The ability of the Listeria organism to avoid the extracellular environment also explains the increased incidence of listeriosis in immunocompromised patients, neonates, and pregnant women. Increased risk of listeriosis has not been associated with deficiencies of immunoglobulins or complement. Nearly one third of those who contract listeriosis infection are pregnant women, with infection occurring most frequently in the third trimester. The symptoms tend to be relatively mild, mimicking a flulike illness, with chills, fever, and muscle aches. Blood cultures, though not always obtained, are positive.
Symptoms usually resolve spontaneously without therapy.
The organism may cause amnionitis as well as precipitate premature labor, leading to septic abortion. Transplacental transmission can also cause the unique clinical syndrome of granulomatosis infantiseptica. The organism can disseminate in utero, forming abscesses and granulomas involving the fetal liver, spleen, lungs, kidneys, brain, and skin. Mortality is high, ranging from 35 to 55 percent. In addition to causing congenital infection, the Listeria pathogen can cause meningitis in neonates seven to 28 days of age.