Listeria: Symptoms, Causes, Risks & Treatment

Listeria is a type of bacteria, particularly Listeria monocytogenes, that can cause a foodborne infection called listeriosis. It is mainly transmitted by eating contaminated foods, such as unpasteurized milk, certain soft cheeses, deli meats, and refrigerated ready-to-eat foods.

Listeria infection can cause symptoms such as fever, diarrhea, nausea, muscle aches, and headaches. Although some people experience only mild symptoms, the infection can become serious if the bacteria spread to the bloodstream or nervous system, causing complications such as sepsis or meningitis.

Listeriosis is especially dangerous for people who are pregnant, newborns, adults aged 65 or older, and people with weakened immune systems. Treatment depends on the severity of the infection and may involve antibiotics, particularly in severe cases or for people at higher risk of complications.

Main symptoms

The most common symptoms of a listeria infection are: 

• Diarrhea
• General malaise
• Nausea and vomiting
• Muscular pain 
• High-grade fever, over 38ºC (or 100.4ºF) 
• Headache
• Loss of appetite
• Confusion
• Fatigue

Gastrointestinal symptoms of listeriosis can appear up to 24 hours after consuming contaminated food or water, and can last between 2 and 3 days.

In more serious and rare cases, Listeria monocytogenes bacteria can spread through the bloodstream and reach the nervous system. This can lead to meningitis, which is an inflammation of the membranes surrounding the brain.

Furthermore, infections in pregnant women can be serious and result in premature birth or infection at the time of delivery.

Who is at higher risk of Listeria infection?

People at higher risk of listeriosis include:

• Pregnant women: Hormonal and immune system changes during pregnancy may increase susceptibility to Listeria infection;
• Newborns: Babies can become infected during pregnancy or around the time of delivery;
• Adults aged 65 or older: Age-related changes in the immune system increase the risk of invasive infection, hospitalization, and serious complications;
• People with weakened immune systems: This includes people undergoing cancer treatment, organ transplant recipients, and those living with HIV or taking immunosuppressive medications;
• People with certain chronic conditions: Diseases such as diabetes, cancer, and chronic kidney or liver disease may increase the risk of severe infection, particularly when immune function is impaired.

People in these groups should take extra precautions when choosing and preparing food. If they develop fever or other symptoms after eating food that may have been contaminated with Listeria, they should seek prompt medical evaluation.

Confirming a diagnosis

A listeria infection is diagnosed by a family doctor, infectious disease specialist or gastroenterologist, based on the person's symptoms, medical history, and possible exposure to contaminated food.

To confirm the infection, the doctor may order blood tests and look for the bacteria in stool samples, gastric washings, the placenta, or amniotic fluid (in the case of pregnant women); additionally, the food suspected of being the source of the infection may also be analyzed.

How it's transmitted

Transmission of listeria occurs mainly through consumption of contaminated food or water. It can be found in unpasteurized milk, dairy products, fruits, greens, meat, fish, seafood, and sausage.

It has a high transmission rate when outbreaks are declared, as this bacteria is easily distributed to many people through contaminated packaged foods. Many food recalls are declared due to the contamination or possible contamination of this bacteria in food products.

This bacteria is resistant to many stress conditions in the processing of food, which is why it can be found in small concentrations in processed food. The presence of listeria in food does not always lead to infections as long as the food is properly stored to prevent bacterial growth.  

Treatment options

Treatment for listeriosis depends on the severity of the infection, the person's symptoms, and their risk of developing complications.

Mild intestinal infections often resolve without antibiotics. In these cases, treatment mainly involves drinking enough fluids to prevent dehydration and resting while symptoms improve.

Invasive listeriosis requires antibiotic treatment, usually in a hospital. Ampicillin given intravenously is commonly used, sometimes in combination with gentamicin. Trimethoprim-sulfamethoxazole may be considered for certain people with severe penicillin allergies, depending on the clinical situation.

Treatment duration varies according to the severity of the infection and whether complications, such as meningitis or bloodstream infection, are present.

People who are pregnant, adults aged 65 or older, and those with weakened immune systems should seek medical advice promptly if they experience symptoms after possible Listeria exposure.

How to prevent listeriosis

To prevent listeria contamination, you should adopt some hygiene measures like: 

• Avoiding unpasteurized milk and dairy products made with unpasteurized milk;
• Reheating deli meats, hot dogs, and cold cuts to 165°F (74°C) or until steaming hot before eating, especially for people at higher risk;
• Washing fruits and vegetables thoroughly before preparing or eating them;
• Avoiding raw sprouts and certain refrigerated ready-to-eat foods when at increased risk of infection;
• Keeping refrigerators at 40°F (4°C) or below and cleaning them regularly;
• Separating raw and cooked foods to prevent cross-contamination;
• Cooking food thoroughly and following recommended food storage times;
• Checking food recalls and outbreak alerts issued by the CDC and FDA.

Listeria can survive and grow at refrigerator temperatures, so refrigeration alone does not eliminate the bacteria. Proper food handling, cooking, and choosing safer foods are essential for reducing the risk of infection.