Health Advisory: Listeria monocytogenes Contamination of Restaurant Milkshakes

This is a health advisory from the Washington State Department of Health (DOH) Office of Communicable Disease Epidemiology

Current Situation in Washington

During an ongoing listeriosis outbreak investigation, Listeria monocytogenes was found in milkshakes (all flavors) sold at Frugals, a drive-in restaurant located at 10727 Pacific Ave S, Tacoma, WA 98444. No other Frugals locations (in Washington or Montana) are thought to be affected. Whole genome sequencing shows that L. monocytogenes found in the milkshakes is closely related to the L. monocytogenes that caused the six outbreak-associated infections.   

Potentially contaminated milkshakes were sold prior to August 8, 2023 when test samples were collected and the restaurant discontinued use of the milkshake machines.

People aged 65 years and older and those with immunocompromising conditions are at risk for developing invasive infection, including sepsis, meningitis, and meningoencephalitis after consuming foods contaminated with L. monocytogenes. Infection in pregnancy can result in miscarriage, stillbirth, preterm labor and sepsis or meningitis in the neonate but pregnant people typically experience a relatively mild “flu-like” illness and can be asymptomatic. Older children or adults with normal immune function may have no or mild symptoms and only rarely develop invasive disease.

Washington State health care providers are requested to:

1. Be alert for possible cases of listeriosis infection in high-risk individuals (aged 65 years or older, weakened immune systems, or pregnant) who consumed milkshakes from Frugals-Tacoma. The incubation period for listeriosis is as long as 70 days so likely exposures would have taken place between May 29 and August 7, 2023. 
2. Review the Centers for Disease Control and Prevention (CDC) framework for patient management of high-risk individuals who may have been exposed to food contaminated with monocytogenes, which can be found below.
3. Promptly report all suspected and confirmed cases to the local health jurisdiction where the patient resides

CDC Framework for Assessment and Medical Treatment of High-Risk People Exposed to Foods Contaminated with L. monocytogenes
(From CDC: Information for Health Professionals and Laboratories)

This section presents a framework for assessment and medical treatment of high-risk people (pregnant women, older adults, and people with weakened immune systems) who may have been exposed to L. monocytogenes by eating contaminated foods.

The risk of invasive listeriosis after exposure to L. monocytogenes is very low; exposure is common, but disease is rare. A study related to a 2011 outbreak linked to cantaloupe estimated the attack rate at roughly 1 case per 10,000 pregnant women who ate the implicated cantaloupe.

Management of pregnant women: The American College of Obstetrics and Gynecology has issued a committee opinion on the Management of Pregnant Women with Presumptive Exposure to Listeria monocytogenes.

Management of people other than pregnant women with elevated risk of invasive listeriosis: Clinicians may consider using the framework below. This framework is not intended either as a practice guideline or as a modification of existing guidelines or practices in managing patients with confirmed or suspected listeriosis. Little scientific evidence is available to inform decisions regarding management of people at elevated risk of invasive listeriosis who have been exposed to L. monocytogenes and who are either asymptomatic or have mild symptoms that could be consistent with early Listeria infection. Therefore, this suggested framework is based on informal solicitation of experts in infectious disease, obstetrics and gynecology, and public health in 2011. Patient management decisions for asymptomatic or mildly symptomatic people are appropriately made on a case-by-case basis, informed by clinical judgment and the likelihood of exposure of the patient. Consultation with a specialist in infectious disease may be considered.

• Exposed, asymptomatic: Most experts believe that no testing or treatment is indicated for an asymptomatic person with elevated risk of invasive listeriosis who ate a product recalled because of monocytogenes contamination. Such a patient should be instructed to return for medical treatment if he or she develops symptoms of listeriosis within two months after eating the recalled product. Symptoms may include fever and myalgias, often preceded by diarrhea or other gastrointestinal symptoms. In older adults and immunocompromised people, symptoms of listeriosis can include headache, stiff neck, confusion, loss of balance, and convulsions.
• Exposed, afebrile, mild symptoms: A person with elevated risk of invasive listeriosis who ate a product recalled because of monocytogenes contamination, who is afebrile and has signs and symptoms consistent with a minor gastrointestinal or flu-like illness, such as mild myalgias or mild nausea or diarrhea, could be managed expectantly; this approach limits low-yield testing. Alternatively, such a patient could be tested with blood culture. If diagnostic tests are performed, some experts would withhold antibiotic therapy unless cultures yielded L. monocytogenes. Others would initiate antibiotic therapy while culture results were pending and then stop treatment if the cultures were negative. The antibiotic regimen could consist of oral ampicillin or amoxicillin, although it is important that both the clinician and the patient know that no effectiveness data exist for this scenario. If the blood culture yielded Listeria, standard antimicrobial treatment for listeriosis, typically including IV ampicillin and gentamicin, would be indicated.
• Exposed, fever and symptoms consistent with invasive listeriosis: An exposed person with elevated risk of invasive listeriosis with fever (>100.6° F, >38.1° C) and signs and symptoms consistent with listeriosis, for whom no other cause of illness is known, should be tested and treated for presumptive listeriosis.
  • Diagnostic testing should include blood culture and other tests, such as culture of cerebrospinal fluid, as indicated by the clinical presentation.
  • The antimicrobial regimen should be the standard therapy for listeriosis, typically including IV ampicillin and gentamicin for 14 to 21 days for nonallergic patients.
  • If blood culture is negative and symptoms resolve, antibiotic therapy may be discontinued.
• Exposed, history of symptoms in past 4 weeks, currently asymptomatic: Most experts believe that no testing or treatment is indicated for an asymptomatic person with elevated risk of invasive listeriosis who ate a product recalled because of L. monocytogenes contamination and experienced symptoms that have resolved. Any such patient should be instructed to return for medical care if he or she develops symptoms of listeriosis within 2 months after eating the recalled product.

Note: The suggested framework above does not include stool culture for L. monocytogenes. Stool culture has not been evaluated as a screening tool and is not recommended for the diagnosis of listeriosis. Ingestion of food contaminated with Listeria occurs frequently because the bacterium is commonly present in the environment. Therefore, intermittent fecal carriage and shedding of Listeria is also frequent (about 5% in unselected populations, but substantial variation exists) and rarely indicative of infection. Stool culture for Listeria may also have low sensitivity and is rarely available in clinical laboratories. Serologic testing for Listeria has poor sensitivity and specificity and is not recommended for diagnosis of listeriosis in individual patients.

Additional Information

Updated January 25, 2024

Please see the following DOH webpages for more information:

2023 Outbreak of Foodborne Listeriosis Associated with Consumption of Restaurant Milkshakes

Listeria bacteria found in milkshakes sold at Frugals restaurant in Tacoma (English)

Se encontraron bacterias de listeria en los batidos de Frugals en Tacoma (Spanish)

To read this and other WA HAN messages, please visit the Washington Health Alert Network webpage.