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Fever is often the only clinical sign of an underlying serious infection in young infants, particularly those in the first few months of life. This chapter focuses on febrile infants 60 days of age or younger. Although most well-appearing febrile infants in this age group have a benign, self-limited illness, approximately 10% have a serious bacterial infection (SBI) with reported rates as high as 15–20% in neonates ≤28 days of age.1–6 While urinary tract infections (UTIs) constitute a majority of SBIs, 1–3% will have bacteremia and/or bacterial meningitis,1–5 termed invasive bacterial infection (IBI).7 Thus, fever is an important sign for identifying infants who require timely evaluation and treatment.

Normal body temperature varies by age, sex, and time of day, making the assessment of fever challenging in this age group. 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 with infants younger than 1 month of age.8 However, despite this individual variation, several studies have demonstrated that a rectal temperature greater than 38.0°C is more than two standard deviations above the mean for age, making this value a reasonable threshold for defining fever in neonates.8–10 Rectal temperature measurement is the standard method for fever determination at this age. Other methods such as axillary or forehead temperature measurements are less reliable 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, the rates of SBI do not differ from infants without a history of fever.11 In contrast, infants who had a documented fever at home by rectal thermometry remain at risk for SBI regardless of the presence or absence of fever when they present to the clinician.11 For infants with only a tactile fever or a fever obtained with use of a nonrectal thermometer, there is no consensus for the need for laboratory evaluation, particularly if the infant did not receive an anti-pyretic. An option of observation in the office or emergency department to evaluate for the development of fever may be considered in these circumstances.

Escherichia coli is the most common organism isolated in cases of both UTI and IBI among febrile young infants in the first 3 months of life (Table 55-1). Febrile infants can be infected by both perinatally acquired organisms, such as group B Streptococcus (GBS) (the second most common organism isolated), as well as community-acquired organisms such as Staphylococcus aureus and Streptococcus pneumoniae. Gram-negative enteric bacilli such as Klebsiella pneumoniae are also pathogens in this age group and are associated with both UTI and IBI.12–15 In a 2002 systematic review of febrile infants <3 months of age, the ...

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