First of all, experts point out that the name listeria, while accurate, isn’t a very precise term for these outbreaks. Listeria is the name of a common family of bacteria that turns up almost everywhere — including in food, water, soil, and the human gut. (1) Not all of these listeria bacteria cause people harm. “It’s a mouthful for sure,” Dr. Harris says of the name Listeria monocytogenes, which is often abbreviated as L. monocytogenes or simply LM. What’s special about LM? Unlike harmless strains of Listeria, LM possesses genes that allow it to penetrate human epithelial cells (those that form the barriers of our bodies’ organs) and slip through the walls of the intestines, where it can then cause an infection, explains Janet Donaldson, PhD, a professor of cell and molecular biology at the University of Southern Mississippi. It’s also important to note that the illness caused by LM is actually termed “listeriosis.” So while a person can be exposed to or infected by listeria bacteria, the resulting infection is called listeriosis. (2) RELATED: The Most Common Culprits of Foodborne Illness It’s not always clear exactly how or why L. monocytogenes contaminates some foods but not others. The LM bacterium is complex, and the presence of other types of microorganisms can both encourage and discourage it to spread. (4) But one thing experts do know is that cooking or heating food kills Listeria monocytogenes. So it’s no surprise that uncooked or “ready-to-eat” foods are the most likely to harbor harmful LM bacteria, Harris says. How sick someone gets after eating a food contaminated with Listeria monocytogenes (or otherwise being exposed to the bacteria) depends on how severe the infection is. Listeriosis is considered “noninvasive gastrointestinal listeriosis” when the infection stays in a person’s gastrointestinal tract. This type is generally more mild, causing common symptoms, like stomach pain and diarrhea. (5) More severe cases are known as “invasive listeriosis,” which is when the infection escapes the gut and gets into a person’s bloodstream, where it causes a much more serious reaction — including blood poisoning, meningitis, and sometimes death. RELATED: How to Prevent Listeria Infection Roughly one in five cases of the more severe type of infection, invasive listeriosis, is deadly, according to the CDC. (6) Major listeria outbreaks are the ones that make the news, and they’re usually linked to specific foods that were somehow contaminated during production, storage, or transportation. But the majority of listeriosis cases are termed “sporadic” — meaning they’re one-offs not linked to a larger outbreak or food contamination issue, according to epidemiology research. (5) In these sporadic cases, experts aren’t always sure exactly how people are exposed, although case reports suggest many of these people had eaten raw or undercooked meat or dairy products. It’s also possible that these sporadic cases stem from people who didn’t cook or store food properly. (5). RELATED: DNA Sequencing Is Changing How Listeria Outbreaks Get Tracked In rare cases, it’s also possible for people to acquire a listeria infection after handling or spending time around farm animals or their manure, according to research published in June 2018 in the journal Foods. (7) Anyone taking immunosuppressant drugs — notably chemotherapy treatments — is also at increased risk for an invasive listeria infection. (9) The CDC estimates that people with cancer are 10 times more likely than healthy adults to develop listeriosis, while people on dialysis are 50 times more likely to become infected. (5) Listeriosis is also a significant threat to pregnant women, who are roughly 10 times more likely than healthy adults to develop an infection. This increased risk is thought to be the result of changes to the cells of the immune system that happen during pregnancy. (10) Pregnant women can also pass the Listeria infection on to the fetus. In these situations, the risks of miscarriage or a stillbirth are high. (11) Healthy people, meanwhile, don’t have as much to worry about when it comes to listeria. While it’s possible for a healthy adult to be infected by L. monocytogenes, that person would have to consume very high quantities of the bacteria — roughly 10 times the amount required to make someone who is at risk sick. (1) Even in those situations, the infection is usually mild and most people don’t require medical attention. (6) A healthy immune system is usually able to produce cytokines and other cellular defenses that eradicate the invading bacteria. (5) RELATED: How Doctors Treat Listeria Infection
