(HealthDay)—A new norovirus strain caused most of the outbreaks of the contagious intestinal illness in the United States between September and December last year, but it is not known if this strain will lead to an overall increase in outbreaks, U.S. researchers report.
Norovirus is highly infectious and is the leading cause of what is commonly called "stomach flu," a gastrointestinal illness with symptoms such as diarrhea, abdominal pain, vomiting, headache, fever and chills.
For the study, U.S. Centers for Disease Control and Prevention researchers analyzed data collected through CaliciNet on norovirus strains linked to U.S. outbreaks and found that 141 of the 266 outbreaks reported during the last four months of 2012 were caused by the GII.4 Sydney strain.
This strain was first detected in Australia in March 2012 and caused outbreaks in that country and several other nations.
"The new strain spread rapidly across the United States from September to December 2012," Dr. Aron Hall, an epidemiologist in the CDC's viral diseases division, said in a CDC news release.
"The proportion of reported outbreaks caused by this strain increased dramatically from 19 percent in September to 58 percent in December," Hall added.
The study was published Jan. 24 in the CDC's Morbidity and Mortality Weekly Report.
Dr. Jan Vinje, director of CaliciNet, pointed out in the news release that "new norovirus strains often lead to more outbreaks, but not always."
New strains of GII.4 have emerged every two to three years over the past decade. "We found that the new GII.4 Sydney strain replaced the previously predominant GII.4 strain," Vinje said.
"Right now, it's too soon to tell whether the new strain of norovirus will lead to more outbreaks than in previous years," Hall said. "However, CDC continues to work with state partners to watch this closely and see if the strain is associated with more severe illness."
More than 21 million people in the United States contract norovirus infections each year and about 800 die. Young children and seniors are most likely to develop severe illness. Norovirus spreads primarily from infected people to others through direct contact, but also spreads through contaminated food, water and surfaces. The illness has often made the news when outbreaks have occurred on cruise ships.
The best ways to prevent norovirus infection include washing hands with soap and water, rinsing fruits and vegetables, disinfecting surfaces, cooking shellfish thoroughly, and not preparing food or caring for others while ill, the CDC suggests.
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More information: CDC report: www.cdc.gov/mmwr
The U.S. National Institute of Allergy and Infectious Diseases has more about norovirus infection.