Researchers have provided insight into a large outbreak in Japan linked to milk served in schools.

The E. coli outbreak in June 2021 involved more than 1,800 cases from 25 schools, but no deaths were recorded.

The O antigens of most E. coli isolates were untypable (OUT). Although major foodborne toxins and pathogens were not detected, a specific E. coli strain, serotype OUT (OgGp9): H18, was isolated from milk samples related to the outbreak and tested patients. Strains from milk and patient stool samples were identified as the exact clone.

Milk cartons were part of school lunches in June 2021 in Toyama City, Japan. According to the study published in the journal Epidemiology and Infection, Pasteurized milk was produced by the T milk factory.

link to milk
Symptoms included abdominal pain, diarrhea, vomiting, and fever. They were consistent with those of diarrheagenic E. coli infection. The O genotype of OUT strains was typed to OgGp9 composed of O genotypes O17, O44, O73, and O106. Strains were tested for anti-O17, O44, O73, and O106 sera, but positive reactions for agglutination were not observed, said scientists.

The strain was closely related to some strains of enteroaggregative E. coli (EAEC) but did not possess typical EAEC virulence factors. 

E. coli OUT (OgGp9): H18 was isolated from 61 of 64 patients. E. coli O18 and O68 were isolated from the three other sick people.

Public health officials determined that the milk cartons produced by the T milk factory served in school lunches on two days in June caused the outbreak. E. coli was mainly isolated from milk cartons in school lunches on the days that caused the outbreak but not in those scheduled to be served on other days.

Nineteen milk samples that tested positive for E. coli OUT (OgGp9): H18 contamination were quantitatively assessed for the level of contamination.

Root cause unknown
T milk factory produced 6,000 to 7,000 cartons of 200 mL, 10 to 20 cartons of 500 mL, and 20 cartons a day from 1,600 to 1,800 kilograms of raw milk. This was pasteurized at 128 degrees C (262 degrees F) for two seconds. The site produced 6,851 200 mL cartons on June 11 and 7,840 on June 14.

Significant differences were not observed among the viable bacterial populations in milk cartons from June 14 to 16. Results indicated that contamination might have occurred after the pasteurization steps.

Milk contamination occurred on June 14 and continued for the next two days. The lack of cleaning on manufacturing lines or sanitary work could be one of the causes of this continuous contamination, said scientists.

The origin of the outbreak strain and the factors for contamination of milk cartons and continuous contamination were not clarified.

However, an inspection found that cross-contamination of pasteurized milk with raw milk by unsanitary handling, insufficient cleaning of raw milk tanks and carton packing equipment, failure in temperature control of pasteurized milk, and structural defects of surge tanks for pasteurized milk were potential risk factors for bacterial contamination.