An outbreak of Shigella in England in 2018 was likely caused by contaminated coriander, according to researchers.

The national food poisoning outbreak highlights the potential for a multi-drug resistant strain of Shigella sonnei to be transmitted via a food vehicle that is distributed across a wide geographic area, according to the accepted manuscript in the journal Epidemiology and Infection.

It was linked to food at multiple restaurants in different areas that were not part of a franchise. Whole genome sequencing helped the identification of potential links between the restaurants.

Poor hygiene practices during cultivation, distribution or preparation of fresh produce were likely contributing factors to the contamination, said researchers.

Found to be related incidents
In March 2018, Public Health England was told of Shigella sonnei infections in people who had eaten at three different catering outlets in England. The outbreaks were initially investigated as separate events but whole genome sequencing showed they were caused by the same strain.

A total of 33 patients, linked to seven different venues specializing in Indian or Middle Eastern cuisine were identified. Five outlets were linked to two or more patients. All outlets used fresh coriander, although a shared supplier was not found.

Two-thirds of confirmed cases were male. The age of patients ranged from 12 to 59 years old. Symptom onset dates for the bulk of them ranged from March 26 to April 3, 2018. Four people were hospitalized for between two and five nights.

Patients were exposed at the implicated venues between March 24 and 31, 2018. The earliest exposure was at a site in Bedford, followed by venues in the West Midlands and two sites in Bradford.

Environmental health officers noted poor temperature control and cleaning standards plus a lack of hand hygiene facilities at one of the West Midlands outlets. It had a food hygiene rating of 1. Following the outbreak, the restaurant was refurbished, and given advice on kitchen routing improvements.

Food traceback investigations revealed fresh coriander leaves were the only common ingredient supplied to all venues attended by patients. In total, 86 percent of cases in the cohort study reported eating dishes containing coriander.

Three venues purchased fresh coriander from local markets and the other bought it from a national supermarket chain. It was not possible to identify wher the coriander was grown.

Likely point of contamination
The outbreak control team said the most plausible explanations for the outbreak were either coriander was contaminated at the point of production or during wholesale distribution.

Bulk supplies of coriander entering the wholesale market are broken down into smaller batches or bunches at multiple locations. This is done by hand, providing an opportunity for contamination by an infected food handler. There was no evidence infected food handlers contaminated the coriander in restaurants as none were known to be sick.

Because of the time-lag between local identification of outbreaks and confirmation by WGS, coriander leaves were not sampled as part of initial outbreak investigations.

Food samples were collected six and nine days after patient exposures, making it unlikely that they were the same batch as people had consumed prior to onset of illness.

Analysis of WGS data also demonstrated a close association between the outbreak strain and isolates from UK cases with recent travel to Pakistan.