According to the CDC, a new strain of norovirus — a stomach bug that causes diarrhea and vomiting  —  was responsible for most outbreaks of the disease in the U.S. in recent months.

Last fall, 53 percent of norovirus outbreaks (141 out of 226) were caused by the new strain, dubbed GII.4 Sydney. Between September and December, the proportion of norovirus outbreaks caused by the new strain increased from 19 percent to 58 percent, the CDC said.

Norovirus outbreaks are common this time of year, typically striking between November and April. The peak usually occurs in January, so it’s too soon to tell whether or not the new strain will cause more outbreaks this season than during previous seasons, the CDC said.
New strains of norovirus typically emerge every two to three years. The current strain was first detected in Australia last March.

“New norovirus strains often lead to more outbreaks but not always,” said Dr. Jan Vinjé, director of CaliciNet, the CDC’s surveillance system for norovirus.

Norovirus is very contagious. Each year, about 21 million people in the U.S. fall ill from the bug, and about 800 die. Young children and the elderly are at highest risk for severe illness, the CDC said.
Fifty-one percent of outbreaks caused by the new strain were spread person to person, 20 percent were due to foodborne illness, 1 percent due to waterborne illness, and 28 percent had an unknown mode of transmission, the CDC said.

The best ways to prevent norovirus infections are hand washing with soap and water; disinfecting surfaces; rinsing fruits and vegetables; cooking shellfish thoroughly; and not preparing food or caring for others while ill, the CDC said.

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