What Is Listeria? Symptoms, Causes, and Treatment

It feels like every week we hear about another food recall due to listeria, with headlines that make the bacteria seem like an ever-present danger lurking in our food supply. The latest news concerns frozen waffles; a manufacturer behind various store brand frozen waffles identified potential listeria contamination during routine testing, and voluntarily recalled some products, despite there being no reported illnesses connected with the foods, per NPR. Earlier this year, an outbreak traced to Boar’s Head brand liverwurst loaves at deli counters across 13 states — resulted in 59 hospitalizations and 10 deaths across 19 states.

While this sounds scary, here’s some context. Since 2004, there have been between two and seven listeria-related food recalls each year. Considering listeria bacteria are everywhere in our environment, this relatively low number is not a sign of rampant danger but rather, an indication of how closely our food is monitored to keep it safe.

Still, the fact that listeria can cause serious illness means that it’s worth being aware of the signs, symptoms, and risks of the bacteria, and understanding the most effective ways to stay safe. So here’s a primer to everything you need to know about listeria and how to prevent it.

What Is Listeria?

Listeria is shorthand for listeria monocytogenes, a hardy bacterium that causes listeriosis, a potentially severe foodborne illness. Listeria monocytogenes is found worldwide, in everywhere from soil to water to animal feces, and can cause illness in humans and other animals.

Unlike many bacteria, listeria can survive and reproduce in harsh conditions like cold temperatures of a refrigerator and without oxygen. This makes it challenging to control if the bacteria contaminate foods or common food prep areas. The good news is that listeria is not invincible. While it can withstand cold, it is no match for heat or disinfectants; cooking food thoroughly and cleaning surfaces can kill the bacteria and prevent infection.

What Are Symptoms of Listeria?

Listeriosis, the illness caused by listeria monocytogenes, presents with symptoms between three days to a month after consuming contaminated food. For most people, symptoms are generally mild and flu-like, and include fever, chills, muscle aches, fatigue, and headache. Gastrointestinal symptoms are also present and include nausea, diarrhea, vomiting, and abdominal pain.

However, in individuals with weakened immune systems, listeria can spread beyond the gastrointestinal tract, leading to invasive listeriosis. This typically causes more severe illness that impacts the nervous system. Invasive listeriosis can cause symptoms like severe headache, stiff neck, confusion, and seizures. Among high-risk individuals, listeriosis can have mortality rates up to 30 percent, making it the third leading cause of foodborne illness-related deaths in the United States, after nontyphoidal salmonella and toxoplasma.

Pregnant individuals are particularly vulnerable; an infection during pregnancy can lead to miscarriage, stillbirth, or preterm birth. This is why listeriosis is considered a significant public health concern, especially for pregnant people and certain other at-risk groups.

The majority of listeriosis cases can be managed at home with rest and hydration. That said, if you have vomiting and diarrhea lasting more than 48 hours, blood in stool or vomit, or if symptoms worsen, you should seek prompt medical care. And if symptoms like confusion, a stiff neck, or convulsions appear at all, you should seek immediate treatment.

The exception is if you’re in a high risk group. In that case, if you develop any of the above symptoms you should seek immediate medical attention, even if it hasn’t been 48 hours or you don’t know whether symptoms are due to listeria or another gastrointestinal infection. Prompt intervention reduces the risk of complications, so when in doubt, it is always safer to see your doctor.

What Foods Are Most Likely to Carry Listeria?

Listeria can be found in a variety of foods, especially those that are improperly manufactured, processed or stored. Common sources include unpasteurized milk and dairy products, unpasteurized juices, raw meat, ready-to-eat deli meats and meat spreads, smoked seafoods, raw sprouts, and other produce items contaminated during processing or storage. Soft cheeses are also associated with listeria, particularly if they are made from unpasteurized milk.

Another source of listeria contamination is food prepared and handled at grocery stores, including deli meats and ready-to-eat products. This is especially a concern if stores do not implement appropriate disinfectant procedures, as listeria can survive on stainless steel and other food preparation surfaces for weeks. The recent outbreak linked to deli counters is a prime example of how listeria can infiltrate food products that are consumed cold or with minimal further preparation.

Who Is Most at Risk For Listeria?

While anyone can contract listeriosis, certain groups are at a higher risk of severe illness, and account for the majority of invasive listeriosis cases. Pregnant individuals, for instance, are about 10 times more likely than the general population to get listeriosis. That’s because pregnancy causes mild immunosuppression — an evolutionary feature to prevent rejection of the fetus. The bacteria can also cross the placenta, and fetal infection can lead to stillbirth, miscarriage, and premature delivery.

People with weakened immune systems, whether due to chronic illness, medications, or other factors, are also more susceptible to severe outcomes if they contract listeria. Infants, who have underdeveloped immune systems, and adults over 65, whose immune function may be diminished, fall into this group as well.

Staying Safe From Listeria

Given the serious nature of listeriosis, it’s essential to take precautions to minimize your risk. The good news? Practicing good food hygiene is quite effective to help protect you from listeria and other foodborne illnesses.

Avoid unpasteurized foods, especially dairy products. Raw meat should be cooked thoroughly, and ready-to-eat meats like hot dogs and deli meats should be reheated before consuming. Wash fresh produce with clean, running water prior to preparing and eating.

Prevent cross-contamination by washing your hands thoroughly with soap and water before and after handling foods. Clean all kitchen surfaces with effective disinfectants like quaternary ammonium compounds (quats) or household bleach, both of which kill listeria. Clean cutting boards and utensils with hot soapy water, and use separate cutting boards when handling raw meats and other foods.

Food storage is the final piece of the puzzle. Make sure your refrigerator is set to 40°F (4°C) or colder. Listeria can survive in the refrigerator, but keeping perishable foods cold slows their growth, minimizing the potential risk of enough bacteria present to cause infection. Store leftovers and raw meats in airtight containers so they don’t potentially contaminate other foods.

Stay Safe But Don’t Panic

While it might seem that we can’t go a day without a headline about listeria, that perception is a result of repetitive news reports, and in reality, should reassure you of the vigilance of our public health systems. Remember, the goal of recalls are always to protect the public using the systems designed to keep our food supply safe. So, while listeria is a concern we don’t want to take lightly, you can continue to enjoy your meals while taking precautions and staying informed.

Andrea Love, PhD, is a biomedical scientist and award-winning science communicator. She has over a decade and a half of expertise in infectious diseases, cancer, and immunology. She excels at translating complex scientific concepts and data for broad audiences. Outside of her career, she is the founder of the science communication organization ImmunoLogic and is the executive director of the American Lyme Disease Foundation. Dr. Love is a PS Council member.