The bacterium listeria has a history of outbreaks in the United States. Learn more about what comprises the bacteria (including common sources), outbreaks throughout history, and both past and present litigation stemming from listeria outbreaks.

What is Listeria?

Image courtesy USDA Photostream | Flickr

Listeria monocytogenes is a bacterium that is found primarily in soil, water, and certain animals such as cattle and poultry. Additional origins include raw milk and products made from raw milk. Listeria outbreaks have originated in food processing plants, and the bacterium has been known to contaminate processed meats.

Listeria is unique in the sense that it can develop and spread in colder temperatures, as opposed to most bacteria, which can only do so in a warm or moist environments. Processes that kill the bacteria include cooking and pasteurization.

Sources of the listeria bacterium include: processed deli meats and hot dogs, unpasteurized raw milk and dairy, refrigerated seafood, pates and meat spreads, and raw sprouts.

What is Listeriosis?

Listeriosis is currently an important public health concern in the United States. The condition is classified as an infection, most often obtained by eating food contaminated with Listeria monocytogenes. Those most at risk for food poisoning from listeriosis include newborns, adults who are elderly, pregnant women, and adults with abnormal immune systems. In rare cases, listeriosis affects others outside of these demographics.

The symptoms of listeriosis are often severe, and include: confusion, fever, stiff neck, weakness, vomiting and diarrhea. The duration of listeriosis often varies, ranging for a few days to a few weeks. Both symptoms and duration of illness are dependent upon the individual affected.

Listeriosis is unique in the sense that risk of contraction varies widely between demographics. For example, the general population acquires listeriosis at a rate of 0.7 per 100,000 people, compared to pregnant women, who are approximately 12 to 20 times more likely to contract the illness, depending upon the source. Those with lupus, kidney transplants, and weakened immune systems also acquire listeriosis at a much higher rate than the general population does.

Pregnant women who contract listeriosis are at risk for miscarriage. Those that deliver a newborn with the illness can pass on bacteremia and meningitis to the child. Because of these factors, it is recommended that pregnant women who have or suspect a listeriosis infection consult a medical professional, who may then decide to monitor the health of the fetus up to delivery.

The History of Listeriosis

Listeria monocytogenes
Listeria monocytogenes | Image courtesy Nathan Reading | Flickr

Historical records of listeriosis span a period of approximately 60 years, although it is likely that the illness existed prior to the ability to reliably track it. After its discovery, listeriosis was subsequently identified as a cause of epidemics and concentrated cases of illness in about 50 species of animals.

It is now understood that modern cases of listeriosis surface with increased frequency among humans, particularly in certain demographics (as described above). Health professionals have determined a correlation exists between animal contraction of the illness and subsequent passage of the illness to the human population.

Food-based listeria transmission was not recognized until later, when food experiments in Canada confirmed transmission of the illness through cheese in the early 1980s. Listeria monocytogenes was included as a legitimate foodborne pathogen shortly after several other outbreaks occurred in the United States and Switzerland. These findings were profound, as they successfully altered the methodology of food safety mechanisms.

Record of Listeriosis Outbreaks

The most significant listeriosis outbreaks on record have been the direct result of transmission through food. As mentioned, the possibility of transmission of listeriosis via foodborne agents was not common knowledge until the early 1980s, which leaves a potentially significant period of time where cases were not analyzed or recorded. Furthermore, the Centers for Disease Control and Prevention (CDC) did not begin regularly tracking outbreaks of foodborne illness until the 1970s.

Following this period of scientific inquiry and discovery, the most significant listeriosis outbreak on record occurred in the summer of 1985. This particular case originated in California and involved the consumption of contaminated Mexican-style soft cheese. Jalisco, the cheese’s manufacturer, employed a non-licensed technician to perform the pasteurization process. The results were tragic: 142 reported cases of listeriosis, of which 52 resulted in death — including 19 stillbirths and 10 infant deaths. The raw milk’s supplier, Alta Dena, was ultimately absolved of any wrongdoing.

The second-deadliest outbreak of listeriosis occurred between January, 1998 and February, 1999. This specific case involved the consumption of contaminated hot dogs and deli meats. In the investigation, CDC officials isolated and analyzed an unopened package of hot dogs, which were found to contain two strains of L. monocytogenes. As a result of the outbreak, at least 50 illnesses were reported in 11 different states. At least 14 adults died and four pregnant women experienced spontaneous abortions or stillbirths.

More recently, at least nine serious cases of listeriosis were reported between 2011 and 2015. So far in 2016, two cases were reported: a multistate outbreak due to raw milk, and another multistate outbreak due to packaged salads.