Listeria: Food Poisoning's Rare but Deadly Germ

Lieutenant Commander Benjamin J. Silk, PhD, MPH


February 27, 2012

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Hello. My name is Dr. Benjamin Silk. As part of the CDC Expert Video Commentary Series on Medscape, I am pleased to speak with you about a rare, but deadly foodborne illness -- invasive listeriosis.

In September 2011, Colorado’s state health department reported to CDC an outbreak of listeriosis. From August to October, the outbreak reached 28 states, and 146 cases of invasive listeriosis were confirmed and reported to public health officials. Ultimately, 30 patients died and 1 woman had a miscarriage. The outbreak made national headlines as the deadliest outbreak of foodborne illness since 1924. With a spotlight on the outbreak, many patients had questions for their healthcare providers. So, here is a review of key points about invasive listeriosis.

Key point 1: Invasive listeriosis is rare but deadly.

  • An estimated 1,600 cases of invasive listeriosis and 260 related deaths occur annually in the United States.

  • An estimated 18% of patients with listeriosis die.

  • Overall, Listeria monocytogenes ranks third as a cause of death due to major known foodborne pathogens in the United States.

Key point 2: Certain well-defined groups are at higher risk for invasive listeriosis.

Higher-risk groups include persons who have compromised cell-mediated immunity:

  • Older adults. In the 2011 outbreak, the median age of patients was 77 years. In fact, the incidence of listeriosis increases steadily with age after about age 50.

  • Patients undergoing transplants or receiving immunosuppressive medications.

  • Persons with other immunocompromising conditions. Well-established conditions that confer risk are AIDS and certain malignancies, especially hematologic. Some studies have found an increased risk with:

    • Liver disease;

    • Kidney disease;

    • Insulin-dependent diabetes; and

    • Alcoholism.

  • Pregnant women. In pregnant women, listeriosis may present as a non-specific, "flu-like illness" or be asymptomatic. The woman does not become seriously ill. However, listeriosis can cause:

Key point 3: For higher-risk groups, food preparation, storage habits and food preferences should be considered modifiable risk factors.

  • Unlike most bacterial causes of foodborne illness, Listeria monocytogenes can continue to grow at refrigeration temperatures. This means that storage times are an important consideration for prevention of listeriosis, particularly for foods that are known to favor growth of Listeria. According to a US Food and Drug Administration (FDA)/United States Department of Agriculture (USDA) risk assessment, some of the most highrisk foods include soft cheeses, unpasteurized dairy products (including raw milk); deli and lunch meats, cold cuts, hot dogs, pâté, and meat spreads.

  • Only 5% of the cases were pregnancy-associated during the 2011 outbreak. By comparison, almost two-thirds of cases were pregnancy-associated during the next largest listeriosis outbreak, which occurred in 1985 and was associated with Mexican-style cheese. Most of the women with pregnancy-associated listeriosis were Hispanic. Differences in the frequency of pregnancy-associated listeriosis during outbreaks indicate that cultural food preferences can affect risk.

Key point 4: Exposed, asymptomatic patients who are at higher risk for invasive listeriosis may not need testing or treatment.

Food products, such as smoked salmon, cheese, sandwiches, and lettuce/leafy greens, may be recalled because testing identified Listeria monocytogenes contamination, even without any associated case of illness. Unfortunately, little scientific evidence is available to inform decisions on clinical management of persons at higher risk for invasive listeriosis who may have been exposed to Listeria monocytogenes, but are asymptomatic. In consultation with a panel of national experts, CDC developed a suggested framework for medical management in these situations. This framework suggests that:

  • No testing or treatment is indicated for an asymptomatic patient who ate a product recalled because of Listeria monocytogenes contamination.

  • Such a patient should be counseled about the symptoms of listeriosis and instructed to return if he or she develops those symptoms within 2 months of eating the recalled product.

Notably, symptoms of listeriosis can differ between risk groups:

  • Older adults and immunocompromised persons typically present with meningitis or septicemia, and they are often severely ill. Common presenting complaints include headache, nuchal rigidity, confusion, loss of balance, seizures, and prostration. These symptoms are often preceded by diarrhea or other gastrointestinal symptoms.

