What is listeriosis?

Listeriosis: A Serious Foodborne Infection

Listeriosis is a serious foodborne infection caused by the bacterium Listeria monocytogenes that disproportionately affects immunocompromised individuals, pregnant women, and the elderly, with potentially fatal outcomes if not properly treated.

Definition and Causative Agent

Listeriosis is an infectious disease caused by Listeria monocytogenes, a short, gram-positive, non-spore-forming bacillus that is facultatively anaerobic and produces a narrow zone of hemolysis on blood agar 1. This intracellular pathogen has the unique ability to:

• Cross intestinal, placental, and blood-brain barriers
• Spread directly from cell to cell, evading host immune responses
• Survive in a wide range of environments, including refrigerated foods

Epidemiology

• Relatively rare but serious infection with high mortality (20-30%)
• Higher incidence in:
  • Pregnant women (18 times higher risk than general population)
  • Elderly individuals
  • Immunocompromised patients (particularly those with HIV/AIDS)
  • Patients with chronic liver disease
  • Patients on immunosuppressive medications

Transmission

Listeria monocytogenes is primarily transmitted through:

• Consumption of contaminated foods, especially:
  • Soft cheeses (feta, Brie, Camembert, blue-veined, and Mexican-style cheese like queso fresco) 2, 3
  • Unpasteurized dairy products
  • Ready-to-eat deli meats and hot dogs
  • Refrigerated pâtés and meat spreads
  • Raw or undercooked seafood
  • Refrigerated smoked seafood
• Environmental sources:
  • Soil
  • Water
  • Decaying vegetation
  • Animal feces

Clinical Presentations

Listeriosis manifests in several forms:

1. Febrile Gastroenteritis:

   • Self-limiting in immunocompetent hosts
   • Fever, diarrhea, nausea, and vomiting within 7 days of exposure 4

2. Invasive Listeriosis:

   • Bacteremia and septicemia
   • Meningitis or meningoencephalitis
   • Focal infections (endocarditis, pneumonia, septic arthritis)

3. Pregnancy-Associated Listeriosis:

   • Mild febrile illness in the mother
   • Can lead to miscarriage, stillbirth, premature delivery
   • Neonatal listeriosis with sepsis or meningitis (20% mortality) 5

Diagnosis

Diagnosis is made primarily through:

• Isolation of L. monocytogenes from normally sterile sites (blood, cerebrospinal fluid)
• Culture on appropriate media
• Serological testing (less commonly used)
• PCR-based molecular methods for rapid identification

Treatment

For invasive listeriosis:

• First-line treatment: Intravenous ampicillin or penicillin G, often in combination with gentamicin 6, 1
• Alternative for penicillin-allergic patients: Trimethoprim-sulfamethoxazole (TMP-SMX) 5
• Treatment duration:
  • 2-3 weeks for bacteremia
  • 3-4 weeks for meningitis
  • At least 4-6 weeks for endocarditis

Prevention

Prevention focuses on food safety practices:

• High-risk individuals (immunocompromised, pregnant, elderly) should avoid:

  • Soft cheeses unless made with pasteurized milk
  • Refrigerated pâtés or meat spreads
  • Hot dogs and deli meats unless reheated until steaming hot
  • Refrigerated smoked seafood
  • Raw or unpasteurized milk and dairy products 2

• Safe food handling practices:

  • Cook foods to proper internal temperatures (poultry to 180°F/82°C, red meats to 165°F/74°C)
  • Wash hands, utensils, and surfaces after contact with raw foods
  • Keep raw meats separate from ready-to-eat foods
  • Consume perishable and ready-to-eat foods as soon as possible 2, 3

Special Considerations

Pregnant Women

• Higher risk of infection with more severe consequences
• Should strictly follow food safety guidelines
• Listeriosis during pregnancy can lead to miscarriage, stillbirth, or severe neonatal infection 5

HIV-Infected Individuals

• Should follow strict food safety guidelines
• Should reheat ready-to-eat foods until steaming hot
• Should avoid soft cheeses, unpasteurized dairy products, and foods from delicatessen counters unless properly reheated 2

Travelers

• Increased risk in developing countries
• Should avoid tap water, ice made from tap water, and raw or undercooked foods 2

Prognosis

• Overall mortality: 20-30%
• Higher mortality in immunocompromised patients and the elderly
• Early diagnosis and appropriate antibiotic treatment improve outcomes but mortality remains significant 7

Listeriosis represents a significant public health concern due to its severity and high mortality rate despite appropriate treatment. Prevention through proper food handling and avoidance of high-risk foods by vulnerable populations is crucial.