(CNN) -- Investigators probing the illness that forced famed chef Heston Blumenthal to temporarily close his award-winning Fat Duck restaurant last month said Friday that 14 staff members and diners were infected with norovirus, a highly contagious stomach bug.
Heston Blumenthal outside the entrance to his Fat Duck restaurant in Berkshire, England.
While the investigation is not yet complete, it is clear that staff at the Michelin-starred eatery worked while infected with norovirus, which goes against official guidance, the Health Protection Agency said.
The HPA said that to date, 529 people have reported being ill after eating at renowned Fat Duck in Berkshire, England.
Blumenthal's voluntarily closed his restaurant in late February after scores of diners reported getting sick. He reopened it last week.
The chef told Australia's Hospitality magazine Thursday that norovirus was the only potential cause of the outbreak that had been found so far after exhaustive tests of the restaurant, staff and customers.
"It is categorically not food poisoning. We know that," Blumenthal told the magazine. "For the last five years we've been sending food off every month for sampling, and I don't know any other restaurant in the country that does that. We also have a company that has been looking after all our health and safety stuff for the last five years."
Noroviruses cause "stomach flu," or gastroenteritis, according to the Atlanta, Georgia-based Centers for Disease Control and Prevention.
Symptoms of norovirus illness usually include nausea, vomiting, diarrhea and stomach cramping -- all of which were reported among diners who fell ill. In most people, symptoms last for a couple of days, according to the CDC.
Health authorities say they detected norovirus infection in six staff and eight diners, along with weaknesses in how the restaurant dealt with staff who were sick.
"Based on staff interviews, sickness records and samples taken, it is clear that staff worked while still infectious with norovirus," the agency said. "HPA guidance states that people should not work in food preparation while they have symptoms (or) are infectious as a result of diarrhea and vomiting to avoid the risk of passing the infection to others."
Health authorities gave the restaurant guidance on identifying and managing staff illness before The Fat Duck reopened, the HPA said.
The HPA's investigation will still take several weeks to finish, said a spokeswoman for the Thames Valley Health Protection Unit, who declined to be named, in line with policy.
People can become infected with norovirus in several ways, including eating contaminated food, touching contaminated surfaces and then putting their hands to their mouths, or having direct contact with an infected person, the CDC says. The virus can spread easily from person to person.
Blumenthal told the magazine that support for the restaurant throughout the ordeal has been "incredible."
"It's affected the restaurant big time because (we) had to cancel 800 people because of the closure. But in terms of the business and people wanting to come in, then no," he said.
Blumenthal spoke to the publication as he was in Melbourne, Australia, for a food and wine festival.
Located in the picturesque village of Bray, The Fat Duck is renowned for such eccentric items as snail porridge, salmon poached in licorice gel, and scrambled egg and bacon ice cream.
Diners must book months in advance to get a table. The restaurant charges 130 pounds ($185) for the tasting menu and 98 pounds ($140) for a la carte.
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