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Listeriosis is caused by the weakly hemolytic, gram­positive bacillus Listeria monocytogenes. It most often occurs in fetuses, in neonates during the first 3 weeks of life, and in older or immunosuppressed adults. The infected fetus usually is stillborn or born prematurely, almost always with lethal listeriosis. This infection produces a milder illness in pregnant women and varying degrees of illness in older or immunosuppressed patients. The prognosis depends on the severity of the underlying disease.


The primary method of transmission in neonatal infection is through the placenta in utero or during passage through an infected birth canal. Other modes of transmission include inhaling contaminated dust; drinking contaminated, unpasteurized milk; coming in contact with infected animals, contaminated sewage or mud, or soil contaminated with stool containing L. monocytogenes: and, possibly, person-to-person transmission.

Signs and symptoms

A person with listeriosis has fever, muscle aches, and sometimes gastrintestinal symptoms such as nausea or diarrhea. If infection spreads to the nervous system, symptoms such as headache, stiff neck, confusion, loss of balance, or convulsions can occur.

Infected pregnant women may experience only a mild, flu-like illness; however, infections during pregnancy can lead to premature delivery, infection of the newborn, or even stillbirth.

Diagnostic tests

L. monocytogenes is identified by its tumbling motility on a wet mound of the culture. Other supportive diagnostic test results include positive culture of blood, spinal fluid, drainage from cervical or vaginal lesions, or lochia from a mother with an infected infant; however, isolation of the organism from these specimens often is difficult. The proportion of monocytes in the blood also increases.


The patient usually receives l.V. ampicillin or penicillin for 3 to 6 weeks, possibly with gentamicin to increase its effectiveness. Alternative treatments include erythromycin, chloramphenicol, tetracycline, and cotrimoxazole.

Ampicillin and penicillin G. with or without gentamicin, are best for treating meningitis caused by L. monocytogenes because they more easily cross the blood-brain barrier. Pregnant women require prompt, vigorous treatment to combat fetal infection.


Pregnant women should avoid contact with wild and domestic animals. Listeria is well controlled in American food products, but food-associated outbreaks have occurred.

Pregnant women should avoid consumption of soft cheeses, deli meats, and cold salads from salad bars. Foreign food products such as non-pasteurized soft cheeses have also been implicated in outbreaks of listeriosis. Food should always be adequately cooked.

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