THURSDAY, Jan. 23, 2003 (HealthDayNews) -- Cases of a stomach virus are increasing on both land and sea in the United States, and researchers say a new, easily transmitted strain of the bug appears to be predominating.
Noroviruses, also known as Norwalk-like viruses, are the most common cause of gastroenteritis in the United States, responsible for about 23 million cases of acute gastroenteritis (AGE) each year. The symptoms are familiar to most people -- nausea, diarrhea and vomiting. The virus is transmitted both via food and, as recent outbreaks indicate, via person-to-person contact and other non-foodborne modes.
Noroviruses gained infamy in recent months with their unwelcome appearance on several cruise lines, a phenomenon that seemed to peak between June and December of 2002. Some 60 percent to 80 percent of outbreaks, however, actually occur on terra firma, says Marc-Alain Widdowson, a Norwalk virus expert and guest researcher with the CDC's National Center for Infectious Diseases in Atlanta.
Since October of last year, the number of cases on dry land have also been on the rise, although no one knows by how much. "There's clearly increased activity, but no national comparison can be made with previous years to assess the degree of the increase," Widdowson says.
Improvements in reporting mechanisms and states' technical capacity for testing stool samples might account for part -- but not all -- of the rise. "When we speak to experienced local health officials, they're seeing a noted increase in activity in their experience," Widdowson adds.
Most of the recent spate of outbreaks are occurring in institutional settings such as long-term care facilities, hospitals, clinics and schools. This points to the likelihood that the germ is being spread by human contact.
In Washington state, for instance, local health officials recorded 10 outbreaks of AGE affecting 354 patients. Ninety-two percent of those cases occurred in four long-term care facilities. Six of seven stool samples from those four facilities tested positive for norovirus.
In New Hampshire, 35 AGE outbreaks, affecting 2,312 individuals, were reported. Lab tests confirmed 29 percent to be norovirus, while 71 percent were attributed to norovirus based on epidemiologic criteria.
New York City also had its share of gastrointestinal illness -- 66 outbreaks affecting 1,700 persons. Two-thirds of stool samples came back positive for the virus.
Since July of last year, almost half (41 percent) of the outbreaks that were genetically analyzed by the CDC were associated with one, newly identified strain of norovirus. So far, no common origin has been identified.
"We have identified what we're provisionally calling a new strain," says Steve Monroe, supervising research biologist at the CDC's National Center for Infectious Diseases. "Molecular fingerprinting shows that this is a unique strain, but it's not radically different from strains that we've seen in previous years."
The strain is being called Farmington Hills strain, after Farmington Hills, Mich., where the first cases were identified.
Farmington Hills strain may passes more easily from person to person than other strains, Monroe says. Not only is the strain predominantly in closed settings, where person-to-person transmission is more likely, but also previous studies have found that the genetic cluster this strain belongs to is statistically associated with nursing-home settings.
No one knows why the virus is stepping up its activity just now, though Monroe speculates that it's just in the cycle of things.
"We have seen a similar pattern in the data in the United States and Europe and elsewhere in 1995 and 1996, so I suspect what we're seeing is a natural cyclical variation in the level of activity," he says. In those two years, one strain accounted for 55 percent of all outbreaks investigated by the CDC.
There's no specific treatment for norovirus, other than rest and plenty of fluids. And getting sick once doesn't mean you'll be protected from future norovirus infections, Monroe warns.
To prevent outbreaks, the CDC recommends following basic food and water hygiene protocols, disinfecting surfaces that you suspect are contaminated, as well as lots and lots of hand-washing.
The CDC and state health departments have their own work cut out for them, encouraging the collection and testing of samples in order to establish a more accurate reporting system.
More information
For more on noroviruses, visit the CDC or the Virginia Department of Health.
