a fever
  • Do not use cold baths or alcohol rubs. These cool the skin, but often make the situation worse by causing shivering, which raises the core body temperature.
  • Take paracetamol as needed.
  • Do not give aspirin to children, use paracetamol instead.

Taking medication to lower a fever can cause the infection to last a little longer. As has been mentioned before, the fever is actually a way of fighting off the infection, and this is being taken away by the medication. Despite this, infections do not last much longer at all if the body temperature is kept low with paracetamol. Most of the time however, people prefer to be comfortable and non-feverish for a tiny bit longer than uncomfortable, hot and unwell for a very slightly shorter illness.

Is Fever Dangerous?

While fever can occasionally be a symptom of a dangerous infection, people are often worried at the fever can get so high as to damage the brain – especially in children. Generally, fever is not considered to be very dangerous by itself, and it is more the cause of the infection that is worrying.

Temperatures of 41oC or more start to be concerning, not only because of the high temperature causing trouble, but also because it is probably the result of a serious infection that needs investigating.

At most levels of fever though, a fever in itself should not be regarded as dangerous. If the child is uncomfortable then Children’s paracetamol given by very carefully following the age-appropriate dosage instructions on the packet can be used to relieve symptoms.

Febrile Convulsions (Febrile Seizures)

Sometimes during periods of fever, infants and young children can suffer from what are known as febrile convulsions (also known as febrile seizures). These are small convulsions (where the child is seen to stiffen their body, twitch their face, arms or legs, lose consciousness or stare blankly) that are caused simply by the temperature being higher than normal.

Suffering from febrile convulsions runs in families, and genetics play a role. They occur in about 3% of the population, and are most common between the ages of 5 months and 5 years. In about one-third of cases they happen more than once. They do not mean the child has epilepsy, and the chances of them having epilepsy in later life are only fractionally higher than the rest of the population.

Generally, febrile convulsions stop by themselves after about 15 minutes, but they can be very distressing to watch. It is important to know that they are completely harmless and are not associated with any long-term brain damage.

Most of the time, investigations such as an EEG or CT are unnecessary because they will not show anything, or change treatment.

If one occurs in your child then do not panic, but always bring your child into hospital to be investigated, to make sure that there is no underlying cause that could be dangerous.

References

  1. Bor DH. Approach to the adult with a fever of unknown origin [online], UpToDate, 2006. Available from: RL Link] (last accessed 24/08/06)
  2. Craig J V, Lancaster G A, Taylor S, Williamson P R, Smyth R L. Infrared ear thermometry compared with rectal thermometry in children: a systematic review. Lancet. 2002;360:603-609.
  3. Dinarello CA, Gelfand JA. Chapter 16: Fever and Hyperthermia [online], Harrison’s Internal Medicine, Access Medicine. Available from: [URL Link] (last accessed 27/08/06)
  4. Guyton, AC, Hall, J.E. Textbook of Medical Physiology. Philidelphia, Harcourt Health Sciences, 2000.
  5. Murtagh, J. General Practice (Third Edition). North Ryde, McGraw-Hill Australia, 2003.
  6. Papadakis A, McPhee SJ. Fever [online], Current Consult: Medicine, Access Medicine, 2006. Available from: [URL Link] (last accessed 27/08/06)
  7. Robinson MJ, Robertson DM. Practical Paediatrics (Fifth Edition). Victoria, Churchill Livingstone, 2005.
  8. Warden et al. Evaluation and Management of febrile Seizures. Annals of Emergency Medicine. 2003: 41 (2); 215-222.
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Dates

Posted On: 14 May, 2005
Modified On: 15 September, 2017
Reviewed On: 5 February, 2008

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