LISTERIAL INFECTIONS is a topic covered in the Harrison's Manual of Medicine.

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  • Etiology and microbiology: Listeria monocytogenes is a food-borne pathogen that can cause serious infections, particularly in pregnant women and immunocompromised individuals.
    • The organism is a facultatively anaerobic, nonsporulating, gram-positive rod that demonstrates motility when cultured at low temperatures.
    • After ingestion of food that contains a high bacterial burden, virulence factors expressed by Listeria allow internalization into cells, intracellular growth, and cell-to-cell spread.
  • Epidemiology
    • Listeria is commonly found in processed and unprocessed foods such as soft cheeses, delicatessen meats, hot dogs, milk, and cold salads; fresh fruits and vegetables can also transmit the organism.
    • There is no human-to-human transmission (other than vertical transmission from mother to fetus) or waterborne infection.
  • Clinical manifestations: Listeria causes several clinical syndromes, of which meningitis and septicemia are most common.
    • Gastroenteritis: can develop within 48 h after ingestion of contaminated foods containing a large bacterial inoculum
      • Listeriosis should be considered in outbreaks of gastroenteritis when cultures for other likely pathogens are negative.
      • Sporadic cases appear to be uncommon.
    • Bacteremia: Pts present with fever, chills, myalgias, and arthralgias. Endocarditis is uncommon and is associated with fatality rates of 35–50%.
    • Meningitis: Listeria causes ~5–10% of cases of community-acquired meningitis in adults in the United States, with case–fatality rates of 15–26%.
      • Listerial meningitis differs from meningitis of other bacterial etiologies in that its presentation is often subacute, with meningeal signs and photophobia being less common.
      • The CSF profile usually reveals <1000 WBCs/μL, with a less marked neutrophil predominance than in other meningitides. Low glucose levels and a positive Gram’s stain are seen in ~30–40% of cases.
    • Meningoencephalitis and focal CNS infection: Listeria can directly invade the brain parenchyma and cause cerebritis or focal abscess.
      • Of CNS infections, ~10% are macroscopic abscesses, which are sometimes misdiagnosed as tumors.
      • Brainstem invasion can cause severe rhombencephalitis, with asymmetric cranial nerve defects, cerebellar signs, and hemiparetic/hemisensory defects.
        • Infection in pregnant women and neonates: Listeriosis is a serious infection in pregnancy.
          • Pregnant women are usually bacteremic and present with a nonspecific febrile illness that includes myalgias/arthralgias, backache, and headache; CNS involvement is rare. Infected women usually do well after delivery.
          • Infection develops in 70–90% of fetuses from infected women; almost 50% of infected fetuses die. This risk can be reduced to ~20% with prepartum treatment.
          • Overwhelming listerial fetal infection—granulomatosis infantiseptica—is characterized by miliary microabscesses and granulomas, most often in the skin, liver, and spleen.
          • Late-onset neonatal disease develops ~10–30 days after delivery by mothers with asymptomatic infection.
      • Diagnosis: Timely diagnosis requires that the illness be considered in groups at risk: pregnant women, elderly pts, neonates, immunocompromised pts, and pts with chronic underlying medical conditions (e.g., alcoholism, diabetes).
        • Listeriosis is diagnosed when the organism is cultured from a usually sterile site, such as blood, CSF, or amniotic fluid.
        • Listeriae may be confused with “diphtheroids” or pneumococci in gram-stained CSF or may be gram-variable and confused with Haemophilus spp.
        • Serologic tests and PCR assays are not clinically useful at present.

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