January 21, 2019
6 Things to Know About Listeria
Ask the Experts

6 Things to Know About Listeria

by Berkeley Wellness  

Recently Blue Bell Creameries and Sabra Dipping Company (hummus) recalled products nationwide due to potential contamination with Listeria, a bacterium that can cause serious, sometimes fatal, illness. John E. Swartzberg, MD, an infectious disease specialist at the UC Berkeley School of Public Health and Chair of the Editorial Board of BerkeleyWellness.com, answers six important questions about the risks of Listeria, and how you can protect yourself.

What is Listeria and how does it get in food?

Listeria monocytogenes is a bacterium that's always around. It's found in the intestinal tract of animals, including chickens and cows, and can contaminate meat and dairy products from the animals. You can also get it from eating fruits and vegetables contaminated from soil. And, of course, once a person has Listeria, they can pass it to other people when they prepare food.

Why is Listeria so dangerous?

Two things make Listeria so risky to people. First, the organism grows really well in a cold environment. Listeria can survive and multiply in a refrigerator. That's important because refrigeration is the main way we protect ourselves from spoiled food, and in the case of Listeria we don't have that tool.

The second thing is that Listeria, unlike most bacteria, can survive inside the cells in our immune system—macrophages—that normally kill bacteria. A healthy person might get sick from Listeria, but that's unusual. Their macrophages can usually control it. But in a person with a compromised immune system, the Listeria can grow, spread cell to cell, and cause disease. So it's a particular risk to people who've had transplants, are getting cancer treatment, have HIV or diabetes, or who are older or sick in general.

It's also a danger to pregnant women, whose immune systems are naturally compromised so their bodies won't reject the fetus. Women are 20 times more likely to get sick from Listeria than other healthy adults the same age.

How do you know if you have Listeria in your body?

Most people who get Listeria never know it. Their immune system contains the bacterium and they feel fine. If the organism does spread in your body, however, it causes listeriosis, a serious, sometimes fatal disease. The symptoms typically start a week or two after you ingested the organisms, although it can happen within days or take as long as a few months. The usual symptoms are fever, muscle ache, and nausea or diarrhea. If the organisms spread to your brain, they can cause confusion, seizures, loss of balance, and the hallmark symptoms of meningitis—stiff neck and headache. In pregnant women, Listeria can infect the fetus, causing miscarriage, premature delivery, or stillbirth.

Which foods are most likely to be contaminated with Listeria?

Unpasteurized foods that you keep cold in the refrigerator for quite a while are risky. Some top examples are:

  • Hot dogs or luncheon meats, unless they're served steaming hot
  • Pates and other meat spreads such as liverwurst
  • Smoked fish, such as lox, smoked trout, and whitefish
  • Raw sprouts
  • Soft cheeses if they're made from unpasteurized milk—queso fresco, feta, brie, and camembert

Never under any circumstances drink unpasteurized milk because it's more likely to be contaminated with Listeria.

What can I do to avoid getting Listeria?

The surest way is to avoid foods likely to have Listeria—especially unpasteurized dairy foods. Buy pasteurized milk and cheeses. If you like cold, processed meats, eat them soon after purchase. Don't let them sit in your refrigerator for weeks. And wash your fruits and vegetables really well under running water. If you use a vegetable brush, wash the brush in the dishwasher after you use it. Also wash your knives before and after you use them. Hard-rind fruit like cantaloupe won't have Listeria on the inside, but your knife can inadvertently drag the organism inside as you slice.

Be sure to clean your refrigerator regularly to avoid cross contamination between foods. And keep the temperature below 40° F because Listeria grows faster at temperatures above 40° F. These steps won't eliminate your risk entirely, but they'll reduce it.

How do I stay informed about food outbreaks and risks?

The Food and Drug Administration has a great website. You can sign up to receive email alerts about recalls and outbreaks. If a particular food has been recalled, the FDA report will tell you the Universal Product Code, next to the barcode, or the Stock Keeping Unit number so you can check the actual product in your refrigerator. It's important to remember that the "sell-by" date means nothing on a food that's been recalled. If the product label says it's good for another two months, you still have to throw it away. A recalled food is not safe.