Listeriosis is predominantly a food-borne disease caused by the ubiquitous gram-positive bacterium Listeria monocytogenes, initially recognized as a foodborne pathogen in the early 1980s. Groups likely to get infected are the elderly, neonates, pregnant women, HIV-patients or those with immunodeficient states. As high mortality rate is accompanying Listeria infections, effective treatment and prevention are essential.

Antimicrobial therapy

Successful treatment of listeriosis with ampicillin or penicillin as a monotherapy has been reported in the medical literature. Nevertheless, since in vitro tolerance or even resistance to penicillin alone has been described, and there are a plethora of studies showing in vitrosynergy and improved clinical efficacy, combination therapy with ampicillin and gentamicin represents the initial regimen of choice.

The duration of therapy for bacteremia should be between one and two weeks, whereas meningitis cases may need to be treated for longer periods of time (up to three weeks). On the other hand, infective endocarditis and brain abscesses necessitate treatment for six to eight weeks. Doses should be varied according to the patients’ altered organ function, with antimicrobial serum monitoring when appropriate.

In neonates, an ampicillin dose of 150 to 200 mg/kg per day for nonmeningeal infections or 300 to 400 mg/kg per day for Listeria meningitis is recommended, however, certain studies suggest that dosages for meningitis should be used in all cases. Furthermore, the higher dose is appropriate for treating listeriosis in immunocompromised hosts.

During pregnancy, the recommended dosage for listeriosis is 2 grams of ampicillin every 6 to 8 hours, which is a dose that provides adequate intracellular penetration and crossing of the placenta. Optimal duration of therapy in pregnancy has not been established, but 3-4 weeks of treatment is considered as a minimum.

Alternative antimicrobial drug and regimes are sometimes needed in certain patient groups may necessitate alternative antimicrobial drugs as a result of allergies or specific diseases. Second-line agents that can be used include trimethoprim-sulfamethoxazole as the best alternative single agent, followed by erythromycin, vancomycin, imipenem and fluoroquinolones. It must be noted that cephalosporins do not show adequate activity against Listeria monocytogenes.

Prognosis and prevention

Untreated listeriosis is fatal within 4 days, and even treatment is unable save the patients with underlying immunosuppression in up to 40% of cases. The bacteriemic and meningitic forms of listeriosis can be cured, but serious complications can ensue despite prompt antimicrobial therapy. After the infection of central nervous systems, sequelae such as strabismus, hydrocephalus and retardation may arise.

Therefore, the best approach is the prevention of listeriosis. General recommendations for prevention are the same as for other foodborne infections, and include thorough cooking of raw food from animal sources, careful washing of raw vegetables before cooking, the avoidance of unpasteurized milk, as well as adequate hand hygiene.

Those at increased risk of acquiring listeriosis should refrain from eating soft cheeses, refrigerated pâtes, refrigerated smoked seafood, meat spreads, luncheon meats and deli meats (unless they are cooked until steaming hot). Cross-contamination of other foods, utensils and food preparation surfaces with fluids from hot dog packages should be avoided.

Considering food consumption patterns and current changes in production, continuous monitoring and improvement of surveillance systems is a way to ensure ongoing public health benefits and provide future development of food safety policy. Doctors (and especially obstetricians) should act as food safety educators, as pregnant women will not likely change their attitude if they do not recognize or believe the source of the information.