Listeria monocytogenes is a foodborne bacterium that causes listeriosis, a rare but serious infection. Each year, about 1,600 people in the United States become ill from listeriosis, with approximately 260 deaths reported, highlighting its potential severity1 . While healthy individuals often experience mild symptoms, vulnerable groups such as pregnant women, newborns, older adults, and people with weakened immune systems face a higher risk of severe illness and complications1 2.
Mild Listeria Infection Symptoms
In healthy individuals, Listeria infection typically causes a noninvasive form of illness known as febrile gastroenteritis. This mild infection usually develops within hours to a few days after consuming contaminated food3 . Symptoms often include:
- Fever3
- Diarrhea3
- Vomiting3
- Nausea and upset stomach2
- Muscle aches and fatigue4
These symptoms generally resolve on their own without the need for medical treatment3 . It is estimated that between 1% and 10% of the population may carry Listeria bacteria in their intestines without symptoms5 . The bacteria can survive refrigeration and even freezing, making ready-to-eat refrigerated foods a common source of infection2 6.
Foods commonly linked to Listeria contamination include soft cheeses, deli meats, hot dogs, and unpasteurized dairy products1 2. The infection mechanism involves ingestion of contaminated food, followed by bacterial colonization in the gastrointestinal tract, where it can cause inflammation and mild gastrointestinal symptoms3 7.
Severe Listeria Infection Symptoms
In high-risk individuals—such as those with weakened immune systems, older adults, and pregnant women—Listeria can cause invasive disease. This occurs when the bacteria spread beyond the intestines into the bloodstream or central nervous system (CNS), leading to serious conditions such as bacteremia, sepsis, meningitis, and meningoencephalitis8 95.
Symptoms of invasive listeriosis often appear within 1 to 4 weeks after exposure but can occur as late as 70 days post-infection1 . Key symptoms include:
- High fever (above 38°C) 10
- Chills and night sweats10
- Severe headache11 4
- Stiff neck11 4
- Confusion or altered mental status, including decreased Glasgow Coma Scale (GCS) scores11
- Loss of balance and coordination4
- Seizures4
- Tachycardia (rapid heartbeat) 10
- Signs of sepsis such as lethargy and poor general condition10
Listeria can also cause rare but severe infections such as rhombencephalitis, an inflammation of the brainstem, which carries high morbidity and mortality12 13. Endocarditis and pneumonia are less common manifestations but have been reported8 14.
Invasive listeriosis is life-threatening, especially for newborns, older adults, and immunocompromised individuals. Early diagnosis and treatment with antibiotics are critical to improving outcomes11 128.
Listeria Symptoms During Pregnancy
Pregnant women are particularly vulnerable to Listeria infection, being about 10 times more likely to develop listeriosis than the general population15 . The infection during pregnancy often causes only mild or nonspecific symptoms in the mother, such as fever and flu-like symptoms including muscle aches and fatigue1 4.
The incubation period for invasive disease in pregnancy ranges from 1 to 4 weeks but can extend up to 70 days after exposure1 . Despite mild maternal symptoms, the infection can have devastating effects on the fetus or newborn, including:
- Miscarriage16 17
- Stillbirth16 17
- Preterm delivery16 17
- Neonatal sepsis and life-threatening infections9 18
Vertical transmission from mother to fetus can occur even if the mother shows no symptoms, complicating early diagnosis18 . Pregnant women may experience mild fever, abdominal or back pain, and rarely gastrointestinal symptoms19 .
Pregnant women are about 10 times more likely to get listeriosis than the general population15 .
Pregnant women are advised to avoid high-risk foods to reduce the chance of infection:
- Soft cheeses, especially Hispanic-style and unpasteurized varieties1 15
- Deli meats and hot dogs unless heated until steaming hot2 15
- Refrigerated pâtés and meat spreads2 15
- Refrigerated smoked seafood unless cooked or canned2 15
- Raw or unwashed fruits and vegetables2 6
Listeria Symptoms in Newborns
Neonatal listeriosis results from vertical transmission during pregnancy or birth and is associated with high mortality and morbidity18 19. Symptoms in newborns can appear at birth or within the first two months of life and may include:
- Fever or hypothermia (low body temperature) 17
- Poor feeding or little interest in feeding17
- Irritability or crankiness17
- Difficulty breathing17
- Low birth weight19
- Jaundice and rash (less common) 17
All reported cases in some series resulted in fetal loss with 100% fetal mortality, although maternal mortality is rare19 . Severe neonatal infections can lead to lifelong complications such as intellectual disabilities, paralysis, seizures, blindness, and organ damage2 17.
Newborns with listeriosis may develop serious infections like meningitis or sepsis, which can cause lifelong health problems or death if not treated promptly17 .
When to Seek Medical Care
Early medical evaluation is crucial for anyone who has consumed potentially contaminated food and develops symptoms suggestive of listeriosis, especially those in high-risk groups such as pregnant women, newborns, older adults, and immunocompromised persons1 4.
Seek medical care if you experience:
- Fever accompanied by muscle aches or flu-like symptoms after eating high-risk foods4
- Severe headache, stiff neck, confusion, or neurological symptoms such as loss of balance or seizures11 4
- Persistent diarrhea and vomiting lasting more than a few days3
- Symptoms during pregnancy, even if mild, such as fever or unusual fatigue15
- Signs of sepsis, including rapid heartbeat, chills, or lethargy10
Prompt diagnosis and antibiotic treatment can reduce the risk of severe complications and death12 11. Pregnant women who suspect exposure should inform their healthcare provider immediately for appropriate testing and management18 15.
Key Takeaways
- Listeriosis is a serious foodborne infection caused by Listeria monocytogenes, affecting mainly pregnant women, newborns, older adults, and immunocompromised individuals1 2.
- Mild infection in healthy people usually causes fever, diarrhea, and vomiting, resolving without treatment3 2.
- Invasive listeriosis can cause severe symptoms including meningitis, sepsis, and neurological complications, with high mortality if untreated11 109.
- Pregnant women often have mild symptoms but risk miscarriage, stillbirth, and neonatal infection16 15.
- Early medical care and antibiotic treatment are critical for improving outcomes in high-risk groups12 11.
Frequently Asked Questions
What foods are most likely to cause Listeria infection?
Listeria commonly contaminates soft cheeses, deli meats, hot dogs, unpasteurized dairy products, refrigerated smoked seafood, and raw or unwashed produce1 26.
Can healthy people get seriously ill from Listeria?
Healthy individuals usually experience mild, self-limiting symptoms like fever and diarrhea. Serious illness is rare unless exposed to a large bacterial dose3 720.
How long after eating contaminated food do symptoms appear?
Symptoms can start within hours to a few days for mild illness, but invasive disease symptoms may appear 1 to 4 weeks later, sometimes up to 70 days post-exposure1 21.
Is Listeria infection contagious?
Listeria is primarily transmitted through contaminated food and is not spread from person to person, except vertically from mother to fetus18 6.
How is listeriosis treated?
Antibiotics such as ampicillin are used to treat invasive listeriosis. Mild cases in healthy people may not require treatment12 1122.
