Skip to main content

All Access Subscription

Get unlimited access to our full publication and article library.

Get Access Now

Interested in Group Sales? Learn more

A new, highly transmissible strain of norovirus first associated with outbreaks on cruise ships is now wreaking havoc in long-term care facilities, the Centers for Disease Control and Prevention (CDC) reports.

Norovirus strain strikes long-term care, cruises

Norovirus strain strikes long-term care, cruises

A novel, highly transmissible strain emerges

A new, highly transmissible strain of norovirus first associated with outbreaks on cruise ships is now wreaking havoc in long-term care facilities, the Centers for Disease Control and Prevention (CDC) reports.

Since July 2002, 41% of outbreaks in which strains were characterized genetically at the CDC have been associated with a single, newly identified strain of norovirus (formerly known as Norwalk-like virus). The strain has been found in various institutional settings over a wide geographic distribution, and no common source of the outbreaks has been identified. The finding of a predominant strain is unusual and contrasts with surveillance data from 1997 to 2000, which rarely detected identical strains from distinct outbreaks, the CDC reports.

The emerging pathogen has been dubbed the "Farmington Hills" strain, after the city in Michi-gan where the first cases were identified. In CDC laboratory surveillance data, six of 11 national outbreaks associated with the norovirus occurred on land in five states (Alaska, Georgia, Kentucky, North Carolina, and Utah). The remaining five outbreaks occurred on cruise ships. No epidemiologic link has been identified between land and cruise ship outbreaks, but the pathogen appears to be more transmissible than typical viruses of this variety. Though norovirus is a common cause of food-borne illness, the reports suggest that the high activity recently noted is occurring in closed settings, where person-to-person spread is the most likely mode of transmission.

"Speculation that this strain might be perhaps more transmissible person to person is reflected by a number of points," says Steve Monroe, PhD, chief of the CDC Viral Gastroenteritis Laboratory. "One, the current predominance of outbreaks in institutional settings and on cruise ships where we assume that person-to-person spread is the most predominant form. The second is that in studies we’ve done and in studies that have been done in the UK, looking at different strains, the genetic cluster that this strain falls into has been statistically associated with outbreaks in nursing home settings. [That] again suggests that there’s something about strains in this cluster that are more likely to cause them to be transmitted person to person."

Two nursing home residents die

Separate data from individual states confirm continuing norovirus outbreaks. In New Hamp-shire, for example, 29 long-term care facilities reported outbreaks of norovirus in 2002. Two long-term care residents died, and 11 other people were hospitalized. Control measures in the long-term care facilities included frequent and thorough hand washing, rapid cleaning of soiled areas, excluding ill staff from work for 48 hours after resolution of symptoms, halting group activities, and stopping new admissions into the facilities. New Hampshire officials also detected an increase in emergency department visits for gastrointestinal illness during December 2002. Since Jan. 1, 2003, an additional 11 norovirus outbreaks have been reported in institutional settings in New Hampshire, the CDC says.

In New York City during the period from Nov. 6, 2002, to Jan. 13, 2003, 66 outbreaks of acute gastroenteritis epidemiologically consistent with norovirus infection were reported. Fifty-one of those outbreaks were in long-term care facilities, nursing homes, or rehabilitation facilities, totaling 1700 people overall.

"It is possible that characteristics of this predominant strain may make it particularly transmissible in certain settings such as nursing homes," says Marc-Alain Widdowson, DVM, a CDC epidemiologist investigating the cases.

"Control of outbreaks of norovirus in closed settings can be difficult due to the low infectious dose of the agent, multiple modes of transmission, and environmental persistence," he explains.

There is a very reliable and complete reporting system for food-borne outbreaks of this type of virus, but only sentinel surveillance for person- to-person transmission. Efforts are under way to develop a surveillance system for nonfood-borne outbreaks of gastroenteritis, which would gather data on epidemiological and laboratory aspects of outbreaks. In the absence of a true surveillance system, there are little historical data to assess the increase in nonfood-borne norovirus, he concedes.

"Whenever assessing this type of data, there are a number of pitfalls," he says. "For instance, clearly there’s been a lot of increased interest in Norwalk viruses because of the cruise ships. So this increased activity could be attributed to increased reporting because of the increased awareness. However, when we speak to experienced local health officials, it’s clear that what they’re seeing is beyond that. What they’re seeing is a very noted increase in activity in their experience. [But] there is no formal way of comparing with previous years. We can’t say how much of an increase this is."

One peculiarity about infection with norovirus in general — from limited studies that have been done in adult human volunteers — is that there does not appear to be long-term protection following infection with this virus, Monroe adds.

"If they were re-challenged, that is experimentally infected again two years later, there was no protection," he says. "The same people who were ill the first time became ill the second time. So while there may be short-term protection over a matter of say, six months, there does not appear to be long-term protection."

That raises the possibility that the norovirus problem may pose a recurrent, persistent threat to both long-term care residents and cruise voyagers.

[Editor’s note: The CDC is encouraging local and state health departments to test for noroviruses when investigating outbreaks of suspected viral acute gastroenteritis. For assistance in testing for noroviruses and for strain characterization, local and state health departments should contact the CDC’s Viral Gastro-enteritis Section. Telephone: (404) 639-3577. E-mail: [email protected].]