New strains of norovirus cause hospital outbreaks

Deaths investigated, national surveillance coming

The emergence of two new strains of norovirus has resulted in increased reports of hospital and long-term care outbreaks, some of which appear to involve the first fatal infections with the virus reported in the United States, the Centers for Disease Control and Prevention reports.1

Beginning in October 2006, emergency department visits for acute gastroenteritis (AGE) also have been increasing. New York has been particularly hard hit. From Oct. 1, 2006, through Jan. 31, 2007, a total of 333 AGE outbreaks were reported in New York — more than four times the number reported during the same period in 2005-2006 (76 outbreaks), the CDC found. Of these 333 outbreaks, 272 (82%) occurred in long-term care facilities and 26 (8%) in hospitals. Of 216 health care facility outbreaks (both hospitals and long-term care) with available data, a total of 7,907 patients and 4,317 staff members were affected. Of these, 207 (2.6%) patients and 20 (0.5%) staff members were hospitalized, and 16 deaths among patients with AGE were reported. In addition, two deaths in Wisconsin were associated with AGE outbreaks in health care facilities. Illness compatible with norovirus infection also was the primary cause of death recorded for a resident of a long-term care facility in North Carolina.

Norovirus infection as a confirmed cause of death has not been reported previously in the United States, and additional investigation into the deaths is ongoing. However, the reports are incomplete and the exact cause of all deaths may be difficult to trace to norovirus. Laboratory testing for norovirus is limited to the state public health laboratories, and norovirus testing is not routinely performed on all specimens from all AGE outbreaks. As a result, the outbreaks likely represent an underestimate and the CDC is setting up a new national surveillance system to get a better picture of the emerging pathogen. In addition to better surveillance, specific protocols are needed to investigate the role of norovirus in diarrheal deaths, particularly among older adults.

"We are looking at the fatal cases to gain more information," says Jacqueline Tate, PhD, a CDC epidemic intelligence officer who investigated the outbreaks. "There is no specific treatment for norovirus. The most important thing is to keep the people well hydrated to replace the fluids that they are losing. It's a matter of treating symptoms and then making sure you have good prevention and control set up in these outbreaks. We don't know what strains were associated with the outbreaks in which the deaths occurred, but this is something that has not been reported to us before. We will certainly be looking closer at these strains."

Dubbed Minerva and Laurens, the two new norovirus strains have previously caused outbreaks on cruise ships. It's not known if they originated onboard or spread from hospitals and nursing homes. "They have been seen on cruise ships, but we're not sure of the direction [of transmission]," she says. "We just started recognizing them in 2006. We are not sure if they came from cruise ships or from the community. They have also been seen percolating in other countries as well, particularly in Europe."

The Minerva strain was detected in 15 (60%) of 25 outbreaks during October-December 2006 on cruise ships and in eight states. During January-June 2007, the same strain caused 66 (54%) of 122 outbreaks on cruise ships and in 19 states. The Laurens strain was detected in 10 (40%) of the 25 outbreaks during October-December 2006 and 33 (27%) of the 122 outbreaks during January-June 2007.

A previous increase in norovirus outbreaks in the United States also was associated with the emergence of new strains. A high number of specimens tested were positive for norovirus, which suggests that the increase in AGE outbreaks was associated with norovirus infection, the CDC concluded. The magnitude and consistency of increases in multiple states suggest an actual increase rather than increased reporting resulting from increased awareness of and testing for norovirus.

"This is a pattern that we have seen in the past," Tate says. "When you have new strains of norovirus circulating, you have an increase in the number of outbreaks at the same time. We are not exactly sure what the connection is between the new strains and the increase in outbreaks. It could be that the new strains are more easily transmitted. It could be that they are more virulent or cause more disease, or it could be that there is a lower immunity in the population for these particular strains."

