Definition. Neonatal sepsis is a clinical syndrome of systemic illness accompanied by bacteremia occurring in the first month of life.
Incidence. The incidence of primary sepsis is 1–5 per 1000 live births. The incidence is much higher for very low birthweight (VLBW) infants (BW <1500 g) with early-onset sepsis of 15–19 per 1000 and late-onset nosocomial sepsis at 21% according to data from the National Institute of Child Health and Human Development Neonatal Research Network. The mortality rate is high (13–25%); higher rates are seen in premature infants and in those with early fulminant disease.
Pathophysiology. Neonatal sepsis can be classified into two relatively distinct syndromes based on the age of presentation: early-onset and late-onset sepsis.
Early-onset sepsis (EOS) presents in the first 5–7 days of life and is usually a multisystem fulminant illness with prominent respiratory symptoms. Typically, the infant has acquired the organism during the intrapartum period from the maternal genital tract. In this situation, the infant is colonized with the pathogen in the perinatal period. Several infectious agents, notably treponemes, viruses, Listeria, and probably Candida, can be acquired transplacentally via hematogenous routes. Acquisition of other organisms is associated with the birth process. With rupture of membranes, vaginal flora or various bacterial pathogens may ascend to reach the amniotic fluid and the fetus. Chorioamnionitis develops, leading to fetal colonization and infection. Aspiration of infected amniotic fluid by the fetus or neonate may play a role in the resultant respiratory symptoms. Finally, the infant may be exposed to vaginal flora as it passes through the birth canal. The primary sites of colonization tend to be the skin, nasopharynx, oropharynx, conjunctiva, and umbilical cord. Trauma to these mucosal surfaces may lead to infection. Early-onset disease is characterized by a sudden onset and fulminant course that can progress rapidly to septic shock and death.
Late-onset sepsis (LOS) may occur as early as 5 days of age. LOS is usually more insidious but it can be fulminant at times. It is usually not associated with early obstetric complications. In addition to bacteremia, these infants may have an identifiable focus, most often meningitis in addition to sepsis. Bacteria responsible for LOS and meningitis include those acquired after birth from the maternal genital tract as well as organisms acquired after birth from human contact or from contaminated equipment (nosocomial). Therefore, horizontal transmission appears to play a significant role in late-onset disease. The reasons for delay in development of clinical illness, the predilection for central nervous system (CNS) disease, and the less severe systemic and cardiorespiratory symptoms are unclear. Transplacental transfer of maternal antibodies to the mother's own vaginal flora may play a role in determining which exposed infants become infected, especially in the case of group B streptococcal infections. In case of nosocomial spread, the pathogenesis is related to the underlying illness and debilitation of the infant, the flora in the neonatal intensive care (NICU) environment, and invasive monitoring and other techniques used in the NICU. Breaks in the natural barrier function of the skin and intestine allow opportunistic organisms to invade and overwhelm the neonate. Infants, especially the premature ones, have an increased susceptibility to infection because of underlying illnesses and immature immune defenses that are less efficient at localizing and clearing bacterial invasion.
Microbiology. The principal pathogens involved in EOS ...
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