ESSENTIALS OF DIAGNOSIS
Ingestion of contaminated food product.
Undifferentiated fever in a pregnant woman in her third trimester.
Altered mental status and fever in an elderly or immunocompromised patient.
Obtain blood and cerebrospinal fluid cultures to confirm the diagnosis.
Listeria monocytogenes is a facultative, motile, gram-positive rod that is capable of invading several cell types and causes intracellular infection. Most cases of infection caused by L monocytogenes are sporadic, but outbreaks have been traced to eating contaminated food, including unpasteurized dairy products, hot dogs, delicatessen meats, cantaloupes, and ricotta cheese. Outbreaks have been associated with significant morbidity and mortality in infected persons.
Five types of infection are recognized:
(1) Infection during pregnancy, usually in the last trimester, is a mild febrile illness without an apparent primary focus and may resolve without specific therapy. However, approximately one in five pregnancies complicated by listeriosis result in spontaneous abortion or stillbirth and surviving infants are at risk for clinical neonatal listeriosis.
(2) Granulomatosis infantisepticum is a neonatal infection acquired in utero, characterized by disseminated abscesses, granulomas, and a high mortality rate.
(3) Bacteremia with or without sepsis syndrome is an infection of neonates or immunocompromised adults. The presentation is of a febrile illness without a recognized source.
(4) Meningitis caused by L monocytogenes affects infants under 2 months of age as well as older adults, ranking third after pneumococcus and meningococcus as common causes of bacterial meningitis. Cerebrospinal fluid shows a neutrophilic pleocytosis. Adults with meningitis are often immunocompromised, and cases have been associated with HIV infection and therapy with tumor necrosis factor (TNF) inhibitors such as infliximab.
(5) Focal infections, including adenitis, brain abscess, endocarditis, osteomyelitis, and arthritis, occur rarely.
At-risk patients (eg, pregnant women) should avoid unpasteurized milk products. Smoked seafoods, cold cuts, hot dogs, and meat spreads also carry risk. Thoroughly cook animal source food and wash raw vegetables.
Ampicillin, 8–12 g/day intravenously in four to six divided doses (the higher dose for meningitis), is considered the treatment of choice. Gentamicin, 5 mg/kg/day intravenously once or in divided doses, is synergistic with ampicillin against Listeria in vitro and in animal models, and the use of combination therapy may be considered during the first few days of treatment to enhance eradication of organisms. In patients with penicillin allergies, trimethoprim-sulfamethoxazole has excellent intracellular and cerebrospinal fluid penetration and is considered an appropriate alternative. The dose is 10–20 mg/kg/day intravenously of the trimethoprim component. Mortality and morbidity rates still are high. Therapy should be administered for at least 2–3 weeks. Longer durations—between 3 and 6 weeks—have been recommended for treatment of meningitis, especially in immunocompromised persons.
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