Why do children get fevers?
Our normal body temperature is around 37°C. Our temperature can go up and down a little, around this figure, during the day. Children's temperatures can easily rise slightly with things like hot baths, exercise and wearing overly warm clothes. Teething often increases a toddler's temperature by 0.5°C.
Fever is a part of the body's natural defences against infection. Fever is created by your immune system under the direction of a part of the brain called the hypothalamus. The hypothalamus acts like a central heating thermostat. Fever happens when the hypothalamus sets the body temperature above its normal level.
It does this in response to an infection with germs, usually because it detects the presence of infectious agents like bacteria or viruses. It is believed that the increased temperature is a protection the body has developed to help fight the germs that cause infections, as they tend to multiply best at normal body temperature.
The mechanism through which the body increases its temperature is by reducing heat loss. We sweat less and feel dry to the touch, we shiver (the movement tends to increase temperature) and, because we feel as though we are cold, we curl up and seek different ways of warming up. The blood vessels in our skin shrink to preserve heat loss, so we look pale. All of this is why, as the temperature is rising to meet the directions of the thermostat, we are hot to the touch but we feel that we are cold. During this stage of a fever your child will not be pleased when you try to cool them down, as they will already feel as though they are cold.
Eventually, the body's temperature reaches the new 'thermostat' setting, and the feeling of being cold goes away. Eventually it reverses, the thermostat setting drops down again towards normal, and the body tries to lose the extra heat it has on board. It does this by sweating and by opening up the blood vessels in the skin so that we are flushed and sweaty.
Children tend to get higher fevers than adults - although this is only true after the age of 6 months. Before that age the immune system of a baby is quite immature. After the age of 6 months the actual temperature, in a fever, is not a good guide to whether or not your child is seriously unwell.
What can cause a fever/high temperature?
The most common causes of fever in children in the UK are viral infections. There are many other uncommon causes. Some of these will show other obvious signs:
- Infections with germs called viruses are the common cause. Viral infections cause many common illnesses such as colds, coughs, flu, diarrhoea, etc. Sometimes viral infections cause more serious illnesses.
- Infections with germs called bacteria are less common than viral infections but also cause fevers. Bacteria are more likely to cause serious illness such as pneumonia, joint infections (septic arthritis), urine infections, kidney infections, septicaemia and meningitis. However, bacteria can also cause fever in less serious infections such as ear infections and infected skin rashes.
- Inflammatory conditions and reactions may cause fever, including Kawasaki disease, some types of arthritis, and reactions to some medicines.
- Immunisations: occasionally children develop a fever after an immunisation. This is because immunisations are generally designed to 'trick' the body's immune system into thinking it sees an infection, so that it develops immunity. Fevers following immunisation are not usually high or prolonged.
- Other types of infection: these include 'tropical' infections such as malaria and dengue fever, and conditions which are more common outside the UK, such as tuberculosis.
- Heat stroke is a possible cause of raised body temperature, although technically this isn't a fever, as the body is being heated from the outside (whereas in fever the body does the heating itself).
How common is fever in children?
Fever and feverish illness are very common in young children, particularly in those aged less than 5 years, and it can be really worrying for parents. It's not always easy to judge how sick your child is, or whether you should ask for medical help.
Three to four out of every 10 parents of children aged less than 5 years say their child has had a fever in the past year. It is probably the most common reason for a child to be taken to the doctor. Fever is also the second most common reason for a child being admitted to hospital and it can be a cause of great anxiety in parents. This leaflet offers guidance with:
- Understanding how best to manage a fever.
- Knowing when to seek professional help or advice.
- Knowing what signs suggest that your child may be seriously unwell, including how to check for signs of lack of fluid in the body (dehydration) and other signs of serious illness.
Whether or not you decide to seek help or advice, you should always give a child with a temperature lots to drink. It is not always necessary to give them paracetamol or ibuprofen.
Further reading and references
Feverish illness in children - Assessment and initial management in children younger than 5 years; NICE Guideline (Updated August 2017)
Feverish child - risk assessment; NICE CKS, September 2013 (UK access only)
Thompson M, Van den Bruel A, Verbakel J, et al; Systematic review and validation of prediction rules for identifying children Health Technol Assess. 2012 Mar16(15):1-100.
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