In 2023, a Growing—and Deadly—Outbreak of Listeria Was Reported in Washington State. How PulseNet Helped Lead Investigators to the Cause

In the first few months of 2023, six people in Washington State were sickened with the bacteria Listeria monocytogenes (L. mono), which can be found in food, water, soil and animals. Three people ultimately died from infection with the bacteria, which tends to hit pregnant people, adults over 65 and/or those with weakened immune systems the hardest.

When the first patient fell ill in February, a specimen was sent to the Washington Public Health Laboratories, where  scientists quickly confirmed the presence of L. mono and then performed whole genome sequencing (WGS) to analyze the organism’s specific DNA.

“We confirmed the L. mono identification by plating the submitted isolate to agar plates and then utilizing biochemical tests and MALDI-ToF as confirmatory steps,” explained Anna Pickett, microbiology supervisor at the Washington State Department of Health who was involved in the investigation. “Most hospitals/clinics can correctly identify L. mono, but we could characterize these bugs thoroughly. Once confirmed, the patient isolates are routed to WGS.”

Those WGS results are then uploaded to PulseNet, a national network of public health and food regulatory laboratories that uses WGS to look at an organism’s unique DNA fingerprint. Public health professionals can access the PulseNet database and look for similarities between specimens, with the aim of linking cases and identifying causes.

PulseNet proves pivotal

In June, there was a second case. That patient’s isolate clustered via PulseNet with the L. mono isolated from the resident in February.

 “These two isolates were indistinguishable from one another,” Pickett explained. “Given the relative rarity of L. mono infections, we designated these two isolates a Washington State cluster code and began the interview process.”

While investigators interviewed patients looking for common denominators—for example, eating at the same restaurants or consuming the same food—cases grew.

“Several patients had underlying health conditions such as cancer and autoimmune diseases,” Pickett said. “By early July, we identified a potential link to a common hospital in Tacoma, and the Food & Shellfish Bacteriology Laboratory performed testing of environmental swabs and food products from the hospital café kitchen. But … we did not detect any Listeria spp. from those swabs/food items.”

A month later, two more cases were detected, bringing the total to six. One patient mentioned having a milkshake from the Tacoma location of Frugals, a local fast-food restaurant chain. This prompted investigators to re-interview surviving patients and/or their family members, some of whom indicated they had consumed milkshakes from the same establishment.

“In early August, the Tacoma-Pierce County Health Department collected environmental swabs from both milkshake machines at Frugals Tacoma, as well as multiple flavors of milkshakes from the two machines,” Pickett noted. “They also collected an unopened bag of milkshake mix. These samples were then tested by the Food & Shellfish Bacteriology Laboratory. We recovered L. mono from all swabs and processed milkshake flavors within the machines. We did not detect L. mono in the unopened bag of milkshake mix. After isolating the L. mono from these products/swabs, we performed WGS. All recovered isolates matched the six patient isolates via PulseNet.”

The cluster was first identified on June 9, 2023. By August 17, matches from the Frugals L. mono isolates were confirmed. “L. mono can take slightly longer to produce symptoms,” Pickett acknowledged, “as the onset time for this disorder is far longer than other enteric bugs (Salmonella, E. coli, etc.).”

After being exposed to L. mono, it can take days or even several weeks for symptoms to appear.

The public health response

Once investigators were able to pinpoint the source of the infection, county officials worked with Frugals to stop selling milkshakes at all its locations.

Additional testing found that the issue was limited to just the Tacoma Frugals restaurant. That establishment removed the impacted machines (thought to be contaminated through improper cleaning) and purchased new ones. Staff at all restaurants operated by Frugals received training on proper cleaning protocol. “After identifying the source, no other cases were linked to this outbreak,” Pickett said.

PulseNet works to connect the dots

Pickett said PulseNet—largely supported by critical funding from the US Centers for Disease Control and Prevention—was “invaluable” in detecting the outbreak.

“In addition to allowing us to connect the first two cases, PulseNet assisted us in identifying every related patient, enabling us to cast a wide net in finding what was causing the infections,” Pickett commented. “Further, once we did identify a potential source, we were able to confirm our suspicions quickly and definitively. Without PulseNet, it would have been extremely challenging to recognize the outbreak quickly. We may have identified a problem once more cases rolled through, but being able to link a case from February to one in June and then immediately investigate saved precious time and, potentially, lives.”

This blog post is part of a series honoring PulseNet’s 30th anniversary. Find more stories about the importance of PulseNet here.