Sept. 10 (Bloomberg) -- The food-poisoning scare at the Fat Duck that caused 529 diners to fall sick earlier this year may have been exacerbated by the slowness of the restaurant to report the problem, U.K. health investigators said today.

The illness was caused by norovirus probably carried in shellfish, an inquiry by the Health Protection Agency and environmental health officers found. The outbreak lasted for at least six weeks between Jan. 6 and Feb. 22 before chef Heston Blumenthal, 43, closed his eatery, the agency said in an e- mailed release.

“Several weaknesses in procedures at the restaurant may have contributed to ongoing transmission including: delayed response to the incident; staff working when they should have been off sick and using the wrong environmental cleaning products,” the agency said. “Delays in notification of illness may have affected the ability of the investigation to identify the exact reason for the norovirus contamination.”

The restaurant in the village of Bray, west of London, reopened on March 12 after working with the agency and officials from the Royal Borough of Windsor and Maidenhead to uncover the cause of the sickness. Blumenthal said in an interview in February that a virus had been doing the rounds in the area and that he initially doubted the problem was specific to the Fat Duck because he was so fastidious about hygiene.

Shellfish Victims

“The organism responsible was norovirus which was probably introduced via shellfish (more diners who ate shellfish dishes reported illness),” the health agency said in the statement.

“Oysters were served raw; razor clams may not have been appropriately handled or cooked; tracing of shellfish to source showed evidence of contamination and there have been reports of illness in other establishments associated with oysters from the same source,” it said.

The Fat Duck, which has held three Michelin stars since 2004, was named the top eatery in the World’s 50 Best Restaurant Awards in 2005. Blumenthal is known for experimental dishes such as Jelly of Quail, Cream of Crayfish; and Sound of the Sea, where diners listen to the sound of lapping waves through headphones while consuming seafood.

“We are glad that the report has finally been published and draws a conclusion to the closure of the Fat Duck and, more importantly, that the norovirus has been identified as the cause,” Blumenthal spokeswoman Monica Brown said today in an e- mailed release.

Client Confidence

“We were affected by this virus during a national outbreak of what is an extremely common and highly contagious virus,” Blumenthal said in the release. “I would like to reassure our guests that they can continue to visit us with total confidence.”

Blumenthal was in France for the Omnivore Food Festival in Deauville when he took the decision to close on Feb. 24.

“It’s really upsetting,” he said at the time. “You know how much time, energy, effort and emotion goes into doing what we do, and I don’t want anyone to leave the Fat Duck with even a headache because food is my life.”

Dr. Graham Bickler, regional director of the health agency said the risks of raw shellfish were well known.

“Individuals infected with norovirus can readily transfer the virus onto foods they prepare,” he said in the release. “The virus will remain viable and capable of causing illness in those foods that are not subsequently cooked. The more intensively that food is handled, the more likely it is to become contaminated by infected food handlers.”

The 13-course tasting menu at the Fat Duck costs 130 pounds ($216). Matching wines are available at 90 pounds, 185 pounds and 285 pounds per person. It’s normal to wait two months for a table and even then these are difficult to secure.

(Richard Vines is chief food critic for Bloomberg News. Opinions expressed are his own.)

To contact the writer on the story: Richard Vines in London at rvines@bloomberg.net.

Last Updated: September 10, 2009 09:15 EDT