PLUS: Investigation into E. coli O157 outbreak linked to cheese continues

Source not identified as PHE declares outbreak over

By Joseph James Whitworth

- Last updated on GMT

Investigations suggested a strong association with mixed salad leaves
Investigations suggested a strong association with mixed salad leaves

Related tags Escherichia coli

An outbreak of E. coli O157 which sickened 161 people has ended, according to Public Health England (PHE).

The agency said as the latest date of onset in a primary case is 5 July, the outbreak has been declared over and it is unlikely that affected product is still on sale.

Investigations identified mixed salad leaves as the likely cause but the source or how contamination occurred is not known.

The outbreak strain was not detected in any samples taken during the investigation but sampling is continuing.

England reported 154 cases, six in Wales and one in Scotland.

PHE said it has enhanced monitoring in place to ensure it detects any recurrence as soon as possible.

Nine affected by HUS and two dead

The World Health Organization (WHO) said six new infections were reported with onset after 29 June. It would be expected that almost all cases with onset of symptoms after 5 July would have been reported by 26 July.

A total of 66 patients sought hospital care, of whom 56 were admitted. Nine had symptoms consistent with haemolytic uraemic syndrome (HUS) and two died.

“To date, no other country in the region has reported an increase in similar or related cases, and the event is currently considered to be an outbreak confined to the UK,”​ said WHO’s European office.

“However, the source of the outbreak has not yet been identified, although continuing investigations suggest a strong association with eating mixed salad leaves.”

The outbreak was characterised by multiple small clusters linked to catering and residential care premises.

FSA investigation focus

The Food Standards Agency’s (FSA’s) investigations focussed on distribution of mixed salad leaves to wholesale and not supermarkets.

A strong association was found to eating mixed salad including rocket leaves from catering establishments such as cafes and restaurants.

When asked if wholesalers that stopped adding imported leaves to mixed salad products could start again and re-start the outbreak, FSA said all salad leaves used by producers involved in this investigation will be produced in accordance with best practice for the sector and product will continue to be tested by the companies on a routine basis. 

Richard Hoskin, head of incidents at the FSA, said consumer protection was the main priority throughout the investigation.

“We are continuing to work hard to identify the source of the outbreak and obtain the necessary assurances from industry that appropriate hygiene controls are in place.”

Necessary assurances include evidence of compliance with UK and EU legislation, industry assurance schemes and hygiene best practice.

Samples were Shiga toxin-producing E. coli (STEC) serogroup O157 phage type 34, positive for the eae (intimin) and verocytotoxin 2 genes but negative for the verocytotoxin 1 gene.

Professor Jeremy Hawker, incident director at PHE, said: “We urge people to remove any loose soil before storing vegetables and thoroughly wash all vegetables and salads that will be eaten raw unless they have been pre-prepared and are labelled ‘ready to eat’.

“These measures may reduce the risk of infection from any E. coli contaminated vegetables and salad but will not eliminate any risk of infection completely.”

Analysis of whole genome sequencing (WGS) data indicated isolates fell within a 5-SNP (single nucleotide polymorphism) cluster, a strain not related to those circulating in the UK bovine reservoir.

The outbreak strain was most closely related (>70 SNPs) to sequences identified in people reporting recent travel to the Mediterranean region suggesting the strain was likely to have been imported.

However, other European countries have not reported similar increases, and the strain has not been reported outside the UK.

The European Centre for Disease Prevention and Control (ECDC) said no other EU country reported an increase in Shiga toxin-producing E. coli O157 or cases related to the UK outbreak.

E. coli linked to cheese update

Meanwhile, Health Protection Scotland (HPS) said confirmed cases of E. coli O157 associated with eating cheese made from unpasteurised milk has risen to 19.

The additional case developed symptoms at the same time as the previous cases, between 2 and 15 July. All patients are recovering at home.

The agency said Dunsyre Blue cheese remains the most likely cause.

Humphrey Errington, founder of the company, told us previously​ that it has tested every batch from the last three months and had not stopped producing cheese.

HPS would not tell us if any cheese had tested positive for the outbreak strain or how many of the 19 cases said they ate the cheese.

The multi-agency Incident Management Team said the outbreak investigation is in line with national guidance.

“This standard approach is applied to every incident, and we are aware of the potential impact on businesses, we are however satisfied that our approach has been proportionate and reasonable based on the evidence.

“It would not be appropriate to respond in more detail at present as investigations have not yet concluded. However a formal outbreak report will be produced by the Incident Management Team after the investigation is declared over.”

When asked about a product link being made from the above outbreak with 19 cases compared to no identified item in the separate produce outbreak, FSA said the complexity of an investigation can vary significantly from outbreak to outbreak.

"During outbreak investigations, being able to make reliable linkages between cases and specific products is dependent upon many factors including for example the consumers’ memory of what they had eaten," ​said the agency.

"Complex food supply chains can also make it harder to trace the source of an outbreak. Mixed salad leaves can contain several types of leaf supplied by many different companies.

"This can make it more difficult to identify the source than if a single product from a particular manufacturer is the likely cause of an outbreak."

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