Foodborne illness investigation underway: A third case of ice cream-related Listeria in 12 months

Public Health is investigating a case of Listeria infection that is linked to two cases identified last year. In all three cases the patients with highly weakened immune systems consumed milkshakes at the University of Washington Medical Center (UWMC) while hospitalized. In the previous two cases identified in 2014, the individuals consumed milk shakes made with a commercial ice cream product that was found later to be contaminated and was subsequently recalled. That product was not consumed by the current case. Samples of the different ice cream product currently used to make milkshakes at UWMC have been negative for Listeria. However, current samples taken from the ice cream machine there tested positive for Listeria, and UWMC immediately discontinued using the machine. listeria_monocytogenes

Dr. Jeff Duchin, Health Officer for Public Health – Seattle & King County answered questions about what these patients had in common and the ongoing investigation.

What did these cases have in common?  All three patients had highly weakened immune systems that put them at high-risk for Listeria infection.  Each of them consumed milkshakes that were made in the same machine. Significantly, they did not all consume milkshakes made of ice cream from the same manufacturer. Twelve months ago, two of the ill people consumed milkshakes made from a bagged ice cream product from a local manufacturer that, upon investigation, was found to be contaminated with Listeria and resulted in a recall of the product. Now, a year later, a third person has fallen ill from consuming a milkshake from the same machine but that was made with a product from a different manufacturer. Samples of this product were tested and did not show Listeria contamination. All three persons recovered from their Listeria infection.

How do you know that these three cases are connected? The genetic fingerprints match. Specifically, pulse field gel electrophoresis analysis (PFGE) of the Listeria isolates match for: 1) the current case 2) the ice cream machine; 3) people who became ill last year; and 4) the product consumed last year. In other words, the same related strain of Listeria is common to all three patients, the previously contaminated ice cream, and the ice cream machine.

 What is your conclusion about the link between the three cases and the cause of the current infection? Our investigation is in progress, so our findings could change. Our current assessment is that the Listeria bacteria were present in the milk shake machine where they contaminated the milkshake that was consumed by the recent case. Listeria can survive for long periods despite efforts to clean and sanitize food-processing equipment. UWMC cleaned and sanitized their machines twice weekly, which is not as frequently as is recommended.

What is the next step of the investigation? Our next step is to find out more about milkshake machines and practices used for cleaning and sanitizing this equipment. We will work with our partners at Washington Department of Health to examine cleaning practices and test various machines.

What kind of ice cream machine was used? The machine at the UWMC used a commercial liquid ice cream product to make soft serve ice cream and milkshakes. As the investigation progresses, we will be examining other machines that use these kinds of ice cream mixes.

My business uses this kind of an ice cream machine. How should I clean it? We encourage operations using soft serve ice cream machines to follow manufacturer instructions for frequency and method of cleaning and sanitizing.

Is there a health risk to UWMC patients from this incident?  We believe the risk to be low. Illness caused by Listeria usually occurs from a few days to a few weeks after exposure, but can be as long as two months. UWMC discontinued use of its machines on November 25. As a precaution, anyone who has been a patient at UWMC, particularly patients who are immune-compromised or pregnant and who have eaten soft ice cream or a milkshake at UWMC should be aware of signs of Listeria infection and monitor their health for 70 days after eating the milkshake.

Patients who have questions about their risk should consult their health care providers. There is no risk to persons who ate hard ice cream products or to patients who consumed milkshakes at UWMC and have not experienced symptoms after 70 days. Even for UWMC patients, the risk of illness is very low: UWMC reports that they have served hundreds of milkshakes each week over the past year and conducted over 22,000 blood cultures on patients and only this one patient tested positive for Listeria.

Is the problem ongoing? No. The problem has been corrected by UWMC by immediately discontinuing the use of soft serve ice cream machines. We conducted a thorough inspection and found no other problems at UWMC food services that we think contributed to this issue.

Should the general public be concerned?  No. The general public is not at risk from this incident because milkshakes from the machine with Listeria were only served to patients at the UWMC.

What do food safety experts say about the risk of listeria in ice cream? Currently, ice cream products, including soft ice cream, are considered very low risk foods for causing Listeria infections. Cases of Listeria from ice cream in the U.S. are very rare.  The Centers for Disease Control and Prevention (CDC) tells us they are aware of only two Listeria outbreaks linked to ice cream. One was the King County 3-person outbreak being described in this blog. There was another multistate outbreak identified in 2015 connected to another ice cream manufacturer in which 10 people became ill and that also lead to a product recall. A different strain of Listeria was found in these two outbreaks showing that the current 3-person outbreak in King County is not connected.

