In a notable incident that took place in June 2021, researchers have shed light on a significant E. coli outbreak in Japan that was linked to milk served in schools. Although the outbreak affected more than 1,800 individuals from 25 schools, thankfully, no fatalities were reported.
The O antigens of most E. coli isolates in this outbreak were untypable (OUT). While major foodborne toxins and pathogens were not detected, a specific E. coli strain, serotype OUT (OgGp9): H18, was identified in milk samples associated with the outbreak and in the tested patients. Notably, the strains found in the milk and in the stool samples of affected individuals were found to be the same.
Milk cartons were a part of school lunches in Toyama City, Japan, during June 2021, and the milk in question was produced by the T milk factory.
The symptoms experienced by those affected included abdominal pain, diarrhea, vomiting, and fever, which were consistent with those of diarrheagenic E. coli infection. The O genotype of the OUT strains was identified as OgGp9 and composed of O genotypes O17, O44, O73, and O106. When these strains were tested with anti-O17, O44, O73, and O106 sera, positive reactions for agglutination were not observed.
While the E. coli strain was found to be closely related to some strains of enteroaggregative E. coli (EAEC), it did not possess typical EAEC virulence factors.
The E. coli OUT (OgGp9): H18 strain was isolated from 61 of the 64 patients, with E. coli O18 and O68 being isolated from the remaining three individuals.
Public health officials traced the source of the outbreak to the milk cartons produced by the T milk factory, which were served in school lunches on two specific days in June. Notably, E. coli was primarily isolated from milk cartons in school lunches on these particular days, while no contamination was found in milk cartons intended for other days.
Nineteen milk samples that tested positive for E. coli OUT (OgGp9): H18 contamination were quantitatively assessed for the level of contamination.
The T milk factory produced thousands of cartons of milk daily, and the milk underwent pasteurization at 128 degrees Celsius (262 degrees Fahrenheit) for two seconds. While significant differences were not observed among the viable bacterial populations in milk cartons from June 14 to 16, results suggested that contamination might have occurred after the pasteurization steps.
Milk contamination was observed on June 14 and continued for the subsequent two days. It is suggested that inadequate cleaning on manufacturing lines or unsanitary handling could be contributing factors to this continuous contamination.
Despite extensive investigations, the origin of the outbreak strain and the exact factors leading to the contamination of milk cartons, as well as the subsequent continuous contamination, remain unclear. However, an inspection did reveal potential risk factors, including cross-contamination of pasteurized milk with raw milk due to 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. These findings raise concerns about the safety of milk production processes and the need for improved hygiene measures to prevent such outbreaks in the future.