New strains may be fueling surge in norovirus outbreaks

Aug 24, 2007 (CIDRAP News) – Norovirus outbreaks in the United States appear to have increased sharply since the beginning of last year, probably fueled by two new strains of the gastrointestinal pathogen, the Centers for Disease Control and Prevention (CDC) reports.

The increase is especially notable in long-term care facilities (LTCFs), the CDC says in today's issue of Morbidity and Mortality Weekly Report. One recent outbreak in an LTCF led to the first recorded fatal case of norovirus, in a 90-year-old in North Carolina, the report says.

Noroviruses can spread via food or through contact with contaminated surfaces. There is no national surveillance system for acute gastroenteritis (AGE) outbreaks in which foodborne transmission is not suspected, but several lines of evidence suggest that norovirus outbreaks have been on the rise, the CDC says.

Outbreaks triple
For example, in reviewing evidence from 24 states that reported at least five AGE outbreaks in 2005 and 2006, the CDC found that outbreaks overall more than tripled, from 372 in 2005 to 1,316 in 2006. Fifty-eight percent (762) of the outbreaks were in LTCFs.

Norovirus was confirmed in only 29% (382) of the outbreaks, but testing and reporting vary by state for nonfoodborne AGE outbreaks. Findings in states that routinely test outbreak specimens indicate that a high proportion of the episodes were caused by norovirus, the report says.

Further, CDC tests of specimens from 126 AGE outbreaks in various settings in 2006 confirmed that 114 (90%) were due to norovirus, the agency says. Investigators found that 87 (76%) of these involved two new GII.4 norovirus variants, called Minerva and Laurens. A new strain has brought an increase in outbreaks at least once before, the report says.

Findings in three states
Health officials in many states began contacting the CDC in late 2006 about apparent increases in norovirus outbreaks, the report says. The agency reviewed data for three of those states—North Carolina, Wisconsin, and New York—and found striking increases.

North Carolina had 17 outbreaks consistent with norovirus infection in LTCFs in 2006, compared with six in 2005 and three in 2004. The outbreaks affected 573 residents and 288 staff members, and 36 patients were hospitalized. One 90-year-old resident died, and the gastrointestinal illness was listed as the primary cause of death.

"Norovirus infection as a confirmed cause of death has not been recorded previously in the United States," the CDC says.

In four of the North Carolina episodes, outbreaks occurred after illness among food handlers, suggesting that foodborne transmission might have been involved.

In Wisconsin, AGE outbreaks reported to the state increased more than fourfold in 2 years, from 23 in 2005 to 106 in 2006, the report states. Eighty-seven (82%) of the 2006 outbreaks were confirmed as norovirus by polymerase chain reaction (PCR).

Of 58 nonfoodborne norovirus outbreaks in Wisconsin in 2006, 45 occurred in LTCFs. These outbreaks included 2,071 cases, with 44 patients hospitalized. Two patients died, but the primary causes of death were not recorded.

New York had 333 AGE outbreaks from October 2006 through January 2007, compared with 76 for the same period in 2005-06, the CDC says. Eighty-two percent (272) of the 2006 outbreaks occurred in LTCFs and 8% (26) in hospitals. For healthcare facilities overall, 7,907 patients and 4,317 staff members became ill; 207 patients and 20 staff members were hospitalized. Sixteen patients died, but the causes of death were not reported.

Boston also saw signs of a recent increase in AGE outbreaks related to norovirus, according to the CDC. Between Dec 1, 2006, and April 1, 2007, the city had 18 outbreaks in colleges, day care centers, and healthcare facilities, compared with two outbreaks in the same period a year earlier. PCR testing confirmed norovirus in eight of the outbreaks.

More surveillance expected
For the AGE outbreaks in late 2006 and 2007 nationwide, the CDC says, "A high proportion of the specimens tested were positive for norovirus, which suggests that the increase in AGE outbreaks was associated with norovirus infection. The magnitude and consistency of increases in multiple states suggests an actual increase rather than increased reporting resulting from increased awareness and testing for norovirus."

The agency says it is unclear whether the surge in outbreaks is a result of increased pathogenicity or transmissibility of the two new strains, lower immunity in the population, or other factors.

The CDC says its data probably underestimate the number of recent norovirus outbreaks, since reporting methods and thoroughness vary by state.

But better national surveillance for AGE outbreaks is on the way, the agency says. Last year the Council for State and Territorial Epidemiologists passed a resolution stating that all AGE outbreaks should be reportable nationally, regardless of whether transmission is foodborne or person-to-person. "This will be implemented in 2008 through the National Outbreak Reporting System," the article says.

The CDC also says that CaliciNet, a CDC database used to collect and compare norovirus sequences, will soon be widely accessible to state and local health departments.

Noroviruses (previously known as Norwalk-like viruses) are the most common cause of AGE outbreaks, according to the CDC. Because patients shed the viruses for a long time after recovery and it takes only a few viral particles to cause illness, LTCFs are susceptible to prolonged outbreaks with high attack rates, the agency says.

CDC. Norovirus activity—United States, 2006-2007. MMWR 2007 Aug 24;56(33):842-6 [Full text]

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