E. coli was behind a large-scale food poisoning outbreak that involved about 3,000 school students and staff in Japan this past year, according to a study.

The investigation found the 2020 outbreak was caused by red seaweed used in a salad contaminated with E. coli O7:H4. The seaweed had been imported in 2017.

E. coli O7:H4 carrying the astA gene for enteroaggregative E. coli heat-stable enterotoxin 1 (EAST1) was detected in fecal specimens from patients and in the red seaweed. Food poisoning cases from E. coli carrying astA for EAST1 usually show relatively mild symptoms, according to the study published in the journal Epidemiology and Infection.

In late June 2020, elementary and junior high school students in Yashio, Saitama, came down with gastroenteritis symptoms such as diarrhea and abdominal pain. Patients were in all 15 public elementary and junior high schools in Yashio and the Yashio Board of Education jurisdiction. A school lunch from a private meal supplier was the common foodstuff eaten by all patients.

As many as 6,732 students, teachers and other staff had consumed the school lunch, and 2,958 of them fell sick. The major symptoms were diarrhea, abdominal pain and fever.

Examining patients and food samples
An investigation involved collection of fecal specimens from 19 patients in eight of 15 schools and the Yashio Board of Education who were not receiving antibiotics. Also, 27 food samples from lunch, served on June 24, 25 and 26, stored by meal type at the Yashio Board of Education and the private school meal supplier’s facility, and nine fecal specimens from food preparation staff were taken. Four swab samples of the kitchen facilities were collected.

Direct examination of culture plates from 18 of 19 patients’ samples and five of nine food workers’ samples showed growth of lactose-fermenting colonies that appeared to be E. coli. Colonies were confirmed for the presence of virulence-associated genes for diarrheagenic E. coli. Fourteen isolates from 14 patients were identified as E. coli O7:H4.

The 27 stored food samples and four swab samples were tested for E. coli O7:H4 carrying astA and it was detected in two seaweed salad samples served on June 26.

The seaweed salad was made from six kinds of seaweed rehydrated with water and boiled vegetables with dressing. Coliforms were found in the red seaweed by the manufacturer during self-inspection. E. coli carrying astA was then detected in the red seaweed, which had been imported in 2017.

Coliforms were not detected during an import inspection nor when it was sold to the manufacturer. No other complaints about the same lot of seaweed have been reported. So, the source of the E. coli contamination of red seaweed could not be identified.

Researchers said it was the first report of large-scale food poisoning caused by E. coli O7:H4.

“Since this particular outbreak was attributed to seaweed, it is recommended that marine products be carefully monitored for contamination.”