Researchers describe non-O157 STEC outbreak traced to salad

Researchers in Finland have detailed an E. coli outbreak in 2016 with more than 230 cases linked to rocket salad, known as arugula salad by U.S. consumers.

In August 2016, an outbreak of Shiga toxin-producing E. coli (STEC) and enteropathogenic E. coli (EPEC) with 237 cases occurred in the Helsinki area of Finland. Gastroenteritis cases were reported at 11 of 12 events between Aug. 19 and 21 that were served by one catering company.

Researchers writing in the Eurosurveillance journal suggested that EPEC could be added to the national surveillance since diagnostics for it are routinely done in clinical labs.

The incubation period for STEC infection was shorter than the usual three to four days with a median of 20 hours. None of the patients developed hemolytic uremic syndrome (HUS). For this reason and based on the mild symptoms, norovirus was suspected as the cause of illness before STEC was revealed in PCR screening tests.

Microbiological and epidemiological investigations suggested rocket salad was the cause of the outbreak.

The rocket was found to be contaminated by E. coli. Stx and eae genes were detected in rocket sampled from an unopened package, and identical STEC and EPEC strains were isolated in rocket-containing dishes and patients’´ samples. During the outbreak investigation, no rocket of the contaminated batch was still on the market.

Two staff members of the catering company had gastroenteritis symptoms at the same time as event participants. They had eaten some of the rocket during preparation of the dishes and had not been working while symptomatic.

Since 1997, six STEC outbreaks with a median of 13 cases have been recorded in Finland. Foodborne outbreaks caused by EPEC are seldom reported.

In the 11 events, 50 different dishes and bread, water, milk, juice, tea and coffee were served. In univariate analysis of each event, six foods served at four of them were significantly associated with illness. Of these, only chicken fillet in oil with fresh herbs and lime spiced salmon contained ingredients common to foods served at several events. In pooled analysis, only food containing rocket was significantly associated with gastroenteritis.

The STEC ONT:H11 and EPEC O111:H8 strains isolated from food samples containing the rocket were identical to patient isolates. Between Aug. 19 and Oct. 2, 2016, 31 samples were positive for STEC and 62 for EPEC by PCR in event participants and staff members. At least three people had both STEC and EPEC as they were EPEC-positive in a control sample that was taken because of primary STEC infection.

“The probability of mixed infections is also supported by the fact that two different strains of STEC and three different strains of EPEC were found in the patients, and both STEC and EPEC were found in the food samples,” said researchers.

In preliminary analysis of food samples E. coli levels of 3,100 colony forming units per gram were found in rocket. Samples of chicken fillet in oil with fresh herbs including rocket and of thyme-marinated roast beef garnished with rocket were stx2- and eae-positive.

In examinations by PFGE, STEC strains isolated from samples of chicken fillet in oil with fresh herbs including rocket and from thyme-marinated roast beef garnished with rocket were identical to STEC ONT:H11 patient isolates. EPEC O111:H8 identical to human isolates were found in these samples that contained unheated rocket.

The premises of the catering company were inspected in August with no major findings. The company had received packages of fresh rocket on Aug. 18 from Denmark with packing in Sweden.

Authorities in Denmark inspected the rocket grower. They found control documentation of the farmer was satisfactory, including the result for lab analyses of samples from rocket for indicator bacteria and for quality of water used in the plant. General maintenance of the production facilities was also adequate.

Based on experience from the outbreak, national surveillance criteria for STEC were updated to meet the practice in reporting labs covering both PCR-positive and culture-confirmed findings. In the described outbreak, 35 percent of the patients’ STEC strains could be isolated from cultures.

The Finnish Infectious Disease Registry (FIDR) reporting criteria had included only culture-confirmed STEC. At the start of the investigation, FIDR could not be used for outbreak detection and investigation purposes because of a reporting delay of more than one week.

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