How do people get infected with Listeria?  Listeria is a foodborne illness. Listeria bacteria are found widely in soil and water and some animals, including poultry and cattle. It can be present in raw milk and foods made from raw milk, and on raw fruits and vegetables. It can also live in food processing plants and contaminate a variety of processed meats. The CDC estimates there are about 1600 reported Listeria infections and 260 deaths in the U.S. each year.

Who is at risk for severe Listeria infections? Healthy people rarely develop serious Listeria infections. Pregnant women and their newborns, older adults, and people with immune systems weakened by cancer, cancer treatments, or other serious conditions (like diabetes, kidney failure, liver disease, and HIV) are more vulnerable to severe Listeria infections.

People with immune deficiency and/or pregnant women and young children should always take precautions to avoid foods that are known to be high risk for Listeria contamination. What foods are typically implicated in causing Listeria infection?

  • Soft cheeses such as feta, brie, camembert, blue-veined, and Mexican-style cheese unless they are made with pasteurized milk.
  • Any unpasteurized milk products
  • Ready-to-eat foods, luncheon meats and cold cuts such as hot dogs, bologna, deli meats, and fermented or dried sausages unless they are cooked until steaming hot to 165° F before eating.
  • Refrigerated meat spreads, pates, or smoked seafood unless they are canned products. Canned fish, meat spreads, or pates may be eaten because these are thoroughly cooked in the canning process.
  • Unwashed fruits and vegetables
  • In addition, people with these conditions should be aware of the potential low risk for Listeria from ice cream products.

Read more guidance here (; and,

What are the symptoms of Listeria infection? Pregnant women and their newborns, older adults, and people with immune systems weakened by cancer, cancer treatments, or other serious conditions (like diabetes, kidney failure, liver disease, and HIV) can develop a life-threatening infection after eating food contaminated with the bacterium (germ) Listeria monocytogenes. People without these risk factors rarely are affected.

A person with a Listeria infection usually has fever and muscle aches, sometimes preceded by diarrhea or other gastrointestinal symptoms. Almost everyone who is diagnosed with Listeria infection has invasive infection, meaning the bacteria spread from their intestines to the blood, causing bloodstream infection, or to the central nervous system, causing meningitis. Although people can sometimes develop the infection up to 70 days after eating contaminated food, symptoms usually start within several days to a few weeks. Listeria infections are treated with antibiotics.

The symptoms vary with the infected person:

  • Higher-risk people other than pregnant women: Symptoms can include fever, muscle aches, headache, stiff neck, confusion, loss of balance, and convulsions.
  • Pregnant women: Pregnant women typically experience only fever, and other non-specific symptoms like chills and headache. However, infection during pregnancy can lead to miscarriage, stillbirth, premature delivery, or life-threatening infection of the newborn.
  • Healthy people. Healthy people rarely develop invasive Listeria infection. However, people exposed to a very large dose of Listeria bacteria can develop a non-invasive illness (meaning the bacteria do not spread into their bloodstream or other sites) with diarrhea and fever.

 You’ve said that the general public is not at risk from this current outbreak but that Listeria is known to be present in other food products. Should the general public be concerned about getting sick from ListeriaThe risk to the general public of getting a Listeria infection is low. Listeria can make anyone sick, though healthy people rarely get a serious illness. People at higher risk are more likely to get a severe, even life-threatening illness. All people can reduce their chances of getting Listeria infection if they:

  • Thoroughly heat leftovers from meals before eating.
  • Thoroughly cook food from animal sources, such as beef, pork, or poultry.
  • Wash raw vegetables thoroughly before eating.
  • Keep uncooked meats separate from vegetables, cooked foods and ready-to-eat foods.
  • Avoid raw (unpasteurized) milk or milk products made from raw milk.
  • Wash hands, knives, and cutting boards after handling uncooked foods.

What should I do if I’ve been exposed to Listeria through a contaminated food? If a person has eaten food contaminated with Listeria and does not have any symptoms, most experts believe that no tests or treatment are needed, even for people at higher risk. If symptoms develop, contact your health care provider for evaluation, especially if you are pregnant or have a weakened immune system or other high-risk condition described above.