Later infections in a deadly Listeria outbreak traced to sandwiches served in hospitals in England could have been prevented, according to an investigation into the incident.

The first outbreak patient in April 2019 was not confirmed for almost two months because Public Health England did not get the isolate for typing from the hospital until early June.

Officials said it was “conceivable” that earlier receipt of the isolate would have raised the possibility of a wider problem sooner and may have led to earlier intervention and prevention of some further cases in the outbreak. Referral of clinical isolates is voluntary. Experts say making such action mandatory should be considered.

Between May 16 and June 14, 2019, nine confirmed outbreak cases of listeriosis were identified in England. Seven people died. Patients were from seven National Health Service (NHS) Trusts. Illness onset dates ranged from April 20 to June 2, 2019. The median age was 75 years old. Six patients were female.

Consumption of prepacked sandwiches supplied by The Good Food Chain was confirmed for eight cases. Of these, six had chicken sandwiches and two ate other sandwiches from this supplier including cheese and egg. Meat was provided by a producer called North Country Cooked Meats. Sandwiches were supplied to 42 NHS Trusts in England, Wales and Scotland.

Listeria found at supplier and producer
It was the eighth outbreak of listeriosis in England and Wales associated with sandwiches purchased from or provided in hospitals since 1999. A review into hospital food in England following the outbreak was published recently and included plans to improve food safety.

Listeria monocytogenes from unopened packs of cooked duck, chicken and ham, sampled from The Good Food Chain’s environment was detected by an external laboratory through routine testing on April 25, 2019. An isolate from chicken was confirmed as the outbreak strain on June 10.

Levels of Listeria monocytogenes ranging from 1,100 to 3,500 colony forming units per gram (cfu/g) were detected in an unopened pack of diced chicken sent to the supplier from the producer’s manufacturing environment. This is above the 100 cfu/g limit in EU regulations. It was later confirmed by whole genome sequencing as the outbreak strain. This strain was also identified from a coronation chicken sandwich sampled from a hospital.

Isolates submitted by a commercial lab confirmed cross contamination of cooked meats between the sandwich supplier and meat producer. The outbreak strain was also isolated by lab services working for another customer of the producer from an unopened diced bacon sample.

Sampling of ingredients and the food processing environments of production and supplier sites during May and June 2020 recovered types of Listeria monocytogenes unrelated to the outbreak strain. However, their detection indicates inadequate cleaning and hygiene, according to officials.

One of the last confirmed patients consumed a cheese sandwich, which was produced by the supplier but had no ingredients from North Country Cooked Meats. This suggests there may have been cross contamination and hygiene failures in the supplier’s and producer’s practices, according to the investigation report.

The Good Food Chain changed chicken supplier in late May, and voluntarily halted production on June 5, with a withdrawal of all products on June 10, 2019. The company ceased trading in late June.

Salford City Council is still investigating the producer, which went into liquidation, and its food safety management systems. Physical inspections did not find serious defects or failures in processing or handling. Production records for 19 batches were reviewed and all of them had some level of missing records or discrepancies.

Issues at hospitals
Investigations of hospital catering facilities identified two hospital trusts had not registered with local authorities as food businesses.

Of eight hospitals implicated in the outbreak, only three had specific controls for Listeria in their Hazard Analysis and Critical Control Point plans, while the others said controls were general for chilled food storage and/or food poisoning bacteria.

Temperature and shelf life controls at each hospital found 2018 Food Standards Agency guidance was not being practiced and there was no alternative procedure to mitigate Listeria.

One local authority found there were situations wher cold chain temperature breaches were permitted. It was common practice across most hospitals to work toward a target temperature of 5 degrees C with a critical limit of 8 degrees C — too high to prevent Listeria growth.

Ready to eat (RTE) food with low amounts of Listeria monocytogenes at the production site may see the pathogen survive or grow during transport and storage to levels that pose a risk for immunocompromised patients.

Findings from the outbreak investigation support previous recommendations that healthcare sites should set a limit, to be met by manufacturers, of undetectable Listeria monocytogenes in sandwiches and their ingredients at the production stage.