A new strain of norovirus, identified last March in Australia, has swept through the United States, causing more than half of the 2012 outbreaks.
GII.4 Sydney is implicated in 53% of the outbreaks that occurred from September to December, Leslie Barclay and her colleagues reported in the Jan. 24 issue of the Morbidity and Mortality Weekly Report (2013;62:55).
The other outbreaks were caused by 10 different GI and GII genotypes, including GII.4 New Orleans, said Ms. Barclay of the National Calicivirus Laboratory at the Centers for Disease Control and Prevention, and her coauthors. However, she noted, the GII.4 Sydney virus appears to have replaced the New Orleans strain as the predominant pathogen.
GII.4 Sydney was implicated in 19% of the September norovirus outbreaks; 46% of the October outbreaks; and 58% of outbreaks in both November and December, Ms. Barclay and her coinvestigators said.
The report noted that 51% of the outbreaks were direct, person-to-person transmission; 20% were foodborne; and 1% waterborne. For the other outbreaks, the mode of transmission remains unknown.
The sudden appearance of a new norovirus strain isn’t unusual or unexpected, Dr. Aron J. Hall said in an interview.
"This group of viruses is constantly evolving. New strains develop all the time," said Dr. Hall, an epidemiologist with CDC’s Division of Viral Diseases. "This is following the same pattern we’ve seen for the past 10 years, with a new strain evolving every 2-3 years."
Since the norovirus season doesn’t normally peak until January, it’s too soon to say whether GII.4 Sydney is more virulent, either in the number or severity of cases it causes.
"Every time a new strain emerges, there’s a potential for increased disease activity, numbers of outbreaks, and severity. But sometimes they simply replace the predominant strain with no increases. We have to wait until we’ve tallied all the numbers before we can get an assessment of this season’s magnitude and impact," he said.
GII.4 Sydney was first identified in Australia, and rapidly moved to Japan, the Netherlands, and the United Kingdom, according to a Jan. 4 report (Eurosurv. 2013;18:1)
In early December and late November, the ProMed disease reporting system reported a 64% increase in confirmed norovirus lab reports in England and Wales, and increased deaths in elderly Japanese patients. Molecular assays pinpointed the cause as the new variant of norovirus, the Eurosurveillance researchers noted.
The virus is highly transmissible; exposure to as few as 18 viral particles can cause illness, according to a presentation by Jan Vinjé, Ph.D., head of the National Calicivirus Laboratory. And since about 30% of cases are asymptomatic, the transmission rate can be extremely high.
"These are the two characteristics that explain why norovirus is such a serious public health challenge," Dr. Hall said. "It takes a very tiny amount of exposure to make someone sick, and someone who is sick is shedding billions of viral particles. They can shed virus without any symptoms, so they don’t know they have it and there’s no way to advise them about how to avoid spreading it."
The illness is characterized by vomiting and/or diarrhea with abdominal cramps and low-grade fever. Most patients recover within 12-72 hours, but about 10% need hospitalization and fluid therapy. Severe illness and/or death are most common among the elderly and people with chronic illnesses.
The United States sees about 21 million cases annually; 800 will result in death. A norovirus outbreak can overwhelm local medical resources; the virus causes nearly 2 million outpatient visits and 400,000 emergency department visits each year. The combined annual cost reaches $2 billion, according to the CDC.
As federal employees, Dr. Hall and Ms. Barclay have no financial disclosures.