Researchers have suggested opportunities are being missed to prevent listeriosis from pre-packed sandwiches.

Eating pre-prepared sandwiches served in hospitals was the most common source of Listeria infections in England and Wales between 1981 and 2015.

Scientists said repeated incidents, despite guidance on reducing the risk of listeriosis in healthcare settings being available, shows lessons are not being learned, with risk factors having similarities to those described previously.

They made the comments as part of a study on a case of listeriosis associated with eating sandwiches in a hospital in 2017. This incident had also already been covered in another report.

A review of hospital food was published in 2020 after a Listeria outbreak in England in 2019 that killed seven people was traced to sandwiches. It made five food safety recommendations including every trust must have a nominated specialist and raising standards of audits for high-risk food manufacturers.

Patient ate company’s sandwiches a dozen times
In July 2017, a case of listeriosis in a 53-year-old man in a hospital in Yorkshire and Humber Region was reported to Public Health England. Analysis by whole genome sequencing of the Listeria monocytogenes from his blood was genetically indistinguishable to isolates from sandwiches collected in December 2016 and produced by a company that had one site in the same region. Whilst in hospital, the patient was given sandwiches made by this company 12 times.

No other cases infected by this Listeria type were detected in the UK between 2016 and 2020.

Contamination of products from the company was detected during unrelated microbiological monitoring of food and predated the investigation of the sick person by 18 months, according to the study published in the journal Epidemiology and Infection.

The business was an approved manufacturer producing sandwiches and salads to a range of premises including the National Health Service throughout England. The company sent, on average, 12,600 salads and sandwiches per day to health care environments but stopped supplying the NHS in September 2019 for “commercial” reasons.

Efforts to investigate and control contamination
Between 2016 and 2020, more than 3,000 samples of food, ingredients and environmental swabs from the company were tested. Listeria monocytogenes contamination rates declined after July 2017 from 31 percent to 0.3 percent for salads and 3 percent to zero for sandwiches. The pathogen was found in salads with beef, pork and cheese as well as tuna, egg, mayonnaise and chicken sandwiches.

Results represent persistent contamination of equipment, food contact surfaces and foods at the manufacturer by a single Listeria monocytogenes strain, said researchers.

A total of 168 finished foods made by the company were collected from two hospitals in 2016 as part of routine microbiological monitoring. Listeria monocytogenes was isolated from eight samples and other Listeria species from 13 but all were under 20 colony forming units per gram (cfu/g).

In December 2016, the local authority inspected the company and collected five sandwiches. Listeria monocytogenes and Listeria seeligeri were detected at below 20 cfu/g in two egg mayonnaise sandwich samples.

Samples from the two hospitals between January and June 2017 found Listeria monocytogenes in 31 of 101 salads tested and none of 104 sandwiches, all at under 20 cfu/g except for a quiche lorraine salad which was at 20 cfu/g. Listeria monocytogenes contamination rates in the second half of 2017 were 26 of 230 salads and three of 238 sandwiches positive. Contamination rates for 2018 and 2019 were 11 of 444 and one of 277 for salads and five of 471 to none of 399 for sandwiches.

Listeria monocytogenes contamination rates in salads and sandwiches collected from the hospitals and the company’s factory declined from July 2017 after the incident and when control measures were implemented. Contamination with other species of Listeria increased between 2017 and 2018 in foods at the hospitals and for sandwiches from the firm but declined in 2019 and 2020.

Swabs of drains and a water sample from a vegetable washer showed contamination between July 2017 and July 2019 with the Listeria monocytogenes type associated with the infected person. A second Listeria monocytogenes strain was detected twice in August 2017 from a drain swab and the butter depositor but was not recovered from any foods or ingredients or from other cases of listeriosis in the UK.

Impact of testing decisions
Listeria seeligeri was recovered in environmental swabs at the production site 14 times between June 2018 and July 2019; from four salads in September and October 2017; and from 24 sandwiches between December 2016 and August 2019. It was also detected from 94 foods at the two hospitals. Listeria welshimeri was found in the production area between October and December 2019 and Listeria innocua was also detected.

Listeria monocytogenes had not been detected in any samples that the company sent to a commercial UKAS ISO 17025 accredited laboratory in the first half of 2017.

Environmental health officers visited the company in July and August 2017 and found procedures were adequate but some changes to layout to expand the production area had recently been implemented. There were concerns including sanitization systems for vegetable washing machines, trolley wheel disinfection before moving from low to high-risk areas and shoe changing procedures. There was evidence of floor-level drainage from a low to high-risk area with a build-up of debris.

Food and environmental samples were taken. While the company’s private lab didn’t detect Listeria monocytogenes the PHE Food, Water and Environmental lab in York did. It was not possible to investigate the reason for this discrepancy, although it may be because of test sensitivity, according to the study.