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Masaharu Nomura and Shigeyoshi Itagaki

The Yomiuri Shimbun

Publication Date : 01-01-2013

The series of mass norovirus infections at restaurants, hospitals and elderly care facilities since autumn has likely been prolonged because of mutated strains first detected in early 2012, as mutations have been detected in more than half Japan's prefectures.

The Yokohama city government conducted an on-site investigation at Yokohama Denentoshi Hospital from Thursday to Sunday, where more than 100 people have been infected with acute infectious gastroenteritis. The epidemic has resulted in four deaths--patients aged 80 to 97 who were reportedly bedridden due to dementia and stroke sequelae.

The epidemic began shortly after three employees began complaining of symptoms resembling those seen in norovirus infections on Tuesday.

"To be honest, we didn't expect the infection to spread this far," Seiji Shibuya, director of the hospital, said at a press conference Saturday night.

There was a large number of cases of acute infectious gastroenteritis, mostly caused by norovirus, last year, following the record figures marked in 2006. Group infections thought to be caused by norovirus have occurred one after another nationwide, including one that caused six deaths at a hospital in Miyazaki Prefecture in December.

Norovirus is said to be a main cause of acute infectious gastroenteritis in winter. It causes severe vomiting and diarrhea for one or two days. Many infections occur in people who come into contact with contaminated vomit or stool, and then infect themselves orally by touching the mouth or face. There is no vaccine or medicine for norovirus.

The number of child patients has increased steadily since mid-October, peaking in the week of Dec. 3-9, according to a nationwide survey of 3,000 paediatric departments by the National Institute of Infectious Diseases. The medical facilities surveyed reported an average of 19.62 norovirus patients during the peak week. Although this dropped to 19.23 in the week of December 10-16, experts say the epidemic is ongoing.

A mutated virus is thought to be behind the epidemic. A mutated strain was first detected in Japan in January 2012 from stool samples from people affected by food poisoning in Hokkaido and Osaka Prefecture. Starting in October, a series of group infections have occurred, including one at a welfare facility in Niigata Prefecture. As of Friday, several mutations have been identified in Tokyo, Hokkaido and 26 other prefectures.

"The group infections possibly occurred because many people lack immunity to the mutated viruses," said Kazuhiko Katayama, head of the Department of Virology II at the infectious disease institute.

Asymptomatic patients

Resistance to the virus can differ from person to person, meaning some people show no signs of infection, or only develop a mild case of diarrhea. These people can spread the disease without knowing it.

Surveys in 2005-07 of 1,855 employees at 55 eateries where cases of norovirus food poisoning occurred showed that 133, or 7 per cent, of the people who had norovirus detected in their stool did not develop symptoms such as nausea.

Cases of acute infectious gastroenteritis normally begin to decline in mid-December, when schools' winter breaks start, but the data cited above is based on figures from paediatrics departments, and thus does not reflect society as a whole.

Cases of norovirus due to food poisoning usually peak in January, likely due to the surge in people eating out for New Year's events.

"Be aware that anyone suffering from diarrhea might be infected [with norovirus]," said Takafumi Tsunoda, director of the infectious disease and internal medicine department at Ebara Hospital in Tokyo.