According to the U.S. Centers for Disease Control and Prevention, most of the current national outbreaks are caused by new strains of norovirus. The emergence of the new strain, which is spreading across the country and on cruise ships, offers a possible explanation for the outbreak over the past month. steep wave of infections from Stomach problems.
Scientists call this winter's dominant norovirus strain GII.17[P17]. Nearly 7 in 10 patient samples in norovirus outbreaks have tested positive for the new strain so far this season, according to a network of public health laboratories called CaliciNet.
“It's too early to tell whether this strain is associated with more severe norovirus disease, but lower population immunity may be responsible for early surges in seasonal increases in this virus,” said Jan Vinjé, director of the CDC's CaliciNet system. .
Vinjé said all norovirus outbreaks in the United States since 2012 have been dominated by a different strain of the virus that scientists call GII.4. This season, only about 7% of norovirus outbreaks have been linked to the GII.4 strain.
Vinjé said the GII.17 strain is behind outbreaks “all over the country” this season. Highly contagious viruses, including cruise ship break out.
The CDC tracked at least six norovirus outbreaks on ships last month, the most reported for that month in more than a decade.
Norovirus is the most common cause foodborne illness In the United States, climbing across the country almost always begins in the winter.
Many cases clear up on their own without testing or treatment from a doctor, so it's impossible to know exactly how many Americans are affected. However, nearly all data tracking the virus shows norovirus at peak levels across the country.
Norovirus test positivity rates across the CDC's network of labs have reached their highest weekly levels in years. Norovirus infection rates are also now higher than previous seasonal peaks, according to testing company BioFire Diagnostics.
In wastewater, samples collected by WastewaterSCAN showed “high” levels of norovirus at nearly every site the company monitors across the country.
While versions of GII.17 have been circulating for decades, it was only in recent years that this strain caused waves of infections. Last year there was a “sudden rise” in GII.17 reports in parts of Europe. Japan and China reported the first wave of epidemics since 2014.
The CDC reported that GII.17 cases increased last season, but still represented a minority of all norovirus outbreaks in the United States at that time
Will GII.17 lead to more or more severe norovirus cases?
More research is needed to clarify whether GII.17 contributes to differences in the severity of norovirus infections. Some previous changes in the virus had occurred, but none caused new or significantly more severe symptoms.
“What we are seeing with noroviruses has happened before. Noroviruses are very diverse,” said Marion Koopmans, director of the Department of Viral Sciences at the Erasmus Medical Center.
Koopmans, who oversees a global consortium of norovirus researchers, warned that there are still gaps in data tracking norovirus cases around the world. More research is needed to prove whether this strain is indeed causing an increase in illness.
“There's not enough surveillance (in my opinion) to really prove that this is causing the increase in cases because we have a lot of norovirus infections every winter and the surveillance is pretty sporadic,” Koopmans said.
When the latest GII.4 strain, known as the Sydney variant, emerged in 2012, preliminary data suggested it was causing an increase in outbreaks. Hospital data later reported by the CDC showed that this did not actually lead to a significant increase or worsening of norovirus infections.
“Thus, the observed increase in outbreak activity during the 2012-13 season may represent random seasonal and state variability rather than a direct result of the occurrence of GII.4 Sydney,” CDC scientists concluded at the time.
Wisconsin health officials said earlier this month they were seeing a record number of norovirus outbreaks, but they told CBS News they did not believe that was causing a major difference in symptoms or duration of illness.
“It doesn't change any of the recommendations around prevention or transmission,” Elizabeth Goodsit said. “It just means we have a strong outbreak season, so there's an opportunity to really emphasize washing your hands, staying home when you're sick, and at least once you've recovered. The importance of 48 hours.
