Researchers have shed more light on the first recorded enteroinvasive E. coli (EIEC) outbreak in Sweden.

The outbreak in the county of Halland in 2017 had 83 self-reported infections and one secondary case attributed to household transmission, based on a study published in the journal Eurosurveillance.

Leafy greens were suspected to be behind the outbreak as this item was in a number of dishes associated with illnesses. There was no microbiological evidence to identify the source or vehicle of infection, but contaminated salad greens have been linked to previous EIEC outbreaks.

All five staff members at the conference and hotel venue that met the case definition had symptom onset at least one day after the first reported case among venue visitors.

The outbreak was identified because of the high number of people ill during a short time and as they all could quickly be linked to the same venue. Two cases and one household contact were laboratory-/confirm/ied.

EIEC or Shigella?
EIEC and Shigella both cause diarrheal disease. Clinical presentations of illness caused by the two pathogens are very similar and commonly involve diarrhea, abdominal cramps, nausea and fever. They share features that make it difficult to distinguish between them in routine clinical lab practice.

Both pathogens are transmitted via the fecal-oral route and infections are frequently associated with having contaminated food and water. While Shigella is linked to large-scale foodborne outbreaks, incidents caused by EIEC are rarely recorded.

A few EIEC outbreaks have been reported in Europe including in Italy in 2012 and the United Kingdom in 2014. These outbreaks affected 109 and 157 probable cases, respectively. The outbreak strain in these outbreaks, EIEC O96:H19, is an emergent type of EIEC.

Shigellosis is notifiable by law in Sweden as in the majority of countries in Europe. Most people are infected abroad. Reporting is not mandatory for EIEC and occurrence in Sweden is not known. It is unknown if EIEC O96:H19 is circulating in Sweden or if this strain was introduced via an imported food item.

Leafy greens link
From the 554 individuals who received the web-based questionnaire, 351 visitors and 47 staff members completed the survey. The case definition was met by 83 individuals, 78 visitors and five staff, of whom 56 were female.

Fourteen patients reported symptoms lasted for one day or less, 11 each reported symptom duration of two or three days, five cases reported four days, but 42 were symptomatic for five or more days. Symptoms reported were abdominal pain, bloody diarrhea, diarrhea, nausea and vomiting.

No strong association was observed between illness and a specific food item. Interviews with kitchen staff informed the outbreak investigation team that a variety of leafy green products, including baby spinach, rocket salad and mixed salads, were used as garnish on or in dishes.

Whole genome sequencing (WGS) found isolates were E. coli serotype O96:H19, a known EIEC serotype, and were classified as sequence type 99.

No food items sent for testing at commercial laboratories or frozen dill tested at the Swedish National Food Agency tested positive for E. coli.

Potential future increase in EIEC outbreaks
Researchers said despite the limited number of lab confirmed cases, they concluded EIEC to be the disease-causing pathogen.

“This is based on the absence of other common gastrointestinal pathogens in the collected stool specimens and the results of the WGS analysis revealing almost identical genomic sequences of the three EIEC isolates.”

The Environmental Health Unit did a post-outbreak inspection of the kitchen and had no remarks. Officers who did routine inspections earlier that year came to the same conclusion.

Trace-back investigations identified a distributor who delivered baby spinach, rocket salad and mixed leafy greens to the venue. The investigation did not find the products were contaminated before arriving at the venue.

Food items were received from a producer who packages pre-washed, ready-to-eat leafy greens in bags, with products coming from Italy and Sweden. The producer did not have any indication from their own microbiological testing of contaminated leafy greens during the reported dates and had not received complaints from other buyers so further control measures were not taken.

Researchers said the O96:H19 serotype of ST99 is considered a new emerging virulent EIEC strain and differs from other traditional EIEC and Shigella strains in phenotypic tests as it is more reactive.

“Emerging EIEC strains such as EIEC O96:H19, which phenotypically resembles E. coli more than Shigella which could enable improved survival abilities, could potentially contribute to an increase in foodborne outbreaks caused by EIEC in the future. This necessitates improved laboratory preparedness and consensus on recommendations for public health measures of PCR-positive Shigella and EIEC fecal samples,” according to the report.