The Tacoma-Pierce County Health Department of Washington has announced that it is working with the Washington State Department of Health and Thurston County Public Health and Social Services to investigate five cases of foodborne illness caused by Listeria monocytogenes in Western Washington, three of which have resulted in death. Whole genome sequencing (WGS) has indicated that the five cases of listeriosis were likely caused by the same source of infection.

At present, a common food source has not been identified, but the investigation is ongoing. Washington Department of Health is working with Pierce and Thurston County local health jurisdictions to gather information from interviews with patients and their families to help identify any common exposures.

All five of the case patients were over 60 years of age, three of the patients died as a result of their infections, and all of the patients were immunocompromised. The onset of illness occurred between February 27 and June 30, 2023. Of the patients, four resided in Pierce County and one resided in Thurston County.

The U.S. Food and Drug Administration (FDA) has not announced any involvement in the investigation as of July 26, 2023.

Update, August 22, 2023: The number of cases in the outbreak has increased to six. The Washington State Department of Health has identified milkshakes served at Frugals restaurant in Tacoma, Washington as the source of the outbreak.