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New strain of norovirus is circulating

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A new strain of norovirus, the cause of a dreaded wintertime intestinal illness, is circulating in the United States, federal health officials said Thursday.

The strain, designated “GII.4 Sydney,” appeared in Australia last March. It is also spreading in Britain, where it caused an early start of the norovirus season. In the United States, it has caused half of the 266 norovirus outbreaks reported since September to a surveillance network run by the Centers for Disease Control and Prevention.

“We don’t yet know whether there is an increased number of cases this year,” said Jan Vinje, the CDC epidemiologist who runs the network. “Normally the season peaks in January, so it’s a little bit too early to tell.”

Norovirus causes vomiting and diarrhea. It is responsible for 21 million cases of illness a year in the United States, and about 800 deaths, mostly among the elderly. There’s no specific treatment for it and no vaccine (although one is under development). Hand-washing, wiping down contaminated surfaces and isolating people who are ill are the typical strategies for combating norovirus.

Outbreaks are notoriously associated with nursing homes and cruise ships, but norovirus also spreads easily through the general population. In the outbreaks since September, 51 percent involved person-to-person transmission; 20 percent were foodborne; and 1 percent were caused by contaminated water. The mode of transmission was unknown in the rest.

About two-thirds of this season’s outbreaks occurred in long-term care institutions and 13 percent involved restaurants, according to the CDC’s Morbidity and Mortality Weekly Report published Thursday.

As a norovirus strain spreads, people become at least partly immune to it. The “herd immunity” that develops favors the emergence of new, slightly different strains against which people have less protection.

“It’s a little like flu,” Vinje said. “The virus is mutating to a ‘fitter’ strain and is attacking a non-immune population.”

There are five main types of norovirus (denoted GI through GV), with most outbreak viruses falling into one subgroup, GII.4. Historically, GII.4 strains are more severe than others. Whether that will be true this year is not yet known.

Virginia has identified six outbreaks caused by GII.4 Sydney this season — four in 2012 and two this month, said Michelle Stoll, a spokeswoman for the state health department. The number of cases is similar to that of outbreaks caused by other strains.

Information about outbreaks in Maryland and the District caused by the new strain was not immediately available.

Lena H. Sun contributed to this report.

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