Federal scientists have discovered that a new strain of norovirus is responsible for the wave of intense gastrointestinal infections that have overwhelmed hospitals, nursing homes, and college dormitories across New England and the nation this winter.
The finding provides an important clue to the severity and breadth of this season's outbreak, which has alarmed disease specialists because so many adults and children have become so sick. Knowing that few people had previously been exposed to the strain, researchers assume virtually everyone is vulnerable to the germ, which can prove especially dangerous to the frail and elderly.
The US Centers for Disease Control and Prevention expects to officially report its findings on the new form of the virus in coming weeks .
Genetic fingerprinting shows that the virus infecting patients is distinct and aggressive: When the CDC tested stool specimens from October through December , 60 percent of those patients were positive for the new form of norovirus.
"When we get these emergent strains of a virus, it's very hard to know where or why they sprang up -- they seem to come up like daffodils in spring," said Dr. Marc-Alain Widdowson , a top norovirus researcher at the CDC in Atlanta.
Disease trackers first detected evidence in January 2006 that a novel type of norovirus might be circulating, when passengers aboard the Minerva II cruise ship fell ill with a particularly virulent gastrointestinal bug. By last fall, researchers' sophisticated genetic testing on stool samples had begun to better distinguish the new strain from previously identified versions.
The federal researchers have christened the strain GII.4 Minerva . Identifying the new strain could prove useful as researchers investigate drugs to treat the viral infection and, potentially, vaccines to prevent contagion , scientists said .
"There isn't any doubt that the public health-communicable disease community has been abuzz about norovirus over the past six months," said Dr. William Schaffner , an infectious disease specialist at Vanderbilt University . "In some jurisdictions, there have been so many outbreaks they can barely tote them up, let alone investigate them. And it's a much more substantial infection this year, so knowing its identity is important for several reasons."
The Northeast and California have been particularly hard-hit, Widdowson said.
Norovirus outbreaks are nothing new aboard cruise ships or in schools. In fact, the virus acquired its name from Norwalk, Ohio, where a virulent virus infected an elementary school in 1968.
But outbreaks in US hospitals and nursing homes had been uncommon -- until this year. In Massachusetts , patients and staff have fallen ill at the VA Boston Healthcare System, McLean Hospital , and, most recently, Hebrew Rehabilitation Center in Roslindale, where 225 residents and 119 staff members became sick over the last two weeks.
The size of the outbreaks and their severity suggested that an especially nasty bug had arrived. In the past, norovirus infections routinely passed after a day and fever was rare. This year, specialists said, some patients were sick for three or four days and developed fevers.
"We have received more reports involving fever and longer duration of symptoms," said Jan Vinjé , a CDC virus specialist and one of the discoverers of the new strain. "I have not had norovirus, but I cannot imagine how you can survive four days of vomiting."
The discovery of the new strain, though, only partially answers the question of why norovirus has proved so threatening this winter.
"When you get a new strain sweeping through, there are questions about why is this such a successful strain," said Emory University infectious disease specialist Christine Moe , whose research group spends much of its time studying norovirus. "And it could be due to a number of reasons."
There could be something about the emergent strain, for example, that makes it hardier, allowing it to survive longer on elevator buttons, telephone receivers, or salad bar counters. The strain may also be more resistant to cleaning agents.
It may also survive longer in its victims, meaning that even after patients recover , the virus would be present in their stools and they could infect others through poor personal hygiene.
Specialists also hypothesize that the new strain may have the capacity to attach itself at more points to a human cell, making it more dangerous to more people.
Determining the reasons for the virulence of the current strain is pivotal, but not easy. Although scientists identified the first strain of norovirus more than three decades ago , they have not succeeded in routinely growing it in laboratories .
That makes the virus more difficult to study and, by extension, makes development of antiviral drugs and vaccines more daunting.
"If we had a system that could grow the virus, we would be able to ask a lot of other questions that we might be able to get answers to," said Dr. Robert Atmar , a virus specialist at Baylor College of Medicine in Houston .
Still, Vinjé said, federal agencies are talking to big pharmaceutical companies as well as smaller boutique firms about exploring pills and shots against norovirus.
Dr. Anita Barry , Boston's director of communicable disease control, said that it is important to recognize that a new strain of norovirus is making the rounds.
"We know with norovirus that when you get one strain, it doesn't necessarily offer protection against another strain," Barry said. "You can get this illness repeatedly, and that's why prevention is really the way that people need to go to prevent this.
"So, please, make sure you wash your hands," she said.
Stephen Smith can be reached at email@example.com.