  • For pregnant women, on the other hand, the main issue is infection of the fetus or newborn. These women typically present with fever and myalgias, often preceded by diarrhea or other gastrointestinal symptoms, but these symptoms may be quite mild. Newborns, on the other hand, present with sepsis or meningitis.

In general, the risk for invasive listeriosis after exposure to Listeria monocytogenes is very low. Thousands of pregnant women consumed the Jensen Farms cantaloupe that caused the 2011 outbreak, but surveillance indicates that the attack rate of listeriosis in pregnant women was extremely low.

In summary, education about foods to avoid and how to safely prepare and store food is a key part of prevention for persons at higher risk for invasive listeriosis. These higher-risk groups are well-defined and can be targeted by clinicians for prevention education. Several web resources for patient education are available on the CDC webpage associated with this presentation.

Two key prevention messages to take away now are:

  • Persons at higher-risk for invasive listeriosis should not eat deli and lunch meats, cold cuts, or hot dogs, unless they are heated to an internal temperature of 165°F or until steaming hot just before serving. Persons at higher risk also should not eat soft cheeses unless they are labeled as made from pasteurized milk.

  • No one should drink raw milk or eat un-aged cheeses or other products made from raw milk. In addition to Listeria monocytogenes, a wide variety of pathogens that are sometimes found in raw milk can make people sick.

Thank you for talking the time to refresh your knowledge of listeriosis and thanks for your efforts to educate your patients about prevention of listeriosis and other foodborne illnesses.

Web Resources

Listeriosis and the Burden of Foodborne Illness

CDC Listeriosis

Scallen E, Hoekstra RM, Angulo FJ, et al. Foodborne illness acquired in the United States--major pathogens. Emerg Infect Dis. 2011;17:7-15.

2011 Outbreak Linked to Cantaloupes

Multistate Outbreak of Listeriosis Linked o Whole Cantaloupes from Jensen Farms, Colorado.

CDC. Multistate outbreak of listeriosis associated with Jensen Farms cantaloupe -- United States, August-September 2011. MMWR Morb Mortal Wkly Rep. 2011;60:1357-1358.

Deadly Listeria Outbreak Halted in Record Time.

Outbreak Response Team. How to Report a Foodborne Illness. Healthcare Professionals.

Prevention Resources for Groups at Higher Risk Checklist of Foods to Avoid During Pregnancy.

FDA. Special Handling for Ready-to-Eat, Refrigerated Foods: Reducing the Risks of Foodborne Listeria.

FDA. The Dangers of Raw Milk: Unpasteurized Milk Can Pose a Serious Health Risk.

FDA. Preventing Listeriosis in Pregnant Hispanic Women in the US.

FDA. Food Safety for Moms-to-Be.

USDA. Protect Your Baby and Yourself From Listeriosis

USDA. Proteja a su bebé y protéjase usted de la Listeriosis (Spanish)

FDA. To Your Health! Food Safety for Seniors.

USDA. Food Safety for Older Adults.

USDA. Food Safety for People with Cancer.

USDA. Food Safety for Transplant Recipients.

USDA. Food Safety for People with HIV/AIDS.

USDA. Eating Defensively: Food Safety Advice for Persons with AIDS.

Lieutenant Commander Benjamin J. Silk, PhD, MPH. Dr. Silk is an officer in the US Public Health Service. He completed his PhD in epidemiology at Emory University's Rollins School of Public Health, and received his MPH from Tulane University's School of Public Health and Tropical Medicine. While training in CDC's Epidemic Intelligence Service, Dr. Silk led investigations of Legionnaires' disease outbreaks and 2009 pandemic of influenza A (H1N1). Currently, Dr. Silk is assigned to the Enteric Diseases Epidemiology Branch at the CDC, where he focuses on surveillance and the epidemiology of listeriosis and vibriosis. Dr. Silk has 16 years of experience in applied public health at the local, state, and federal levels.


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