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Fever in a young infant is often the only clinical sign of an underlying serious infection. This is particularly true for infants younger than 2–3 months, since they lack many of the clinical signs typically used by clinicians to judge general appearance. Although most well-appearing febrile infants in this age group have a benign; self-limited illness, as many as 10% have serious bacterial illness, including 3% with bacteremia and bacterial meningitis.19 Thus, fever is an important symptom for identifying infants who need immediate evaluation and treatment.

The definition of what constitutes fever in this age is debatable. Normal body temperature varies with a variety of factors including age, sex, and time of day. There may be as much as a 0.5°C difference between the physiologic nadir in the early morning and the peak in the early evening. Older infants appear to have slightly higher basal body temperature compared to infants younger than 1 month.10 However, despite this individual variation, several studies have shown that rectal temperatures more than 38.0°C are greater than two standard deviations above the mean for age.2,3,10 It is important to emphasize that rectal temperature is the standard method for fever determination at this age. Other temperature-taking methods such as axillary or forehead measurements are unreliable and should not be used. Often, the parent will report a subjective fever because the infant “felt warm” or had “fever to touch.” In these cases, if the infant was afebrile when examined by the clinician, there was no increase in serious bacterial illness.11 However, if the infant had a documented fever at home by rectal thermometry, the infant remains at risk for serious bacterial illness regardless of the presence or absence of fever when the infant presents to the clinician.11 One important caveat is the possibility of environmental factors as a cause of elevated body temperature in the infant, which often happens in the summer especially if the infant is bundled in warm clothing.

The most common organisms associated with fever in young infants are shown in Table 51–1. In infants younger than 4 weeks, infection is usually caused by organisms acquired perinatally—group B Streptococcus (GBS), gram-negative bacilli (Escherichia coli, Klebsiella), Listeria monocytogenes, and herpes simplex virus (HSV). By 6 weeks of age, the etiology shifts to community-acquired organisms—Streptococcus pneumoniae and less commonly Neisseria meningitidis and Haemophilus influenzae type B. During the winter months, common viral causes are influenza type A or type B and respiratory syncytial virus (RSV).

Table 51–1. Common Bacterial Pathogens in Cases of Bacteremia & Bacterial Meningitis

Infants younger than ...

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