Fever, a pyrogen-mediated increase in body temperature, may have a multitude of disparate causes, including infection, inflammation, or malignancy. Therefore the most important goal when confronted with a febrile patient is to determine the underlying cause. Owing to a differential risk of disease, the cutoff point of abnormal temperature is determined by the age of the child and the presence of any underlying immunodeficiencies. Although there is some overlap, in this chapter the discussion of fever is divided into two age groups: (1) neonates and young infants and (2) older infants and toddlers.
FEVER IN NEONATES AND YOUNG INFANTS
Fever frequently prompts the medical evaluation of neonates (younger than 28 days) and young infants (aged 29–90 days). In this age group, fever is generally defined as a temperature greater than 38.0°C (100.5°F). Up to 15% of neonates and young infants with fever have a serious bacterial infection (SBI). The risk appears to be lower in certain subgroups, such as those with normal laboratory studies or with bronchiolitis.
The most common SBIs in febrile infants are urinary tract infection (UTI; 5%–10%), bacteremia (1%–2%), and meningitis (0.5%–1%). Bacterial pneumonia, gastroenteritis, septic arthritis, osteomyelitis, cellulitis, omphalitis, and mastitis also occur. Infecting organisms vary by the site of infection (Table 95-1). Escherichia coli, the most common cause of UTIs, has recently replaced group B streptococcal (GBS) as the most common etiologic organism for bacteremia in 1- to 3-month-old infants due to perinatal prophylaxis against group B Streptococcus (see Prevention below).1 UTIs caused by Staphylococcus aureus and Citrobacter species are often associated with urinary tract abnormalities and extrarenal sites of infection. Early-onset (age younger than 1 week) GBS infections are typically associated with bacteremia, pneumonia, and meningitis; late-onset (older than 1 week) infections include septic arthritis, osteomyelitis, cellulitis, and adenitis. Streptococcus pneumoniae is responsible for 5% of cases of meningitis in neonates and a slightly greater proportion in infants 1 to 3 months of age.2 Brain abscesses are more likely in cases of meningitis caused by Citrobacter koseri, Enterobacter sakazakii, Bacteroides fragilis, and Serratia marcescens. Bacteremia complicates Salmonella gastroenteritis in 5% to 10% of cases.3 In contrast to older children, young infants with Salmonella bacteremia rarely have other immunocompromising conditions. Omphalitis, an infection of the umbilical stump, affects 0.2% to 0.7% of newborn infants; it occurs most often among hospitalized preterm infants undergoing umbilical catheterization. Gram-negative organisms have emerged as an important cause of omphalitis since the introduction of antistaphylococcal cord care. Predominant pathogens causing omphalitis now include S. aureus, group A β-hemolytic streptococci, E. coli, Klebsiella pneumoniae, and Proteus mirabilis; anaerobes are involved in one-third of cases. S. aureus causes most cases of neonatal mastitis.4 Case reports occasionally implicate group A β-hemolytic streptococci, GBS, and enteric gram-negative rods.