Michigan Restaurant at Center of 11-Year Salmonella Outbreak

While most outbreaks of foodborne illness peak and recede, one southern Michigan restaurant struggled with an intermittent Salmonella outbreak for more than a decade.

From September 2008 to July 2019, there were 35 primary cases and one secondary case of Salmonella Mbandaka ultimately traced to the restaurant by the local public health department, William Nettleton, MD, medical director of the Kalamazoo County Health and Community Services Department, and colleagues reported in the August 20 issue of Morbidity and Mortality Weekly Report.

The unusual persistence of the outbreak was due to a complex interplay between the restaurant environment and asymptomatic food workers, Nettleton told MedPage Today.

“It was very challenging to identify the source of the outbreak,” he said in an interview. “Typically with Salmonella or other types of enteropathogens, there’s a foodborne vehicle. People get sick over a period of days to weeks, and once the source is eliminated, people stop getting sick. You get the traditional bell curve.”

“This was different,” he said. “The sporadic incidence made it very challenging.”

The restaurant initially made it on the county health department’s radar in 2012, when the Michigan Department of Health and Human Services sounded an alarm about Salmonella Mbandaka cases occurring intermittently in the county since 2008.

Kalamazoo health officials at the time launched a hypothesis-generating questionnaire, and by 2014 they’d homed their sights on the restaurant in question, after five known cases reported a meal there.

An interview with the restaurant revealed no further epidemiologic link at the time. But over the next 4 years, the establishment was hit with continued cleanliness and maintenance citations. As cases continued to mount, the department developed another questionnaire that included specific questions about this restaurant in particular.

With that, more cases were identified and an intensive investigation began in 2018, with the department taking employee stool samples along with environmental samples and analyzing them in parallel.

Genetic testing revealed five isolates of the outbreak strain from four employees among the 100 who were sampled. None of them had symptoms at the time of sample collection or in the weeks preceding.

Nearly half (49%) of the 80 environmental samples turned up in the outbreak strain as well. Positive samples were found in many places in the kitchen, including cooking, preparation, dish-washing, storage, and employee restrooms.

The employee and environmental samples matched the strain found in sick patrons as well, Nettleton said: “They were all highly related” on whole-genome sequencing.

Just before the environmental sampling, the facility had closed for renovations to the kitchen, flooring, walls, and major equipment. After further renovations to the flooring and equipment in the fall of 2018, which included a norovirus standard cleaning, sampling continued to detect Salmonella Mbandaka, the researchers reported.

“How recalcitrant it was, was quite something,” Nettleton said.

By late 2018, the restaurant voluntarily and permanently closed. All food, dishes, storage, soft goods, chairs, and tables were destroyed. Metal food production equipment was extensively cleaned, quarantined, and re-sampled before being re-deployed. The building was deemed ineligible for food production or storage relicensure.

In 2019, 8 months after the restaurant had closed, the outbreak strain was detected in a person who’d eaten at the restaurant 3 weeks before it shuttered. That person reported chronic, intermittent diarrhea since that time.

Among the 35 cases, the mean age was 57 (range 1.5 to 90 years), 67% reported vomiting or diarrhea, 33% reported urinary tract infection, and six of the patients (17%) had been hospitalized.

Many of the cases (42%) had the outbreak strain isolated in urine, and 33% reported urinary symptoms without diarrhea or vomiting — findings consistent with previous reporting on the Salmonella serogroup that Mbandaka belongs to, the researchers said.

When the restaurant was in mitigation, asymptomatic employees needed two negative cultures before returning to work. The duration of shedding ranged from 31 to 123 days.

Nettleton noted that while asymptomatic carriage of Salmonella typhi species is well characterized, less is known about asymptomatic carriage of non-typhoid species, like the strain in this outbreak.

He added that while previous codes omitted it, the 2017 FDA Food Code included asymptomatic nontyphoid Salmonella infection as a food worker condition of restriction.

“Further adoption of the 2017 FDA Food Code will aid public health professionals in disrupting nontyphoidal Salmonella transmission in restaurant settings, particularly as more protracted outbreaks are identified,” the researchers concluded.