Epidemiology

Neonatal infections are usually classified according to time and mode of onset in three categories: (1) prenatal, (2) perinatal (early onset), and (3) nursery-acquired (late onset). The division in time between early and late onset is usually 2 to 7 days of age (Table 94.1). Different investigators have divided early-onset from late-onset infections at different days of life but most early-onset infections are evident during the first day of life. Infections that begin within the first month of life are considered neonatal, but many intensive care units for neonates provide continuing care for infants several months of age with complex problems that are the result of prematurity and complications of neonatal disorders. Therefore, neonatal nursery-associated infections may occur in infants up to a year or more of age. Bacterial infections due to rapidly dividing high-grade pathogens that set in substantially before birth usually result in a stillbirth. Often it is difficult to distinguish infections acquired shortly prior to birth from those acquired as a result of contact with maternal vaginal, fecal, or skin flora during delivery.

Neonatal sepsis occurs in approximately 2 to 4 per 1000 live births in the United States. World-wide reports vary from <2 to 50 per 1000 live births. The rates of early-onset sepsis have fallen to <1.0/1000 in the United States and Western Europe. Risk factors noted in Table 94.1 have a very strong predictive influence on infection rates. Full-term infants born without incident have a very low incidence of infection, lower than any other population of hospitalized patients. Infants susceptible to early-onset postnatal infections are primarily those born prematurely. Those premature infants born to mothers with an infection or whose membranes rupture more than 18 hours before delivery may have an infection rate of 20% or more. In extremely premature infants extra vigilance is required for early recognition and treatment of infection. Premature infants are much more likely to develop sepsis as a consequence of the amnionitis caused by ascending infection than are full-term infants.