By: Ryan Robinson, PhD


Due to the public nature of recent outbreaks most consumers are likely already familiar with Listeria monocytogenes, the tiny Listeria organism is a significant cause of severe foodborne illness. What consumers may not know is how dangerous listeriosis (the illness caused by the listeria). They also may be unaware of just how troublesome and difficult this infection is to prevent and eradicate.

Unlike many other sources of foodborne illness Listeria survives and thrives at temperatures consistent with those used to refrigerate food. At room temperature, Listeria monocytogenes can even move from contaminated food to uncontaminated food by using a whip-like tail appendage known as a flagellum.

Listeria monocytogenes and the associated illness listeriosis are serious, dangerous infections that prompt about 1500 hospitalizations annually. The mortality rate among tracked cases is above 15%.  Pregnant women, the elderly, and young children at particularly high risk for severe complications from infection (CDC Listeria Surveillance Program).

Traditionally, Listeria monocytogenes has been associated with unpasteurized dairy products, cold deli meats, and uncooked hot-dogs. Most educated consumers are aware of the need to be exceptionally discerning when consuming these items. Unfortunately, there is significantly lower public awareness of other less-traditional sources of Listeria contamination.

Frozen produce, smoked seafoods, and packaged food products are often overlooked sources of contamination. Recent reports that a popular brand of Cashew Butter marketed under NOW health foods “Nutty Infusions” brand is a stark reminder of the vigilance required in monitoring and surveilling for Listeria contamination and outbreaks.

What is the difference between Listeria contamination, listeriosis infection, and a listeriosis outbreak?

Listeriosis infections are categorically dangerous, and have the potential to develop into a severe public-health crisis. Due to the inherent danger from contaminated food products, the FDA mandates that food manufacturers in the United States consistently sample products and monitor for Listeria contamination.

If the presence of Listeria is detected in sampled, a contamination event is recorded, production stops, and any potentially contaminated food is recalled by the manufacturer. In the best-case scenario, food is recalled from distributors or grocers before it ever reaches the consumer.

If an individual does fall ill and seek medical attention, the incidence is recorded and referred to state health authorities and the CDC. The CDC meticulously tracks reported data regarding illnesses in the US and abroad. When more of a specific type of illness occurs than normal, the issue is investigated as an outbreak, and epidemiologists attempt to trace the source of the illness.

Outbreaks of any illness are frightening occurrences, and particularly troubling when the effect our food supply. The good news is that reported Foodborne Illness outbreaks, while newsworthy and attention-grabbing, are actually quite rare in the United States. The bad news is that many types of foodborne illness are increasing in frequency, and that the frequency of outbreaks may not appropriately represent the total risk of illness to the general population.

Not every illness is an outbreak

It’s important to note that contamination events can be exceptionally harmful, even if they don’t result in an outbreak. The early symptoms of serious illnesses like listeriosis often mimic common symptoms from influenza and other forms of foodborne illness (fever, body-ache, nausea, vomiting) so infected individuals may eschew medical treatment, or the illness may be initially misdiagnosed.

Furthermore, bureaucratic overhead may prevent timely recognition of an outbreak. Even if the illness is correctly diagnosed, it may not be reported rapidly to the CDC or to state health authorities. If local health care providers are lapse in their reporting, it may not be reported at all.

Finally, there is some subjectivity in defining an outbreak. The definition of an outbreak stipulates that the illness must occur at a higher prevalence than it does normally, so most isolated incidents of foodborne illness are not classified as outbreaks, and do not gather large-scale national attention.

How often to Listeria contamination events occur?

Unfortunately, Listeria contamination is identified frequently in food manufacturing and processing facilities across the US. The recent cashew butter contamination incident is just one in a list of contamination events that have been characterized, and several other recent noteworthy contamination events have occurred in recent history.

For 2017, the FDA has recorded more than 30 documented mandatory recall events for food potentially contaminated with the dangerous Listeria monocytogenes bacteria. These recalls span all manner of different food products, but frequent offenders include packaged cheeses and frozen produce.

How often to listeriosis infections and listeriosis outbreaks occur?

Most Listeria contamination is caught before consumption of contaminated food products, but listeriosis infections, and listeriosis outbreaks occur more frequently than anyone would prefer.

The CDC indicates that about 1600 people are infected with Listeria annually. Most cases are not part of a confirmed listeriosis outbreak.  Declared listeriosis outbreaks are less common than isolated incidents, and historic data provided by the CDC indicates that an average of 2-3 listeriosis outbreaks occur annually.

How do we protect our families?

Listeriosis is particularly difficult to treat, and the best possible method for protection is prevention. Obviously, consumers are advised to pay close attention to FDA mandated recalls, and to immediately discontinue use of any recalled products. There are also a variety of steps you can take in your own kitchen to help prevent the spread of Listeria monocytogenes.

The FDA suggests offers the following guidelines:

  1. Chill foods properly – refrigerated foods should be below 40 degrees (Fahrenheit) and frozen foods should be below 0 degrees.
  2. Heat meats properly – Especially hot-dogs and dairy meats. The internal temperature should reach at least 165 degrees Fahrenheit.
  3. Keep your refrigerator clean – Spills, drips, and dribbles provide an opportunity for Listeria to grow and spread to other food.
  4. Watch the expiration date! – Ready to eat food should be eaten or discarded by the sell by or best-by date labeled on its packaging. Remember that Listeria can survive and even grow at refrigerated temperatures; the longer it stays in the fridge, the more opportunity Listeria has to grow.

As with any serious illness, affected individuals are urged to seek medical attention immediately, and to report potential foodborne illnesses to the FDA.