In July 2024, deli meats from a Boar's Head facility in Jarratt, Virginia killed 10 people and hospitalized at least 59 across 19 states. CDC's outbreak investigation traced the source to one slicing line operating with documented sanitation failures. The plant closed permanently. The outbreak was the largest US listeria event in a decade, and it followed the same pattern as the 2011 cantaloupe outbreak and the 2014 caramel apple outbreak before it.
Listeria monocytogenes punches above its weight as a foodborne killer. Salmonella infects far more people, but listeriosis hospitalizes 94 percent of confirmed cases and kills 20 to 30 percent of the severe ones. This post complements foodborne disease outbreaks and the outbreak investigation framework, inside the pandemic preparedness 101 hub.
Key Takeaways
- Listeria causes around 1,600 illnesses and 260 deaths per year in the US, with 94 percent hospitalization rate.
- Pregnant women are 10 to 20 times more likely to develop invasive listeriosis than the general population.
- High-risk foods: deli meats, soft cheeses, smoked fish, refrigerated pâté, raw sprouts, melons, and unpasteurized dairy.
- Listeria grows at refrigerator temperatures, unlike most foodborne bacteria.
- Symptom onset can be days to 70 days after exposure, making traceback difficult.
- Reheating ready-to-eat foods to 165 degrees F kills Listeria.
What is Listeria?
Listeria monocytogenes is a gram-positive bacterium that lives in soil, water, animals, and food-processing environments. Most healthy adults exposed to it shrug the infection off as a mild GI illness or no symptoms at all. In pregnant women, newborns, older adults, and immunocompromised people, it can invade the bloodstream and CNS, causing listeriosis.
The bacterium has two clinical patterns: febrile gastroenteritis (mild, self-limiting) and invasive listeriosis (bacteremia, meningitis, sepsis). Invasive cases are the ones that get counted in CDC PulseNet outbreak reporting.
Listeria is unusual among foodborne pathogens because it grows at refrigerator temperatures. Most bacteria need warmth; Listeria multiplies, slowly but reliably, at 35 degrees F. A contaminated deli meat held in proper cold storage for a week is more dangerous than one consumed fresh.
What foods cause listeriosis outbreaks?
Ready-to-eat foods that contact bacteria after cooking and are then consumed without further heating cause most outbreaks. The food categories with the highest historical risk:
- Deli meats and hot dogs: sliced at retail or in factories with contaminated equipment
- Soft cheeses: brie, camembert, queso fresco, blue cheese, especially unpasteurized
- Smoked fish: cold-smoked salmon, smoked trout
- Refrigerated pâtés and meat spreads: contamination during packaging
- Raw sprouts: alfalfa, mung, clover; bacteria thrive during germination
- Melons: cantaloupe and honeydew, contaminated through cut surfaces
- Unpasteurized dairy: raw milk, raw milk cheeses
- Refrigerated dips and salads: once contaminated, refrigeration does not save you
The 2024 Boar's Head outbreak hit deli liverwurst and other sliced meats. The 2011 Jensen Farms outbreak hit cantaloupe, killing 33 people across 28 states. The 2014 Bidart Bros caramel apple outbreak killed 7. The same pattern repeats: ready-to-eat product, post-cook contamination, distribution to many states before recall.
Who is at high risk?
Four groups bear nearly all of the severe disease burden: pregnant women, newborns, adults over 65, and people with weakened immune systems.
| Group | Relative risk vs healthy adults | Typical outcome |
|---|---|---|
| Pregnant women | 10 to 20x | Miscarriage, stillbirth, neonatal sepsis |
| Newborns | High | Sepsis, meningitis, death |
| Adults 65+ | 4 to 5x | Bacteremia, meningitis |
| Immunocompromised | 100 to 300x | Sepsis, encephalitis |
| Healthy adults | Baseline | Self-limiting GI illness or asymptomatic |
For pregnant women, even a mild flu-like illness can be vertically transmitted to the fetus with severe consequences. CDC estimates 1 in 7 listeriosis cases occurs during pregnancy. About 20 percent of perinatal listeriosis cases end in stillbirth or neonatal death.
What are the symptoms?
Symptoms in adults range from mild gastroenteritis to severe invasive disease.
- Mild form (24 to 72 hours after exposure): diarrhea, fever, muscle aches. Self-limiting.
