Because there are few symptoms that are unique to listeriosis, doctors must consider a variety of potential causes for infection, including viral infections (like flu), and other bacterial infections that may cause sepsis or meningitis. 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.
Listeriosis is usually a self-limited illness—which means that most infected individuals will improve without the need for medical care. But for those patients with a high fever, a stool culture and antibiotic-treatment may be justified for otherwise healthy individuals. Although there have been no studies done to determine what drugs or treatment duration is best, ampicillin is generally considered the “preferred agent.” There is no consensus on the best approach for patients who are allergic to penicillin.
For those persons who suffer a Listeria infection that does not resolve on its own, the complications (or sequelae) can be many. The most common is septicemia (bacterial pathogens in the blood, also known as bacteremia), with meningitis being the second most common. Other complications can include inflammation of the brain or brain stem (encephalitis), brain abscess, inflammation of the heart-membrane (endocarditis), and localized infection, either internally or of the skin.
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.
Death is the most severe consequence of listeriosis, and it is tragically common. For example, based on 2009 FoodNet surveillance data, 89.2% of Listeria patients ended up in the hospital, the highest hospitalization rate for pathogenic bacterial infection. In persons 50 years of age and older, there was a 17.5% fatality rate—also the highest relative to other pathogens.