Regardless, it's clear that norovirus is no longer a problem confined primarily to the high seas. "It can be a source of hospital outbreaks," she says. "It is similar to the cruise ship setting in that you have a closed population or a population living at close quarters. This virus is easily spread from person to person and through contaminated surfaces. That's what we see on cruise ships and that's what we are seeing in long-term care facilities and hospitals."

Indeed, long-term care settings are particularly vulnerable to outbreaks due to the low infectious dose of norovirus (fewer than 10 viral particles), its persistence in the environment, and the close quarters and interaction typical among residents. Noting that many of the outbreaks were associated with breaches in infection control measures, the CDC reiterated recommendations to prevent norovirus transmission. (See recommendations.) Those include frequent hand disinfection, contact precautions with patients and use of bleach or another appropriate disinfectant. Many long-term care facilities used disinfectants that had limited effectiveness against norovirus (e.g., quaternary ammonia compounds) during the outbreaks.

"This is a very hardy virus," Tate says. "Disinfectants that typically work on other agents do not work for norovirus. People need to use an appropriate cleaner such as bleach or any of the disinfectants that are approved by the EPA as effective against norovirus."

Several outbreaks in a long-term care facility in North Carolina were preceded by symptomatic illness in food handlers, reminding that norovirus can be a foodborne pathogen as well as transmitted through direct contact and the environment. "We believe [food handlers] were the mode of transmission for those particular outbreaks," she says. "They should not work when they are symptomatic and 24-72 hours after the symptoms resolve." At least two outbreaks in North Carolina were preceded by illness among staff members who also worked at other long-term care facilities with reported norovirus outbreaks.

Transmission also goes the other way too, so it was not surprising to see health care workers infected, some severely enough to be hospitalized. Thirteen health care workers either visited the emergency department or were hospitalized for severe dehydration in a recently reported outbreak at Johns Hopkins Hospital in Baltimore. "Health care workers really do need to be on the lookout for norovirus infections, and if there is an outbreak, hospitals need to address it very aggressively," says hospital epidemiologist Trish Perl, MD, one of the investigators in the study.1 "Our experience shows that people can get very sick and that it costs a lot to fix the problem and address disruptions to staffing."

The 2004 outbreak involved 355 cases that affected 90 patients and 265 health care workers and that were clustered in a coronary care unit and psychiatry units. Attack rates were 5.3% (seven of 133) for patients and 29.9% (29 of 97) for health care workers in the coronary care unit (CCU) and 16.7% (39 of 233) for patients and 38.0% (76 of 200) for health care workers in the psychiatry units. Thirteen health care workers (4.9%) required emergency department visits or hospitalization. The investigation showed that many of the initial health care workers in the cardiac unit who became ill had attended a social event outside of the hospital, where one of the nonstaff guests was already experiencing symptoms, the authors noted. Others likely became ill after touching a patient chart that had been handled by another ill colleague.

Detected noroviruses had 98%-99% sequence identity with representatives of a new genogroup II.4 variant that emerged during 2002-2004 in the United States (e.g., Farmington Hills and other strains) and Europe. Aggressive infection control measures, including closure of units and thorough disinfection using sodium hypochlorite, were required to terminate the outbreak. No deaths were reported.

Total hospital costs for a three-month norovirus outbreak — including extra cleaning supplies, staff sick leave, diagnostic tests, replacement staff, and salaries and lost revenue from closed beds — were estimated at more than $650,000. Calculations of costs associated with the cleanup included expenses for cleaning supplies ($96,000), staff sick leave and overtime ($89,000), plus lost revenue from closing the units and echocardiogram laboratory to new patients ($418,000). Indeed, nearly 460 hours of sick leave were used by staff on the CCU, 138 hours in the echocardiogram lab, and more than 2,000 hours in psychiatry services, they found.


  1. Centers for Disease Control and Prevention. Norovirus Activity — United States, 2006-2007. MMWR 2007; 56(33):842-846.
  2. Johnson CP, Qiu, H, Ticehurst JR. Outbreak management and implications of a nosocomial norovirus outbreak. Clin Infect Dis 2007; 45:534-540.