• Kyodo News

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The record-breaking nationwide outbreak of gastroenteritis caused by norovirus may stem from a mutation of the disease, an expert said Wednesday.

The highly contagious virus has typically been transmitted by eating raw oysters and clams, but this season’s epidemic comes mostly from human-to-human infections and can best be explained by a possible outbreak of a new virus strain by, for example, mutation, said Shigeo Matsuno, a senior researcher at the National Institute of Infectious Diseases.

“The cause . . . is not eating raw oysters,” Matsuno said. “It must be human-to-human infection in most cases. It is natural to think that the infection spread so rapidly because many people have no immunity to the virus.”

Oysters and other bivalves often cause norovirus infection because they concentrate the virus after taking it in along with plankton in the water.

Apart from bivalve consumption, infection occurs orally when viruses in substances excreted or vomited are somehow transmitted, and in places where people are in close contiguity, such as nursing homes and schools.

Oral infection with only a small number of norovirus organisms — less than 100 — is known to cause gastroenteritis that brings stomach pain, severe diarrhea and vomiting.

Some 3,000 medical institutions across Japan reported they treated 65,638 infected patients between Nov. 27 and Dec. 3, a record-high average of 21.8 per institution, compared with 19.8 logged in the preceding week, also a record. Norovirus infection statistics have been compiled since 1981.

In previous instances, the number of cases reached a peak after mid-December.

With the rapid increase, the institute is concerned that the outbreak of norovirus gastroenteritis may show “the largest-ever spread” and is calling for people to wash their hands with soap as a way to prevent an epidemic.

On Monday, Prime Minister Shinzo Abe urged Health, Welfare and Labor Minister Hakuo Yanagisawa to initiate action against the epidemic.

A public junior high school in Kitami, Hokkaido, closed Tuesday following a suspected norovirus outbreak, with 58 of the school’s 416 students, and one teacher, suffering symptoms that include vomiting and abdominal pain since Saturday.

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