Febre

O que é uma febre?

Body temperature fluctuates slightly throughout the day. Normal temperature varies depending on the person, the body site where temperature is measured, and the time of day. A temperature above the normal range is known as a fever (or pyrexia).

It is usually agreed that a person with a temperature of 38 degrees (38°C) or higher has a fever. However, a temperature over 37.5°C can be considered to be a mild or low grade fever. Fevers occur in both children and adults and are very common, particularly in children. Fevers are often accompanied by sweating. Also see Febre em crianças.

The normal body temperature changes throughout the day. These changes can be caused by exercise, eating, sleeping and even the time of day. The temperature is usually highest in the early evening and lowest in the early hours of the morning.

The average body temperature, taken with a digital thermometer in your mouth (oral temperature), is 37°C (98.6°F), but anywhere between 36.5°C (97.7°F) and 37.2°C (98.96°F) is considered normal. Armpit temperatures tend to be 0.2°C to 0.3°C lower than this.

High temperature (fever) is caused by the release of certain chemicals by the immune system, usually as a result of infection or inflammation.

Most fevers are caused by infections or other illnesses. Viral infections are the commonest causes of a fever. The high body temperature makes it more difficult for the germs (bacteria or viruses) which cause infections to survive.

Fever caused by infections

Common conditions that can cause fevers include:

• Respiratory tract infections and colds: see the separate leaflet called Common cold (Upper respiratory tract infections).

• Gripe (influenza): see the separate leaflet called Flu.

• Infecção do intestino (gastroenterite): see the separate leaflets called Gastroenterite em crianças e Gastroenterite.

• Infecções de ouvido: see the separate leaflet called Ear infection (Otitis media).

• Infection of the tonsils: see the separate leaflet called Tonsillitis.

• Kidney or urinary tract infections: see the separate leaflets called Infecção urinária em crianças, Cistite em mulheres, Infecção urinária na gravidez, Infecção urinária em homens, Infecção renal (Pielonefrite) e O que os rins fazem?

• Common childhood illnesses: for example, chickenpox (varicella), scarlet fever, rubella and whooping cough (pertussis). See the separate leaflets called Catapora em crianças, Escarlatina, e Coqueluche.

If developing a fever after having been abroad, it is important to seek medical advice. Certain infections that can be caught abroad (such as Malária, typhoid or yellow fever) can cause a fever.

A child's temperature can also be raised when their teeth start to develop (their teething period), following some vaccinations, or if overheating because of too much bedding or clothing. See the separate leaflets called Erupção dos dentes, Imunização e Morte súbita infantil.

Fever caused by other conditions

Fever can be caused by conditions other than infections but this is less common. For example, blood clots in the leg or lung can occasionally lead to a fever developing. See the separate leaflets called trombose venosa profunda e Embolia pulmonar.

There are some health conditions that cause inflammation in the body and these can also cause fevers at times. See the separate leaflets called Artrite reumatoide e Lúpus eritematoso sistêmico.

There are also some types of cancer which can lead to a fever developing. See the separate leaflet called Cancer.

Sometimes the cause of the fever is not clear. This is called a fever of unknown origin.

A high temperature (fever) is very common, particularly in children. Around 3 out of 10 young children have a fever every year. Older adults are less likely to develop a high temperature when they get an infection; however, older adults who do develop a fever with an infection have been shown to be more likely to recover quickly.

There are different ways of taking a temperature with a thermometer. A thermometer can be placed either under your armpit or in your ear. The forehead thermometers are no longer recommended as they are less accurate. Rectal thermometers are no longer usually used in the UK.

As a parent it can feel worrying if a child has a high temperature (fever). However, having a fever is very common and usually settles by itself without treatment.

Signs of fever

Using a thermometer is the best way to tell if there is a fever. However the following will give a rough idea:

• The forehead may feel hot when felt with the back of the hand. This is much more sensitive to temperature than the palm.

• Cheeks may be flushed.

• Feeling hotter than others around.

Note: Fever usually responds well to medication such as paracetamol and ibuprofen. However, these medicines do not "cure" a fever and, as soon as the medication wears off, the temperature will increase again. This is normal and, if wanting to reduce the fever, then regular medication will need to be given.

Fever in adults

A high temperature (fever) is usually associated with physical discomfort and most people feel better when a fever is treated. However, depending on age, physical condition and the underlying cause of the fever, it may be best not to treat the fever.

Many experts believe that fever helps the body fight off infections naturally.. There is some evidence that fever reduces the severity of infections and helps faster recovery. However, paracetamol and ibuprofen will both reduce the fever if needed. It is important to drink plenty of fluids to avoid becoming dehydrated from sweating.

If there is an obvious cause for the fever, for example a flu-like illness with aches and pains and headaches, then treating the symptoms with paracetamol or ibuprofen will also help the underlying fever. If there is a bacterial infection, for example a urine infection, then antibiotics will be needed to treat the infection but these will not reduce the fever itself.

Tests may sometimes be needed if there is no obvious cause for the fever.

Testing may include:

• Exames de sangue.

• Exames de urina.

• X-rays to determine the cause.

Febre em crianças

In children, paracetamol and ibuprofen are only recommended if the child is distressed with the fever.

Warm (tepid) sponging is no longer recommended for children to treat a fever. Drinking plenty of fluids is very important during a fever. Keeping fluid levels up is important to reduce the risk of dehydration. It is also important to make sure they are not wearing too many clothes - one layer is fine.

Nota: in children, giving paracetamol or ibuprofen does not reduce the risk of a seizure caused by a fever occurring. See the separate leaflet called Febrile seizure (Febrile convulsion).

What are the types of fever?

Common types include continuous fever, where the temperature stays high with little variation, intermittent fever, where it comes and goes, and remittent fever, where it rises and falls but never fully returns to normal.

What is a fever dream?

A fever dream is a vivid or unsettling dream that happens when you have a fever. The changes in brain activity and sleep patterns can make your dreams feel more intense, strange, or even frightening. They usually pass once the fever settles.