Listeria Infection: Symptoms & Treatment
A Listeria bacterium infecting tissue.
Credit: CDC/ Dr. Balasubr Swaminathan; Peggy Hayes

Listeriosis is an infection caused by eating Listeria monocytogenes bacteria, according to the Centers for Disease Control and Prevention (CDC). Over the past few years, the bacteria have been responsible for dozens of food product recalls, numerous hospital visits and even deaths. In fact, according to the CDC, Listeria infections cause 1,600 illnesses and 260 deaths each year in the United States.

A Listeria infection is typically caused by eating contaminated foods, according to the Mayo Clinic. The bacteria live naturally in soil and water in the environment, and it is possible for vegetables to become contaminated by Listeria when growing in contaminated soil, or when manure is used a fertilizer, Dr. Robert Glatter, an emergency physician at Lenox Hill Hospital in New York City told Live Science.

Other common ways someone can come into contact with L. monocytogenes is by consuming meat products obtained from animals that carry the bacteria but don’t show any symptoms. This is particularly true if the meat was not cooked. Foods that are made from unpasteurized milk are also common culprits, said John R. Palisano, a professor of biology at Sewanee: The University of the South in Sewanee, Tennessee.

After being consumed, the bacteria travel through the gastrointestinal tract and eventually to the bloodstream. "Listeria-producing toxins can actually damage cells," said Dr. Jane Frederick, FACOG, a specialist in reproductive endocrinology and infertility at the HRC Fertility in Orange County, California. "One of Listeria’s favorite places to invade is the nervous system. This invasive bacteria can grow there, leading to meningitis or even encephalitis."

The bacteria have a trick to infect a patient. "Listeria monocytogenes is sneaky because it hides inside our cells, and it can travel from cell to cell without leaving the intracellular environment, making it very hard for the immune cells and immune molecules in our blood to “see” it and attack it," Dr. Aileen M. Marty a professor of infectious diseases at the Herbert Wertheim College of Medicine, told Live Science. "Its deadly weapons are powerful poisons (toxins) that it can release and cause us great harm." 

For healthy people, a Listeria infection can simply cause a few undesirable symptoms. The symptoms include headache, fever, chills, upset stomach and vomiting, according to the National Library of Medicine. Pregnant women usually only experience mild, flu-like symptoms, though, accord to the New York State Department of Health.

Listeriosis is dangerous for those with a weakened immune system and older adults, though, according to the Mayo Clinic. In an outbreak caused by contaminated salads in early 2016, for example, the average age of those affected was 81.

A Listeria infection is also dangerous for pregnant mothers. "One of the reasons it is so scary for pregnant mothers is that these special proteins help the bacterium cross the 'placental barrier' that normally protects the growing baby from germs in the mother’s blood," said Marty. Contracting a Listeria infection while pregnant can cause stillbirth, miscarriage, premature delivery or a life-threatening infection for the newborn.

Because listeriosis has symptoms that are much like the flu, a doctor will need to run tests beyond just a physical examination. A blood test is the most common way to determine if a person has an infection. A doctor may also order urine or spinal fluid tests, as well, according to the Mayo Clinic.

In many cases, those infected will simply need to let their immune system fight through the disease. Those with more severe cases, newborns or those who are pregnant are often prescribed antibiotics, according to the Mayo Clinic.

The best way to prevent a Listeria infection is by using careful food preparation techniques. Here are tips for avoiding the bacteria:

  • All meat should be properly cooked. 
  • Refrigerate or freeze cooked foods within two hours of preparation.
  • All milk products should be made with pasteurized milk to minimize the risk of contaminating food. 
  • Meat that is properly cooked at a factory can become contaminated after cooking, but before packaging, so unopened hot dogs or luncheon meats should not be stored in the refrigerator for more than two weeks, and opened packages for less than one week. 
  • Food handlers should wash their hands for at least 20 seconds with warm soapy water when working with food.
  • Don’t use the same knife or cutting board for vegetables after using these items to cut uncooked meat. Also, knives used on vegetables and fruit should not be used to cut cooked meat unless they are properly cleaned. 
  • Unlike most bacteria, Listeria monocytogenes can grow in refrigerators. Use caution when storing foods that might be contaminated.
  • Do not eat foods that have been recalled for Listeria contamination. Follow the manufacturer’s instructions on returning or disposing of the product. 
  • Wash all fruits and vegetables with cold, running water. "Even if the skin of vegetables or husk of fruit, like melons, is not eaten, they should be properly cleaned before they are cut up," Palisano added.
  • Sanitize all food preparation areas.
  • Throw away leftovers after four days.
  • Reheat leftovers to at least 165.2 Fahrenheit (74 Celsius). 

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