Fever in Babies

What is fever?

Fever caused by bacterial or viral infection

Fever is usually a sign that your baby's body is fighting an infection. Bacteria and viruses usually thrive at a temperature near our normal body temperature. When we have a fever, our body temperature is elevated, which makes it harder for bacteria and viruses to survive. Fever also activates the immune system and sets the infection-fighting white blood cells into action.

It is important to realize that while fever is serious in newborn babies, it is not necessarily a bad thing if the baby is over three months of age. Fever is the body's way of fighting infection, so it is actually a good thing. Scientists believe that, in response to invaders such as viruses and bacteria, the body's white blood cells produce a chemical that tells a part of the brain called the hypothalamus to turn up the body temperature. A higher body temperature enables the immune system to fight infections more efficiently. Fever may also help to enhance the production of antiviral substances in the body.

Usually, fever is associated with common illnesses such as colds, sore throat, or ear infections, but occasionally it can be a sign of something more serious.

Fever caused by heat illness

Sometimes fever is not a response to illness, but rather it is caused by heat exhaustion or heat stroke. Heat exhaustion is a severe heat illness with symptoms of dehydration, fatigue, weakness, nausea, headache, and rapid breathing. It occurs when people in a hot climate do not drink enough water. Heat stroke is a life-threatening medical emergency where the body becomes so hot that is no longer able to regulate its temperature.

Heat stroke can occur if a child is dressed too warmly in hot weather, if he exercises too much in the heat without drinking enough water, or if he is left in a car on a hot day. Heat stroke can cause the body temperature to rise above 41°C (105.8°F), which can lead to brain damage if not treated immediately. The symptoms of heat stroke are as follows:

  • flushed, hot, dry skin with no sweating
  • temperature of 41°C (105.8°F) or higher
  • severe, throbbing headache​
  • weakness, dizziness, or confusion
  • sluggishness or fatigue
  • seizure
  • decreased responsiveness
  • loss of consciousness

How to take your baby's temperature

There are two ways to take your baby's temperature: rectally or under their armpit. The most accurate of these methods is the rectal way; however, many parents do not find this approach very appealing. Here are a few tips for taking your baby's temperature:

Measuring temperature rectally using an electronic thermometer

  • Lie your baby on their back and bring their knees up over his abdomen. It is much easier to take the temperature if two people are doing it.
  • Make sure the thermometer is clean.
  • Dip the thermometer in some water-soluble jelly.
  • Insert the thermometer about 2.5 cm (1 inch) into your baby's bottom.
  • Wait for the thermometer to take the reading. This is usually indicated by a beep. Read the temperature carefully and write it down in a notebook.
  • Clean the thermometer after use with soap and water.
  • The normal range for a temperature taken rectally is 36.6°C to 38°C (97.9°F to 101°F)

Measuring temperature under the armpit

Armpit temperature
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An armpit temperature of 37.2°C (99°F) or higher means a fever.

  • Place the bulb of the thermometer in your baby's armpit, and hold their arm down alongside their body. Make sure the bulb is completely covered in the armpit.
  • Wait for the thermometer to take the reading.
  • The normal range for a temperature taken under the armpit is 36.7°C to 37.5°C (98.0°F to 99.5°F).

Oral thermometers are not recommended until about four years of age. Ear thermometers should not be used in newborn babies and young infants because they tend to give inaccurate readings in the very young. Ear thermometers can be used in children over two years old. Fever strips, which are placed on the child's forehead, are also not recommended due to inaccuracy.

Treatment of fever in young infants

In babies three months of age or under, fever can be a cause for concern. If you notice a temperature that is even just slightly above the normal range – 38°C (101°F) taken rectally or 37.3°C (99.1°F) taken under the armpit – bring your newborn baby to the doctor as soon as possible.

Treatment of fever in older infants

Treating a fever due to infection

Most fevers are caused by viruses and will get better without treatment. Because of this, many doctors do not recommend reducing a fever in infants over six months of age unless the fever is over 38.5°C (101.5°F). However, if the infant is having aches and pains from the fever, acetaminophen can be used to make them feel more comfortable.

If a fever is found to be caused by a bacterial infection, the infection should be treated with antibiotics. Antibiotics work to destroy the bacteria, and in the process lower the fever. Sometimes antibiotics and acetaminophen​ are used simultaneously to treat the fever. Fevers that shoot up past 41.5°C (106.7°F) are rare and should be treated immediately.

Treating a fever due to heat illness

If a fever is due to heat exhaustion or heat stroke, it can be dangerous and requires immediate attention. Heat exhaustion can be treated by bringing the child indoors, loosening their clothing, encouraging them to eat and drink, and giving them a cool bath. Heat stroke is a medical emergency and should be treated immediately by a physician. While waiting for medical help, bring your child indoors, remove their clothing, and sponge them with cool water.

Douglas Campbell, MD, FRCPC

Andrew James, MBChB, MBI, FRACP, FRCPC