A chipotle chili product was the most likely vehicle of infection in the largest recorded norovirus restaurant outbreak in the United Kingdom, according to researchers.

During October and November 2016, more than 1,000 customers and staff reported gastroenteritis after eating at 23 different locations of the Wahaca restaurant group in the U.K. Norovirus genogroup II.6 (GII.6) was identified from 30 of 48 samples from staff.

The outbreak coincided with a new menu launch, according to the study published in the journal Eurosurveillance.

“We recommend that multi-branch restaurants with central suppliers and kitchens are vigilant to the possibility of contaminated ingredients entering their supply chain and the potential for rapid spread of pathogens. Food business operators should ensure that appropriate hazard analysis and critical control point processes are in place, particularly for new ingredients and ready-to-eat foods and consider the potential for cross-contamination within preparation areas in risk assessments,” the researchers wrote.

Product from outside the EU
Involvement of all 23 branches suggested the vehicle of transmission was a nationally distributed item that had either been newly introduced or modified for use in the new menu. The most likely vehicle was found to be a new chipotle chili product imported from outside the European unio that was used uncooked in implicated dishes.

Public health implications could have been more serious had the outbreak been caused by a more virulent pathogen than norovirus, which is generally a mild and self-limiting illness, according to the researchers.

The likely source was a contaminated batch of a nationally distributed ingredient, but analytical studies were unable to implicate a single ingredient.

The investigation highlighted challenges in identifying the vehicle of transmission from a large, complex menu with multiple ingredients used in numerous dishes with possible undocumented variation between restaurant locations.

Environmental health investigations identified one chipotle product which had been labelled as another chipotle product in some locations.

“It had been newly imported from outside the European unio for the new menu, was not tinned and was not cooked during initial processing. It is plausible that chipotle product B may have been mistakenly used in the chipotle mayo in place of chipotle product A in some branches and this may explain the variation in attack rate,” according to researchers.

Illnesses after new menu launch
On Oct. 27, 2016, Public Health England (PHE) received reports of diarrhea and vomiting affecting 10 staff at a London location of the restaurant chain. In the following days, customers from all branches and staff members from 22 sites reported gastrointestinal symptoms to the company’s head office.

The day before, a new menu was introduced at all branches with more than 70 dishes, 12 of which had never been served before.

In total, 287 staff members and 825 customers reported gastrointestinal illness to the company. The first report of illness on Oct. 19 was in a staff member and the last recorded illness onset date was Nov. 11. Customers first reported illnesses following consumption of food at one branch on Oct. 15 and the last consumer to report an illness dined on Nov. 10.

Four retrospective cohort studies were conducted, one among staff of the restaurant group and three in customers at four locations.

Staff and customer cases
Fifty-eight percent (589 out of 1,029) of staff completed the questionnaire, of whom 125 were categorized as cases. The age range was 16 to 55 years, with 57 percent  of the patients being female.

Of 159 customer responses in the combined customer cohort study, 37 were male with ages ranging from 7 to 65 years. There were 58 confirmed customer cases.

The chipotle chili was the only ingredient in common between two dishes implicated in the multivariable analysis of the staff cohort study and one dish implicated by the combined customer study. Recipes for salmon and chicken tostadas included chipotle mayo. The recipe for chicken wing glaze included a chipotle chili paste.

All dishes associated with gastrointestinal illness in the multivariable analysis were prepared in the salad section, meaning cross-contamination could have played a role in transmission, researchers reported.

Customer cohort studies found a strong association between gastrointestinal illness and consumption of dishes containing poached, ready-to-eat, vacuum-packed chicken from a new supplier, introduced for the menu change.

Food chain investigations found no evidence to implicate chicken as a vehicle for norovirus transmission and it was supplied to other U.K. restaurants, but there were no reports of similar outbreaks or of gastrointestinal illness from staff responsible for processing it.

Wahaca voluntarily closed 16 of its locations in the U.K. and discarded fresh and partially used produce. Those restaurants carried out deep cleaning, including environmental fogging. Staff were advised to stay off work for 72 hours following their last gastrointestinal symptoms and were offered paid sick leave to encourage policy adherence. Potential vehicles of norovirus transmission identified by epidemiological studies were removed from the menu.

Researchers were unable to test the potential vehicles identified for norovirus, because accredited methods are only available for limited food items, not implicated in the outbreak.

The team said development of sensitive laboratory methods for testing food specimens for viruses during outbreaks would be valuable in future investigations.