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.
Treatment: Listerial Infections
Ampicillin (2 g IV q6h) is the drug of choice for the treatment of listerial infections; penicillin is also highly active.
- Many experts recommend the addition of gentamicin (1.0-1.7 mg/kg IV q8h) for synergy.
- For penicillin-allergic pts, trimethoprim-sulfamethoxazole (15-20 mg of TMP/kg IV daily in divided doses q6-8h) should be given. Cephalosporins are not effective.
- Neonates should receive ampicillin and gentamicin, dosed by weight.
The duration of therapy depends on the syndrome: 2 weeks for bacteremia, 3 weeks for meningitis, 6-8 weeks for brain abscess/encephalitis, and 4-6 weeks for endocarditis. Early-onset neonatal disease can be severe and requires treatment for >2 weeks.
Prognosis: With prompt therapy, many pts recover fully.
- However, permanent neurologic sequelae are common in pts with brain abscess or rhombencephalitis.
- Of live-born treated neonates in one series, 60% recovered fully, 24% died, and 13% were left with neurologic or other complications.
Prevention: Pregnant women and other persons at risk for listeriosis should avoid soft cheeses and should avoid or thoroughly reheat ready-to-eat and delicatessen foods, even though the absolute risk posed by these foods is relatively low.
For a more detailed discussion, see Pollard AJ: Meningococcal Infections, Chap. 180, p. 995; Hohmann EL, Portnoy DA: Listeria monocytogenes Infections, Chap. 176, p. 982, in HPIM-19.