- Invasive form (1 to 70 days, median 21 days): fever, severe headache, stiff neck, confusion, loss of balance, convulsions. Can progress to meningitis or sepsis.
- Pregnancy: flu-like illness in the mother, with fetal infection that may go unnoticed until miscarriage or stillbirth.
- Neonatal: early-onset sepsis in the first week of life (acquired in utero) or late-onset meningitis at 1 to 4 weeks (acquired during birth or from environment).
The long incubation window is the investigation nightmare. A patient hospitalized in May may have been exposed in early April. Recalling what you ate 6 weeks ago is unreliable, which is why molecular subtyping at CDC PulseNet drives traceback.
How is listeriosis treated?
Listeriosis is a bacterial infection treatable with antibiotics. Ampicillin or penicillin, often combined with gentamicin, is the standard regimen for invasive disease. Treatment typically runs 2 to 6 weeks depending on severity and site of infection.
Listeria is intrinsically resistant to cephalosporins, which are first-line empirical antibiotics for many bacterial infections. Clinicians treating suspected bacterial meningitis in older adults often add ampicillin specifically to cover Listeria. Missing it has cost lives.
Pregnant women with confirmed listeriosis receive IV ampicillin to reduce fetal risk. Early treatment improves outcomes; late treatment after fetal infection has begun is often unhelpful.
How do you reduce your risk?
Risk reduction targets the high-risk foods if you are in a high-risk group. For most healthy adults, listeriosis is so rare that aggressive avoidance is not warranted. For pregnant women and immunocompromised people, the calculus is different.
Practical rules during pregnancy or immunosuppression:
- Reheat deli meats and hot dogs to 165 degrees F (steaming hot) before eating
- Skip unpasteurized soft cheeses; pasteurized soft cheeses are fine
- Avoid refrigerated pâtés and meat spreads (canned or shelf-stable are fine)
- Avoid raw sprouts entirely
- Wash melons under running water before cutting; clean the knife between cut and skin
- Skip cold-smoked fish unless cooked in a casserole or pizza
- Keep refrigerator at 40 degrees F or below and clean spills immediately
Food recalls happen often. CDC and FDA maintain active recall pages; signing up for FDA email alerts gives you advance notice when a product has been pulled.
How are outbreaks detected?
CDC PulseNet links labs across all 50 states. When a clinical lab cultures Listeria from a patient, the isolate gets whole-genome sequenced and uploaded. Matching genomes across patients suggests a common source. PulseNet has detected outbreaks involving as few as 3 to 5 cases that would never have been recognized as related otherwise.
The investigation then follows the 10-step framework: patient interviews about food exposures, comparison against a database of typical purchases, traceback through distributors and retailers, environmental sampling at suspect production facilities.
The 2024 Boar's Head investigation traced the outbreak strain in plant environmental swabs collected over a year before the human cluster surfaced. Plant sanitation failures had been documented in 69 USDA inspections in the 12 months before the outbreak. Detection systems work; enforcement is the harder problem.
FAQ
Can you get Listeria from leftovers?
Yes, especially from leftovers in the refrigerator for more than 3 to 4 days. Listeria slowly multiplies in cold storage. Reheating leftovers to 165 degrees F kills the bacteria. The risk is much higher for cold-served foods that were already contaminated and stored cold.
Is pasteurized soft cheese safe in pregnancy?
Yes, generally. Pasteurization kills Listeria, and pasteurized brie, camembert, feta, and goat cheese are considered safe. Read the label. Imported and artisanal cheeses are more often unpasteurized. The pasteurization status is the determining factor, not the texture.
Why is Listeria more dangerous than Salmonella?
Salmonella infects far more people but mostly causes self-limiting gastroenteritis. Listeria infects fewer but invades the bloodstream and central nervous system far more often. Hospitalization rate is 94 percent for confirmed Listeria versus around 20 percent for Salmonella. Mortality follows the same pattern.
How long after eating contaminated food do symptoms appear?
Anywhere from 1 day to 70 days, with a median around 21 days. The wide window makes recall of what you ate unreliable. Patients are often unable to identify the specific exposure even when investigators ask about every food eaten in the past month.
Should I avoid all deli meats during pregnancy?
Cold deli meats are a top exposure source for pregnancy listeriosis. The safe option is to heat them until steaming (165 degrees F) before eating, which kills the bacteria. Pre-packaged sealed deli meats have a lower contamination rate than freshly sliced at-counter products, but heating before eating remains the strongest safeguard.