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Listeria monocytogenes is a deadly pathogen. Outbreaks of this disease can carry death rates exceeding 20% – higher than Salmonella, E. coli O157:H7 or Campylobacter. It targets the elderly, pregnant women, newborns, and people with weakened immune systems – the most vulnerable sectors of the population. So, why do so many people know so little about it?

Perhaps, because most Listeria monocytogenes infections pass under the radar screen – rarely causing symptoms in healthy people.

Some history

Though identified in 1926 and recovered from a newborn in 1936, the pathogen wasn’t linked to foodborne disease until the early 1980s. The earliest recorded outbreaks of Listeria monocytogenes were traced to cole slaw (in Canada), Mexican-style cheese (in the USA), and pasteurized milk (in the USA).

What is Listeria monocytogenes, and where is its natural habitat?

Listeria monocytogenes is widely distributed in nature – in soil, in coastal waters, and in a variety of animals and birds. While it prefers oxygen for growth, it can grow in the presence of just a minimal amount of oxygen. It is able to survive high salt levels and dry conditions. And it grows in refrigerated foods.

How is Listeria monocytogenes transmitted? What is the incubation period of the infection?

Most infections with Listeria monocytogenes develop as a result of eating a contaminated food. A healthy adult may develop very mild symptoms – muscle aches, nausea, diarrhea – within 48 hours of having become infected, or may not experience any symptoms at all. A pregnant women, even though only mildly ill herself, may transmit her infection to her baby in the womb; or, the baby can become infected during delivery as it passes through her birth canal. Listeria monocytogenes also can be spread person-to-person (e.g., from a patient to a caregiver).

What is listeriosis? How long does it take to develop?

The consequences of a Listeria monocytogenes infection are far more serious for vulnerable individuals. In these people, the pathogen can invade organs and systems beyond the digestive system, causing a syndrome known as listeriosis. It can take two to six weeks for listeriosis to develop in a susceptible adult – even longer in a few cases. Infants who contract the infection in the womb will show symptoms within a week of birth; newborns who are infected during delivery usually take 2-3 weeks to develop signs of illness.

What are the symptoms of Listeria monocytogenes infections and listeriosis?

Gastrointestinal symptoms:- In most individuals, an infection with Listeria monocytogenes is limited to the digestive system. Symptoms – if they occur at all – are very mild, and consist mainly of nausea, diarrhea and muscle aches.

Invasive listeriosis:- The elderly, the newborn, pregnant women and people with a weakened immune systems (including cancer patients undergoing therapy and people infected with HIV) are susceptible to invasive listeriosis, in which the microbe escapes from the digestive tract and infects other organs and systems in the body. In this case, the pathogen may cause bacteremia (bloodstream infection). Symptoms of invasive listeriosis can include fever and muscle aches, headache, stiff neck, confusion, loss of balance, and convulsions. Newborn infants who became infected in the womb or during delivery may develop meningitis.

What is the prognosis of Listeria monocytogenes infections?

The mild form of gastrointestinal infection usually is self-limiting and needs no treatment to ensure a full recovery. The story is far different for invasive listeriosis. The death rate in Canada’s 2008 Listeria monocytogenes outbreak topped 35%; the death rate in the Jensen Farms cantaloupe outbreak is 18%, based on CDC’s most recent data. Some pregnant women miscarry; others give birth to infected babies. Historically, approximately one-half (50%) of babies infected in the womb or during delivery are stillborn or die as a result of the disease – usually from meningitis.

What foods carry Listeria monocytogenes?

In theory, any food may be contaminated with Listeria monocytogenes. Historically, most outbreaks have been traced to smoked seafood, ready-to-eat meats, other ready-to-eat dishes, cheeses (often, but not always, made from unpasteurized milk), and milk. Salads and produce items also can carry the microbe and have been the source of outbreaks. And now we know that cantaloupes may carry the pathogen.

How can susceptible people protect themselves from infection?

First, by paying attention to food recall announcements and immediately discarding any recalled food or returning it to the store. Secondly, by avoiding foods that are most likely to be contaminated with Listeria monocytogenes.

CDC offers the following recommendations for people at high risk of contracting invasive listeriosis:

  • Meats
    • Do not eat hot dogs, luncheon meats, cold cuts, other deli meats (e.g., bologna), or fermented or dry sausages unless they are heated to an internal temperature of 165°F or until steaming hot just before serving.
    • Avoid getting fluid from hot dog and lunch meat packages on other foods, utensils, and food preparation surfaces, and wash hands after handling hot dogs, luncheon meats, and deli meats.
    • Do not eat refrigerated pâté or meat spreads from a deli or meat counter or from the refrigerated section of a store. Foods that do not need refrigeration, like canned or shelf-stable pâté and meat spreads, are safe to eat. Refrigerate after opening.
  • Cheeses
    • Do not eat soft cheese such as feta, queso blanco, queso fresco, brie, Camembert, blue-veined, or panela (queso panela) unless it is labeled as made with pasteurized milk. Make sure the label says, “MADE WITH PASTEURIZED MILK.”
  • Seafood
    • Do not eat refrigerated smoked seafood, unless it is contained in a cooked dish, such as a casserole, or unless it is a canned or shelf-stable product. Refrigerated smoked seafood, such as salmon, trout, whitefish, cod, tuna, and mackerel, is most often labeled as “nova-style,” “lox,” “kippered,” “smoked,” or “jerky.” These fish are typically found in the refrigerator section or sold at seafood and deli counters of grocery stores and delicatessens. Canned and shelf stable tuna, salmon, and other fish products are safe to eat. “

You can find information on the Jensen Farms cantaloupe recall at: Did Your Supermarket Sell Colorado Cantaloupes?

For more information on Listeria monocytogenes and other food-borne pathogens, visit the CDC website or read Food Safety: Old Habits, New Perspectives.

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