When a person is infected and develops symptoms of Listeria infection, the resulting illness is called listeriosis. Only a small percentage of persons who ingest Listeria fall ill or develop symptoms. For those who do develop symptoms as a result of their infection, the resulting illness is either mild or quite severe—sometimes referred to as a “bimodal distribution of severity.”
On the mild end of the spectrum, listeriosis usually consists of the sudden onset of fever, chills, severe headache, vomiting, and other influenza-type symptoms. Along these same lines, the CDC notes that infected individuals may develop fever, muscle aches, and sometimes gastrointestinal symptoms such as nausea or diarrhea. When present, the diarrhea usually lasts 1-4 days (with 42 hours being average), with 12 bowel movements per day at its worst. Most healthy adults and children who consume contaminated food experience only mild to moderate symptoms. The infection is usually self-limited, since, in healthy hosts, exposure to Listeria stimulates the production of tumor necrosis factor and other cytokines, which activate monocytes and macrophages to eradicate the organism. Few people with normal immune function go on to have more severe, life-threatening forms of listeriosis, characterized by septic shock, meningitis and encephalitis.
Early diagnosis and treatment of listeriosis in high-risk patients is critical, since the outcome of untreated infection can be devastating. This is especially true for pregnant women because of the increased risk of spontaneous abortion and preterm delivery. Depending on the risk group, rates of death from listeriosis range from 10% to 50%, with the highest rate among newborns in the first week of life.
Methods typically used to identify diarrhea-causing bacteria in stool cultures interfere or limit the growth of Listeria, making it less likely to be identified and isolated for further testing. On the other hand, routine methods are effective for isolating Listeria from spinal fluid, blood, and joint fluid. Magnetic-resonance imaging (MRI) is used to confirm or rule out brain or brain stem involvement.
Invasive infections with Listeria can be treated with antibiotics. When infection occurs during pregnancy, antibiotics given promptly to the pregnant woman can often prevent infection of the fetus or newborn. Babies with listeriosis receive the same antibiotics as adults, although a combination of antibiotics is often used until physicians are certain of the diagnosis.