Effective Traceback Investigation Strategy Can Quickly Identify a Foodborne Outbreak Source, Explains Case Study

A traceback investigation is the first step in identifying the source of a foodborne outbreak. When people are falling sick, time is of the essence. The faster the outbreak source is discovered, the faster the situation can be remedied (e.g., recall, restaurant closure, public health advisory). Fewer people fall sick.

Some traceback investigation strategies are better than others.

A recent Centers for Disease Control and Prevention (CDC) Morbidity and Mortality Weekly Report (MMWR) case study examines how not only the right questions need to be asked. The right format can make a difference between a lot of data to pick through and more pointed information leading to identifying an outbreak source.

The goal of a traceback investigation is to identify the source as quickly as possible. Lessons learned from this case study could help future traceback investigations.

The Case: A Salmonella Livingstone Outbreak in Utah, Late 2023 to Early 2024

The case used in “Notes from the Field: Rapid Linkage of a Salmonella Livingstone Outbreak to a Restaurant, Using Open Ended Interviews and Patient Purchase Histories – Utah, 2023-2024” explored a Rare Salmonella Livingston outbreak.

What started as a cluster of five illnesses from residents of two neighboring Utah counties through routine foodborne disease surveillance activities grew to 11 cases. Six patients sought treatment at an emergency department. Two patients were hospitalized

The majority of patients reported gastroenteritis symptoms including diarrhea (10 patients), abdominal pain (seven patients), vomiting (four patients), and bloody diarrhea (two patients).

Three patients (27%) developed urinary tract infections. Rare for non-typhoidal Salmonella infections, but consistent with a previous Salmonella Livingstone outbreak.

One patient developed a serious and life-threatening bloodstream infection.

Illness onset was reported between October 13, 2023 and January 20, 2024.

Patient samples underwent genetic testing (core-genome multilocus sequence testing) and genetic similarity was revealed.

Four non-stool specimens, including three urine samples and one blood sample, contained the outbreak strain.

Investigators compared the outbreak strain against other isolated reported from other states in the national database (National Center for Biotechnology Information) to determine if this was a local issue or a part of a larger, multi-state outbreak.

There was no connection to a larger outbreak and no patients reported traveling outside of the state in the week prior to becoming sick.

This suggested a local exposure.

Initial Patient Traceback Investigation Interviews Were a Dead End

Initial outbreak interviews were routinely conducted, not unlike other patient traceback investigation interviews.

Patients were asked about potential exposures. But no common exposure was reported.

This initial investigation was unsuccessful.

With additional patients falling sick, a source needed to be identified.

And quickly!

Follow-up Traceback Investigation Consisted of Open-Ended Interviews and Purchase Histories

Starting January 16, 2024, health officials took a different approach. Patients were re-interviewed with traceback investigation surveys. However, these questions were open-ended interviews. Additionally, purchase histories were also obtained.

The pieces were quickly put together.

By January 17, 2024 four patients had reported eating at the same restaurant. By January 19, 2024, a total of eight confirmed or probable cases reported eating at that same restaurant.

Why was this not discovered in initial interviews?

“No common meal was reportedly consumed” at the restaurant.

Restaurant Traceback Investigation Activities

Once a common source was identified, public health officials closed the restaurant on January 19, 2024 to prevent additional cases.

Investigators performed an inspection and collected environmental samples at the restaurant on January 22, 2024.

Local and state officials collected 71 samples altogether from food and the restaurant environment.

A composite swab is obtained by rubbing a moistened sample collection swab on multiple areas. This allows for a wider area of sampling with reduced burden of sample testing. Composite samples allow for more prompt test results.

The outbreak strain was discovered in seven composite environmental swab samples.

These positive samples included:

• Cleaning equipment
• Three-compartment sink and washing machine
• Drying rack, wooden stools, and trash can
• Utensils shelf
• Stove handles
• Sauce bottles
• Outdoor Dumpster

Composite food samples from sauces from the grill station and vegetables/other ingredients from the ingredient preparation area also contained the outbreak strain.

Additionally, employee interviews were conducted and stool samples from all nine employees were analyzed by polymerase chain reaction (PCR) testing, culture, and genomic sequencing.

The outbreak strain was discovered in the stool sample of an employee that began working at the restaurant on January 16, 2024, reported eating multiple meals at the restaurant, and developed symptoms on January 20, 2024.

Widespread Contamination at Restaurant Determined Source of Salmonella Livingstone Outbreak

Environmental and food sampling results confirmed the restaurant as the source of the outbreak and suggested widespread contamination at the restaurant.

Guided cleaning and sanitation were performed, and the restaurant re-opened on January 29, 2024. According to the case report, as of June 10, 2024, there have been no additional outbreak strain infections reported.

Lessons Learned

This public health jurisdiction learned a powerful lesson as a result of this Salmonella Livingstone outbreak.

Initial, routine patient interviews were not sufficient to identify a common exposure. “Open-ended interviews and patient purchase histories enabled prompt identification of a restaurant source and led to closure of the restaurant within three days.”

Investigators lost nearly a week from the initial failed traceback investigation patient interviews.

Could some of these illnesses have been prevented?

Maybe so.

Traceback investigations are a dynamic process. While certain procedural activities are routine, investigators act as detectives, following the evidence to the source of the outbreak. This case study is a prime example of how investigators can tweak those initial patient interviews to gather more information leading to faster results.

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By: Heather Van Tassell (contributing writer, non-lawyer)