1. What every clinician should know
Clinical features and incidence
Listeriosis is mainly a food-borne infection, and sporadic cases as well as epidemics have been linked to contaminated food. Uncooked meats and vegetables as well as unpasteurized milk can be sources of Listeria monocytogenes. Exposure and transient colonization of the gastrointestinal tract is common, occurring in up to 44% of pregnant women, but invasive disease is rare. Pregnancy is a risk factor for listeriosis, with up to 30% of all listeria infections occurring in pregnant women. Immunocompromised patients (e.g., AIDS, steroid therapy) are at higher risk for infection. The prevalence varies from 9 to 17/100,000 live births.
Exposure to listeria can be lessened if pregnant women avoid ready-to-eat meats (cold cuts and fermented meats e.g., Bologna and salami) and dairy products made with unpasteurized milk. Cross contamination of food can be prevented by cleaning all utensils and surfaces after preparing meat dishes or cutting unprepared foods.
2. Diagnosis and differential diagnosis
The nonspecific nature of the presenting symptoms associated with maternal listeriosis make the diagnosis difficult. Pregnant women with listeriosis present with mild flu-like symptoms. Fever, backache, headache, vomiting/diarrhea, myalgias and sore throat may occur. Importantly, up to 30% of gravidas with listeriosis will be asymptomatic. Clinicians should be particularly suspicious for listeria infection if the patient develops evidence of chorioamnionitis (fever, uterine tenderness, fetal tachycardia) in the absence of rupture of the fetal membranes.
The diagnosis is confirmed most often by culture of the maternal blood or amniotic fluid. The obstetrician should consider ordering both a complete blood count looking for a leukocytosis and a blood culture in pregnant women presenting with flu-like symptoms or chorioamnionitis not associated with rupture of the membranes, as this is the best way to confirm a diagnosis of listeria infection. L monocytogenes is a facultative anaerobe Gram-positive rod.
More than likely, gravidas presenting with the flu-like symptoms noted above indeed have a viral syndrome. The recent epidemic of H1N1 influenza should ensure that clinicians are cognizant of the symptoms and diagnostic tests available for evaluation of pregnant women with influenza-like symptoms in order to not only rule out influenza but also identify those women who are candidates for the initiation of antiviral therapy. A normal urinalysis can rule out the possibility of urinary tract infection or pyelonephritis as a cause of these symptoms.
Review of the patient’s diet can provide clues to increase the probability of listeriosis. A history of eating unheated foods such as deli meats or hot dogs, raw vegetables and foods made from unpasteurized milk such as Mexican queso cheese should suggest the possibility of listerosis. A blood culture can be recommended in these patients and is the only way to rule out L monocytogenes bacteremia.
Parenteral antibiotic therapy should be initiated in those patients with confirmed maternal listeriosis and should be considered in those gravidas at high risk for listeriosis based on dietary history, once blood and amniotic fluid cultures are obtained. Although most gravidas with listeriosis will be in labor and go on to deliver, occasionally the diagnosis is confirmed in women not in labor and with no clinical evidence of chorioamnionitis. In these patients it is acceptable to initiate antibiotic therapy without concurrent induction of labor. However, if her condition deteriorates, or clinical improvement does not occur within 24 to 48 hours or if fetal surveillance is not reassuring, delivery, usually initiated with an induction of labor, is appropriate. In those rare patients treated without delivery, repeat blood and amniotic fluid cultures should be performed to prove microbiologic cure of the infection.
The antibiotic regimen of choice is a combination of ampicillin (>6 g/day) and gentamicin (1.5mg/kg every 8 hours). This combination is synergistic in vitro against L monocytogenes, and ampicillin crosses the placenta promptly. L moncytogenes is resistant to cephalosporins and clindamycin. For women with immediate hypersensitivity to penicillin, trimethoprim/sulfamethoxazole is an alternative although not optimal given its association with neonatal kernicterus when used immediately prior to delivery. Penicillin desensitization may be a better option.
Serious maternal disease is rare in otherwise immunocompetent gravidas. However, meningoencephalitis and endocarditis have been reported as complications of maternal listeriosis.
5. Prognosis and outcome
Fetal transmission is common (~70%) but not an invariable result of maternal listeriosis. If the fetus is infected, it commonly results in spontaneous abortion or stillbirth at early gestations. In two large series, preterm labor and delivery occurred in 50% of cases with 11% of fetuses being stillborn. One third of the fetuses were notable for nonreassuring fetal surveillance during labor. Approximately two thirds of neonates born with listeriosis will make a complete recovery with antibiotic therapy; 13% will survive with significant neurologic sequelae. There is a 25% neonatal mortality associated with neonatal listeriosis.
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