HPR volume 18 issue 2: news (7 March 2024)

Question 1

Update on reporting of non-O157 STEC infections and an outbreak of Shiga toxin-producing E. coli (STEC) O145 in the UK, February 2024

Accepted Answer

In December 2023 the UK Health Security Agency (UKHSA) reported (1) unseasonably high levels of Shiga toxin-producing Escherichia coli other than serogroup O157 (non-O157 STEC) case notifications during October to December of that year. Since then, reporting has returned to levels comparable with previous years. Investigations into the drivers of that unseasonal increase in reports of non-O157 STEC continue, as well as into individual outbreaks of STEC that were reported in the December 2023 report.

The investigations into the concurrent foodborne STEC O145: H28 outbreak associated with unpasteurised cheese – being undertaken by UKHSA, the Local Authority (LA) Environmental Health Team, Public Health Scotland (PHS), Food Standards Agency (FSA), Food Standards Scotland (FSS) and the Animal Plant Health Agency (APHA) – are nearly completed.

In total 36 confirmed cases and 1 probable case have been reported across England (n=29) and Scotland (n=8) since late July 2023, with most cases falling ill in November 2023. The last reported primary case had a symptom onset date of 23 December 2023 (see figure below).

Based on epidemiological, food chain and microbiological investigations, the vehicle was identified as an unpasteurised cheese produced in North West England. The food business concerned carried out a product withdrawal and recall on 24 December 2023 (2) and updated on 27 December 2023 (3) which ensured all recalled products were removed from sale in a timely manner.

Subsequent work between the food business, the LA Environmental Health team and the FSA allowed refinement of the product withdrawal and recall notice to include only the batches likely associated with illness, which resulted in a further update of the product withdrawal and recall by the food business on 9 February 2024 (4).

The FSA and FSS both published updated Product Recall Information Notices (PRIN’s) on 24 (2) and 27 December (3), and again on 9 February (4), with supporting news stories to amplify messaging about each of the product recalls.

Epidemiological curve of cases by week of sample collection and country of residence*

*Date of sample collection unavailable for 1 case.

Twenty cases were female (54%), with ages of all cases ranging from 7 to 81 years (median age of 36 years). For those with information available (n=31), 65% (n=20) reported bloody diarrhoea, 58% (n=15) were admitted to hospital and a further 11% (n=4) also attended hospital for their symptoms. One case developed haemolytic uraemic syndrome (HUS) and subsequently died.

In the early part of the investigation, analysis of routinely collected case questionnaire data indicated many of these cases had travelled by train in the week before their symptoms started. Further interviews and analyses carried out by UKHSA and PHS identified that 16 out of the 17 cases contacted had consumed a specific menu item, which contained unpasteurised cheese, while on board trains operated by the same train company. In addition, 1 case who had not travelled by train reported consumption of the same brand of unpasteurised cheese which they purchased from a local shop. Analysis of information on other foods consumed by cases did not identify any other commonalities. Therefore, based on the epidemiological and food chain links identified during the investigation, the food business undertook a product withdrawal and recall as a precaution on 24 December 2023, and the FSA published a PRIN on the same day.

The infectious dose for STEC is low; therefore transmission can occur not only via consumption of contaminated food but from person-to-person contact and from contact with animals and/or their environment. This is the likely explanation for the fact that not all cases that were interviewed reported consuming the specific implicated cheese – at least 4 further cases were determined to have acquired infection through either person-to-person or environmental transmission. Overall, 5 cases were lost to follow up, so no exposure information was available, and the remaining 9 cases did not provide sufficient detail to determine whether they had consumed the implicated cheese or not.

In every outbreak a proportion of cases will not be contactable during the investigation and the inability of some cases to accurately recall specific foods or dishes eaten, and/or where the food had been purchased in the seven-day period prior to onset of illness, are known limitations in foodborne disease outbreak epidemiological investigations. Therefore, investigations also focus on gathering microbiological and food chain tracing evidence. Over the following weeks further investigations were conducted to identify any other common links, identify the source of the contamination of the product and ensure appropriate health protection and food safety measures were implemented, as described below.

Firstly, 43 samples of cheese were tested by UKHSA Food, Water and Environmental Microbiology laboratories and STEC DNA was detected in 2 samples, although only one could be cultured and therefore sequenced. The strain identified (STEC O109: H16) was not the outbreak strain and did not match any human disease cases. No STEC was detected in the bulk milk tank sample or filter samples used by the food business when producing cheese.

Secondly, APHA undertook an advisory and sampling visit to the farm which produced the implicated cheese. In total, 28 environmental samples including fresh and aged cattle faeces from various locations on the premises and 2 water trough sediment samples were collected and tested for STEC by APHA laboratories. Two of the cattle faecal samples tested positive for STEC O145. These isolates were sequenced at the UKHSA Gastrointestinal Bacterial Reference Unit and both isolates were found to be genetically indistinguishable from the outbreak strain detected in the human cases.

While the outbreak strain was not detected in the milk tank or cheese samples, this is not unexpected considering the limitations of microbiological sampling during outbreak investigations. A negative test does not prove that raw ingredients or a specific food is not contaminated. The very low level of bacterial load required to cause STEC infection (approximately 10-100 organisms), in combination with sporadic and uneven distribution of contamination, can make detection challenging. The detection of the outbreak strain in the faeces samples from the dairy herd (on the premises which supplied the milk used to make the unpasteurised cheese) provided additional evidence in support of the epidemiological and food chain investigation findings on which the food safety and health protection actions taken in December 2023 were based.

Since the investigation began, the food business has been working closely with the local authority, FSA and APHA as well as a trade association with expertise in cheese manufacturing to improve labelling and implement additional measures to control and monitor for contamination of the milk used to produce the relevant cheese products.

Consumption of unpasteurised milk carries an inherent risk of contracting STEC and other gastrointestinal pathogens, so the FSA and FSS advise that vulnerable groups – such as those who are very young, elderly, pregnant or immunocompromised – should not consume it. All products made with raw milk are legally required to be labelled with the words ‘Made with raw milk’ so as to ensure that consumers have the information they need to make an informed choice.