What you need to know about the risk of food poisoning from Listeria

This week, the Centers for Disease Control and Prevention announced an expanded recall of dairy products and other foods possibly contaminated with the bacteria Listeria monocytogenes.

The recalled products – all with ingredients manufactured by California-based Rizo López Foods – include cheese, yogurt, sour cream, salad dressings and packets and other foods that use Rizo López products. They are sold under many brand names and by many supermarkets, including Trader Joe's, Whole Foods Market and Marketside.

Most people who ingest listeria do not become very ill, if they develop symptoms at all. But certain high-risk individuals can become seriously ill. About 1,600 people In the United States, serious listeria infections, known as listeriosis, develop every year — and about 260 of those infections are fatal, said Laura Gieraltowski, an epidemiologist in the CDC's outbreak response and prevention division.

In the current outbreak, there have been 26 confirmed cases in 11 states, including 23 hospitalizations and two deaths.

Here's what you need to know about listeria to stay safe.

How does listeria get into food?

Listeria bacteria occur naturally in the soil and therefore also end up on or in things that come into contact with the soil, such as plants and animals.

“We find it almost everywhere in nature if we look for it,” says Catherine Donnelly, a professor emeritus at the University of Vermont who has studied listeria for decades.

The bacterium is also “a tough germ, and it's tough – it can survive for a long time,” explained Dr. Gieraltowski out. Food contaminated with listeria can deposit the bacteria on counters, deli slicers, and other surfaces where food is prepared and processed, where it can linger and contaminate other foods.

“It forms biofilms, which are structures that allow it to adhere to things like stainless steel,” said Dr. Donnelly.

Cantaloupes are often involved in listeria outbreaks because the “rind has all these bumps and grooves that are really great little places for bacteria to hide in” and they can't wash off, said Dr. Gieraltowski. When people then cut into it, listeria is transferred to the blade and the parts of the fruit they eat.

The current outbreak mainly affects foods containing soft cheeses, such as ricotta and cotija, which have a neutral pH and contain a lot of water that the bacteria can use to multiply, said Jasna Kovac, a professor of food science at Penns State University. Listeria does not grow nearly as well in harder cheeses such as Cheddar and Parmesan, because these are more acidic and contain less water.

What do the symptoms look like and how can you prevent an infection?

It can take anywhere from a few days to as long as three months for symptoms to appear because the bacteria grow so slowly in the body, Dr. Donnelly out.

Most people who become ill from contaminated food will have flu-like symptoms for a few days – fever, diarrhea, vomiting – or may have no symptoms at all. They probably won't need any kind of treatment and will start to feel better on their own, said Dr. Gieraltowski.

But people who are 65 or older, pregnant or have a weakened immune system are at greater risk for listeria infections that spread from the intestines to the blood, said Dr. Kovac. When this happens, people tend to experience more severe flu-like symptoms and may need to be hospitalized. A fetus can also become infected, because listeria bacteria can cross the placenta.

For these reasons, the CDC recommends people in high-risk groups avoid food often contaminated with listeria, including unpasteurized soft cheeses, ready-made deli salads and deli meats, meats and cheeses that have been sliced ​​in a deli but not reheated. (The bacteria can be killed by heating food to a sufficiently high temperature.)

Another way to minimize your risk is to refrigerate food within two hours of cooking, said Dr. Kovac.

And always keep your refrigerator at or below 40 degrees. Many people store their plants at slightly higher temperatures, which allows listeria to thrive, explains Dr. Donnelly out. A few degrees cooler can make 'a huge difference'.