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Listeria monocytogenes is a food-borne pathogen that causes disease primarily in neonates, pregnant women, the elderly, and the immunocompromised host.1 It is worldwide in distribution and is acquired relatively frequently in developed countries due to consumption of refrigerated, contaminated, ready-to-eat food, mostly dairy products and cold cuts.2 Listeriosis is a zoonosis of many animal species. In humans, it causes epidemic and sometimes sporadic outbreaks of febrile gastroenteritis.3 Systemic infection results from passage of the organism across the intestinal mucosal barrier by endocytosis, coupled with its ability to evade immune surveillance by cell-to-cell spread; deficiencies in T-cell immunity such as in pregnancy and immunosuppression increase the risk of listeriosis.4,5 Extraintestinal disease results from hematogenous dissemination with particular predilection for central nervous system and placental infections.

Neonates are typically infected transplacentally or by birth through an infected birth canal.

Clinical Manifestations

The bacteremic illness in the mother presents with a nonspecific illness (flulike or gastrointestinal symptoms) and may progress to amnionitis, preterm labor, or septic abortion in 3 to 7 days.6 Perinatal listeriosis results in neonatal death or stillbirth in 22% of the cases. Neonatal listeriosis has an early and a late onset presentation. Neonates with early onset disease usually present at 1 to 2 days of age, are born prematurely, and typically exhibit a septiclike picture, although respiratory distress, pneumonia, and, rarely, meningitis and granulomatosis infantisepticum are described. The latter is manifested by diffuse granulomas in the liver, skin, and placenta as well as other organs. Late onset disease typically presents at 2 weeks of age, most commonly as meningitis. The case fatality rate in neonates is 20% to 30%.4 Population-based studies show that 88% of listeriosis in children younger than 5 years occurs before 1 year of age, half of which presents on the first day of life.7In 2006, 58 of 884 cases reported in the United States were in children younger than 5 years, whereas the majority of cases occurred in the elderly.8 After the neonatal period, invasive listeriosis most commonly presents as bacteremia without a source or as meningitis (30–55% of cases) leading to neurologic sequelae in 30%. Other forms of central nervous system infection include meningoencephalitis, cerebritis, brainstem or spinal cord abscesses, and brainstem involvement (rhombencephalitis).

Diagnosis and Treatment

The diagnosis is established by positive blood or cerebrospinal fluid (CSF) cultures. Peripheral leukocytosis is common. In meningitis, the CSF is usually purulent with polymorphonuclear cell predominance, an elevated protein level, and often a low or normal glucose.4,9 Rarely, the CSF is devoid of inflammatory cells.10 The CSF Gram stain is positive in only 40%. Concomitant blood cultures are positive in about two thirds of the patients. Real-time polymerase chain reaction on blood specimens may speed up the detection of neonatal sepsis, including that secondary to L monocytogenes.11


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