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Public Health - Seattle & King County


Listeriosis is an infection caused by the bacteria Listeria monocytogenes. Persons at increased risk for severe infections include immunocompromised persons, the elderly, pregnant women, and newborn infants. The bacterium is unusual among foodborne pathogens in that it multiplies at low temperatures, including in refrigerated foods. Transmission occurs primarily through ingestion of contaminated drinks and foods, including raw (unpasteurized) or contaminated milk, soft cheeses such as queso fresco, vegetables, and ready-to-eat meats. During pregnancy, infection can lead to spontaneous abortion, stillbirth, or premature birth. Transmission during delivery can cause severe, often fatal, infections in the newborn, even if the mother is asymptomatic.

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Listeriosis in King County

Purpose of surveillance:

  • To identify common source outbreaks
  • To identify and eliminate sources of transmission, including contaminated food products

Listeriosis case reports

Local epidemiology:

Six cases of listeriosis were reported in 2014, compared to a five-year average of eight cases per year. All cases required hospitalization. Three cases occurred among pregnant women; of these, one delivered successfully without complications, one resulted in fetal death, and the third delivered a premature infant who was also infected.

Two cases occurred among persons taking immunosuppressive medications who had both consumed milkshakes while inpatient at a King County healthcare facility; this product was later confirmed to be contaminated with L. monocytogenes and was recalled. Another case was linked to a statewide outbreak caused by consumption of Central American soft cheeses, in which one Washington resident died. The implicated company had been the source of a previous listeriosis outbreak in 2010.

Each year in Washington state between 11 and 29 cases of listeriosis are reported, including up to five deaths.