What is Listeria and why is it showing up in fresh produce?
By Diane Wright Hirsch, MPH, RD
Yet another outbreak of Listeria monocytogenes has been attributed to fresh produce: bagged lettuce in this case. You may recall the 2011 outbreak associated with cantaloupe that turned out to be the deadliest Listeria outbreak ever in the US. And, Listeria was also the cause of an outbreak tied to apples (and caramel apples) in 2015.
It’s not only produce that has been associated with Listeria contamination. In 2015 alone, there were over 45 recalls of various FDA regulated food products, including pet foods (raw/frozen for the most part), deli salads, cheeses, sprouts, processed salmon products, and ice cream (Blue Bell, Jeni’s). Recalls also were carried out for apples and apple slices, spinach, and frozen vegetables.
So what is Listeria monocytogenes (LM) and where is it coming from?
LM is a bacterial species that was first described in 1926 after an outbreak in guinea pigs and rabbits. For many years, it was known as a disease of animals, including small ruminants, often called “circling” disease. One source of the disease is spoiled silage, causing illness during the winter and spring months after long-term storage of silage. When I first started working in food safety education, it was known as an “emerging pathogen” as it was just starting to become a big problem in the human food system.
The first documentation of a major human outbreak was in 1981. This Canadian outbreak was attributed to coleslaw made from cabbage likely contaminated by sheep manure. The outbreak resulted in 41 cases of illness, 34 of which were pregnant women. There were 18 deaths.
Since then, Listeria has been the cause of outbreaks in dairy products, including a variety of cheeses, processed meats such as hot dogs and cold cuts, sprouts, and, most recently fresh produce. Increased testing over time by government agencies including the United States Department of Agriculture Food Safety Inspection Service (USDA/FSIS) and the US Food and Drug Administration (FDA), has also resulted in many recalls that may have helped to reduce outbreaks from products that are removed from the marketplace.
So why is this now a problem for fresh produce?
Listeria has become pretty much ubiquitous in the food-processing environment. While it’s certainly present in the soil, water, and animal feces, the big problems often arise from food that is processed and/or held in a processing or storage facility that is wet and cold. This is where the bug can grow and multiply if not dealt with by employing scrupulous sanitation programs.
Some of the characteristics of Listeria that make it a particular problem are:
- While most consumers are likely to think that it is what they ate in the last 48 hours that made them sick, Listeria has a long incubation period—as much as 3 weeks. This means that when you eat a food contaminated with Listeria, you may not feel ill or experience some of the worst complications, such as miscarriage until weeks later. Foodborne illness experts often start an investigation with a food recall. Do you remember what you ate for lunch on a Tuesday two or three weeks ago?
- Foods that may be more likely to be contaminated with Listeria are ready to eat foods such as cold cuts, other deli foods, cheese or some fresh fruits and vegetables. Therefore, we do not cook it away.
- While most people are at least somewhat aware of the risk of Listeria from hot dogs, cold cuts or unpasteurized cheeses, they are less likely to connect it to bagged lettuce or cantaloupe. In the 1990s there was a string of outbreaks related to hot dogs and cold cuts. In 2003, FSIS passed a Listeria Rule that addressed many of the problems in that industry.
- Listeria likes cold temperatures-found in refrigerators, coolers and cool processing rooms. Wet AND cold processing environments are Listeria’s
- Inadequate or ineffective sanitation practices can miss the Listeria that might be hiding in floor drains or biofilms, which are a substance not unlike plaque on your teeth that forms when bacteria adhere to a surface, then create a matrix that can build up over time and protect the bacteria from routine cleaning practices. Periodically, these biofilms can break open, releasing the bacteria to contaminate a seemingly clean surface.
Several Listeria outbreaks have been traced to packing facilities (such as the recent apple and cantaloupe outbreaks) where cold, wet environments that are not cleaned adequately can be the problem. Floor drains, conveyor rollers, dunk tanks can all be sources of this bug. Perhaps a contributing condition might be the presence of animal manure on the farm.
Yet another issue is that consumers are demanding that their fresh produce is provided in ever more convenient forms—cleaned, chopped lettuce in bags, sliced apples, cut celery, coleslaw mix, to name just a few. The more handling, the more opportunity for contamination.
So what is a consumer to do?
First, know your risk. Listeria is a bit of a chameleon. We are all routinely exposed to LM, due to its ubiquitous nature. Yet we do not all get sick from it. A young adult with a healthy immune system may make it through a bout of Listeriosis with barely a symptom. According to the Centers for Disease Control, “at least 90% of people who get Listeria infections are in a higher risk group. Healthy children and adults occasionally get infected with Listeria, but they rarely become seriously ill.”
So, if you are 65 or older, have a compromised immune system due to illness or medical interventions, and if you are pregnant, avoid foods that are more likely to be contaminated with LM. Do not eat foods from the deli case, cold cuts or hot dogs unless you heat them to 165 degrees F. Do not drink raw milk or eat cheeses made from raw milk. Fresh produce is trickier. You certainly can cook most fruits and vegetables, but you may not want to cook everything—though you could add spinach and other greens to a hot soup. So the next best thing is to handle fresh fruits and vegetables safely.
- Rinse raw fruits and vegetables, thoroughly under cool running tap water before eating, cutting, or cooking. Even if the produce will be peeled, wash it first. There is no need for special veggie washes. Studies have shown that cold water alone (along with brushing when appropriate) can remove 85-98% of bacteria.
- Scrub firm produce, such as melons and cucumbers, with a clean produce brush (wash the brush in a dishwasher after using).
- Dry the produce with a paper towel.
- Store fresh fruits and vegetables in a clean, DRY, location. Always refrigerate once cut. Store away from raw meats, poultry, fish or eggs.
And, keep your refrigerator clean.
Remember that Listeria monocytogenes likes cold, wet environments—even your home fridge. Using an appliance thermometer, make sure that your refrigerator is at 40°F or lower and the freezer 0°F or lower.
Be sure to clean up all spills in your refrigerator right away–especially juices from hot dog and lunch meat packages, raw meat and poultry, and, of course, those rotting fruits and veggies that lie forgotten at the bottom of the drawer. Regularly clean the inside walls and shelves of your refrigerator with hot water and liquid soap, then rinse and pat dry.
And finally, be sure to monitor your fruit and veggie storage drawer. Throw out anything that is beginning to spoil, is moldy or has other signs of decay.
For more information on safe handling of produce and preventing foodborne illness, go to www.foodsafety.uconn.edu or contact the Home and Garden Education Center at firstname.lastname@example.org or 1-877-486-6271.