Home > Babies and Infection > Babies and Infection – My Baby has a Fever! (Part 1)


September 17, 2011 Leave a comment Go to comments

My baby has a fever!  What should I do?  The first article in a two-part series will define what a fever is, discuss the mechanism of a fever and provide a small insight to a doctors perspective on fever according to age groups.

What is a Fever

First of all it is important to define fever.  A fever is defined as a “rectal” temperature greater than 38°C (100.4° F); “oral” temperature above 37.5°C (99.5°F).  Rectal temperatures are generally 0.4°C (0.7°F)  higher than oral readings because of mouth breathing which cools the oral temperature.  Ear thermometers have been gaining popularity due to its ease of use in that it requires little cooperation from the patient.  However its use have been shown to be less accurate due to improper positioning of the device and also ear wax interfering with the reading.  As for temporal artery thermometers, enough data have not been compiled to make a judgement on reliability.

Is it truly a fever?  Sometimes people make a mistake in diagnosing their baby with fever because their child is too bundled up.  A great way to know whether your child is dressed properly is to dress them similar to the adult.  If the adult feels comfortable wearing short sleeves and shorts, then the child will also feel comfortable in shorts.  One should take some layers of clothes off and take the temperature again after about 15~30 minutes.  If it is still high then one should suspect a fever.  [Extra note:  Teething causes fever.  However teething does not cause fever over 38.4°C (101.1°F)]

A fever is a sign telling you that the body is fighting an infection.  There are other causes for increase in body temperature besides viruses and bacteriavaccinations, trauma, cancer, brain hemorrhages, autoimmune and metabolic diseases, and certain medications.  In these cases, distinction between fever and hyperthermia must be made.  Treatment modalities between fever and hyperthermia are very different.  Hyperthermia is an uncontrolled increase in body temperature that exceeds its ability to lose heat.  In other words, the body cannot cool itself faster than the amount of heat produced.  The hypothalamic set point is the same in hyperthermia (while it is increased in fever).  For the sake of simplicity we will focus our efforts on fever.

The mechanism of fever involves an elevation of body temperature due to an increase in the hypothalamic set point, located in the brain.  The brain controls body temperature.  The hypothalamus resembles a thermostat in your home except that the thermostat in the brain is set to 37.2 ±0.7°C.  The highest point of fluctuation is seen in the early evening while the lowest point is reached in the morning.  Speaking in biochemical terms, the concentration levels of prostaglandin E2 in the brain determines the temperature settings of the thermostat in the brain. The higher the concentrations of PGE2, the higher the set point and therefore higher temperature in the body.  Infections, microbial toxins, mediators of inflammation and immune reactions causes greater increase in the overall levels of PGE2 resulting in fever.  PGE2 is one of the basis for using drugs like acetaminophen (Tylenol) and ibuprofen (Advil) – the drugs reduce PGE2 production lowering the hypothalamic set point.

When a higher set point is achieved, the brain signals the body to hold in heat by constricting the blood vessels in the peripheral portions of the body.  The hands and feet becomes cold and blood is displaced from the skin to the internal organs.  This causes the nerves in the skin to sense that the body is cold and shivering is initiated.  This is why the person feels cold when in actuality the body temperature is increasing.  Vasoconstriction continues to occur until the temperature of the higher set point is reached.

There are three age groups that are considered by physicians when assessing fever in children.

  1. Neonates (less than 28 days)
  2. Infants 1~3 months
  3. Infants and children 3~36 months

Babies less than 1 month with a fever are especially worrisome because they have an immature immune system and physical findings are not reliable.  An immature immune system will not react as strongly to the offending agent versus a mature one.  It is clinically difficult to distinguish it from a bacterial or a viral illness especially when there are no visible causes for the fever.  It is almost certain the child will be hospitalized with a possibility of blood, urine and cerebrospinal fluid being cultured.  The child will also most likely receive intravenous antibiotics from the start.

Babies from 1 month to 3 months with a fever without any localizing signs are more likely to have fever caused by a virus.  Most viral diseases follow a seasonal pattern ie. RSV and influenza A virus are more common during the winter, while the Enterovirus infection usually occurs in the summer and fall.  A serious bacterial disease is a possibility and therefore a trip to your pediatrician is needed.  If your baby looks very sick, hospitalization will be immediately advised.  Several tests and antimicrobial therapy may also be initiated.  In infants that do not look ill, outpatient with close observation will be at hand – this will also depend on the “low risk criteria” defined by several medical guidelines.

Babies from 3 months to 36 months of age with a fever without any localizing signs are most likely caused by a virus and most of the time do not require hospital admittance unless the child appears gravely ill.

In all cases the doctor will consider the age group and try to pinpoint the cause of the fever ie. otitis media, sinusitis, pneumonia, urinary tract infections, meningitis and so on.  If one is not found then the physical state of the child will be assessed – hospitalization or out-patient treatment will then be decided.

The part one of My baby has a Fever gave a background primer on fever.    Part two will discuss how to identify a fever that needs immediate attention to the doctor.  Management of fevers will also be discussed.

References and Additional Readings:

1.  Kliegman, R., et. al. Nelson textbook of Pediatrics, 19th Ed. PA:  Elsevier, 2011.

2.  Hay, W. et. al. Current Diagnosis & Treatment: Pediatrics, 20ed.  OH:  McGraw Hill, 2011.

3.  Longo, D. et. al. Harrison’s Principles of Internal Medicine, 18 ed. OH:  McGraw Hill, 2012

4.  American Academy of Pediatrics. American Academy of Pediatrics.2011. 15 Sept. 2011 http://www.aap.org

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