CDC: New norovirus strain fueling US outbreaks

Jan 24, 2013 (CIDRAP News) – A new norovirus variant that emerged in Australia last year has been the leading cause of norovirus outbreaks in United States over the past few months, and federal health officials today warned that illnesses could increase in the months ahead.

In an update today on norovirus activity from September through December, the US Centers for Disease Control and Prevention (CDC) said the new variant has spread rapidly nationwide, causing an increasing number of cases and accounting for 53% of all norovirus outbreaks. The report appears in the latest issue of Morbidity and Mortality Weekly Report (MMWR).

Norovirus, sometimes called the winter vomiting disease, is highly contagious and typically peaks in January in the United States. It is the leading cause of gastroenteritis outbreaks, including foodborne ones. Some health officials have said the norovirus activity this winter, on top of a brisk flu season, is partly to blame for reports of heavy patient loads at hospital emergency departments in some parts of the country.

The CDC estimates that norovirus is responsible for 21 million infections and about 800 deaths in the United States each year, with young children and older adults most vulnerable to severe illness.

The new genotype II.4 (GII.4) variant, named Sydney, was identified in Australia in March 2012 and recently made news headlines in United Kingdom, where the norovirus season started early with the new variant as the dominant strain. Other countries that reported similar increases in activity due to the new variant were the Netherlands, Japan, Australia, France, and New Zealand, according to a Jan 3 Eurosurveillance report. Though the same strain was implicated in Belgian and Danish outbreaks, those two countries did not experience the same increase in cases.

The UK Health Protection Agency (HPA) said in an update today that the number of lab-confirmed norovirus cases in England and Wales is dropping, but is still 49% higher than at the same point last year. At the end of December, it reported that norovirus infections were 83% ahead of the same time in 2011.

Aron Hall, DVM, MSPH, an epidemiologist with the CDC's Division of Viral Diseases, said in an e-mailed press release today that the new strain spread rapidly across the United States from September to December. "The proportion of reported outbreaks caused by this strain increased dramatically from 19 percent in September to 58 percent in December," he said.

The CDC reported that just over half of the infections resulted from person-to-person transmission, with 20% involving foodborne spread and 1% traced to water. The transmission mode wasn't known in 28% of the outbreaks. The two most frequent settings were long-term care facilities and restaurants.

According to data from CaliciNet, the electronic lab surveillance network the CDC uses to collect information on norovirus activity, the GII.4 Sydney strain appears to have replaced the previously dominant GII.4 New Orleans strain in the United States.

Jan Vinje, PhD, director of CaliciNet, said in the press release that emergence of a new strain often, but not always, leads to more outbreaks. For example, when the GII.4 New Orleans strain emerged it did not spark an increase in norovirus outbreaks. According to today's CDC report, over the past decade new GII.4 strains have emerged every 2 to 3 years.

Hall said it's too soon to tell if the Sydney strain will lead to more outbreaks compared to past years, but he said the CDC and its state partners are watching the patterns closely to see if the strain causes more severe illness.

In the MMWR report the CDC urged healthcare providers to remain vigilant for possible increases in norovirus activity in the coming months because of the emergence of the new strain.

See also:

Jan 24 MMWR "Notes from the Field" report

Jan 4 CIDRAP News item "Report: Emerging norovirus variant may portend severe season"

Jan 24 HPA seasonal infection weekly update

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