Symptoms of Listeriosis
The incubation period (time between ingestion and the onset of symptoms) for Listeria ranges from three to 70 days and averages 21 days (Bryan, 1999).
A person with listeriosis may develop fever, muscle aches, and sometimes gastrointestinal symptoms such as nausea or diarrhea (CDC website, 2009). If infection spreads to the nervous system, symptoms such as headache, stiff neck, confusion, loss of balance, or convulsions can occur. In immune-deficient individuals, Listeria can invade the central nervous system, causing meningitis and/or encephalitis (brain infection). Infected pregnant women ordinarily experience only a mild, flu-like illness; however, infection during pregnancy can lead to miscarriage, infection of the newborn or even stillbirth.
Newborns may present clinically with early-onset (less than 7 days) or late-onset forms of infection (7 or more days) (Bortolussi, 2008). Those with the early-onset form are often diagnosed in the first 24 hours of life with sepsis (infection in the blood). Early-onset listeriosis is most often acquired from the mother through transplacental transmission. Late-onset neonatal listeriosis is less common than the early-onset form. Clinical symptoms may be subtle and include irritability, fever and poor feeding. The mode of acquisition of late-onset listeriosis is poorly understood.