“The listeria outbreak is believed to be linked to Boar’s Head deli meats. The company last week announced a recall of several types of deli meats, including all liverwurst products, as well as some types of ham, salami and bologna.”
Kara Seymore, digital journalist, Editor in Chief, Patch
“Listeria can live in any place where food is grown, packaged, stored, transported, prepared or served. Our research team has even found listeria in organic lettuce harvested from a backyard garden.”
Yvonne Sun, Associate Professor of Microbiology, University of Dayton
Recently, there has been another widespread outbreak of listeriosis. This outbreak is linked to contaminated deli meats—primarily Boar’s Head liverwurst. (In my July 3, 2022 post, “Some Like It Cold: Listeria Outbreak from Contaminated Ice Cream,” I wrote about a listeria outbreak linked to contaminated Big Olaf Creamery ice cream.) By the end of July, Boar’s Head Provision Company had recalled more than 7 million pounds of its meat products. Nonetheless, as of August 8, 2024, the responsible pathogen, Listeria monocytogenes, had sickened 43 people in 13 states, all of whom were hospitalized and three died.
In this week’s Germ Gems post, I provide a brief summary of listeriosis and highlight the modus operandi of the stealthy pathogen Listeria monocytogenes.
What is listeriosis (a recap)? Listeriosis is a major foodborne illness caused by the bacterium L. monocytogenes. The pathogen was isolated from a patient with meningitis in 1919. Then in 1926 E.G.D. Murray, a Canadian clinician and microbiologist, and his colleagues first described Bacterium monocytogenes as a zoonotic infection affecting rabbits. In 1940, the genus was changed to Listeria to honor Joseph Lister who pioneered sterile surgery. (Lister had little or nothing to do with L. monocytogenes or listeriosis but made other major contributions to the field of antiseptic medicine.) But it wasn’t until 1981 when an outbreak of listeriosis hit Halifax, Nova Scotia that L. monocytogenes was recognized as a major cause of foodborne illness.
Epidemiologists linked the 1981 Nova Scotia listeriosis outbreak to the consumption of contaminated coleslaw. (The cabbage was contaminated with L. monocytogenes-containing sheep manure.) Now, outbreaks are caused most commonly by the consumption of improperly processed deli meats, as in the recent outbreak linked to contaminated Boar’s Head liverwurst, or the consumption of unpasteurized milk products, such as raw milk, raw milk cheeses, and ice cream. (One of the many sneaky properties of L. monocytogenes is its ability to grow at freezing temperatures, meaning that refrigeration can’t be counted on to prevent infection.)
Modus operandi (pathogenesis) of L. monocytogenes. L. monocytogenes is a gram-positive bacillus that can be found in moist environments, soil, water, and decaying vegetation. When it gets itself into a human host, however, it becomes an intracellular opportunist. This means it has the ability (some would say, audacity) to survive within cells of the immune system called macrophages. Unless these macrophages become activated by another group of immune cells called CD4 lymphocytes, the bacillus grows happily in this intracellular environment.
The strategies that L. monocytogenes uses to evade host defenses are incredibly elegant. First, it creates an intracellular niche that prevents recognition by the immune system, and it then uses various proteins to escape from vacuoles within macrophages into the cytoplasm where it proliferates. Fortunately, equally sophisticated host defense mechanisms co-evolved with the bacterium that mediate the killing of intracellular bacteria.
Healthy individuals infected with L. monocytogenes experience mild gastroenteritis with nausea, diarrhea, and abdominal pain. People with weakened cell-mediated immunity, however, are at increased risk of developing severe listeriosis. This group includes young children, the elderly, organ and bone marrow transplant recipients, people taking immunosuppressive medications, and, very importantly, pregnant women who can pass the infection on to their unborn child resulting in miscarriage and stillbirth. (See “Battling the silent killer: An insight into listeriosis,” Journal of Bacteriology Infectious Diseases, 2023).
Symptoms of severe listeriosis include fever, muscle pain, and loss of appetite. If the bacterium spreads to the nervous system, meningitis or encephalitis can occur manifested by confusion, headache, stiff neck, and loss of balance. Key diagnostic tests include cultures of blood and cerebrospinal fluid. Prompt therapy with an antibiotic that penetrates cells, such as ampicillin, amoxicillin, or trimethoprim/sulfamethoxazole, is imperative. In vulnerable (immunocompromised) patients, listeriosis has a mortality rate of 20-30%.
The fallout. The economic burden of foodborne illness is estimated at $51 billion annually. Listeriosis is now recognized as an important biohazard in the food industry. New approaches to preventing L. monocytogenes foodborne disease are under investigation, including ionizing irradiation to reduce or eliminate pathogens, not only L. monocytogenes, but also E. coli 0157:H7, Campylobacter jejuni, and Staphylococcus aureus on foods. Additionally, two bacteriophage-based products are available that target L. monocytogenes in food products.
In my opinion, what’s needed now is public health leadership like that provided by Sir Joseph Lister, the founder of antiseptic surgery and pioneer of preventive medicine..
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