Abstracts & Commentary
Synopsis: Recent outbreaks of acute gastroenteritis (AGE) on land and at sea have occurred and are due to a predominant strain of norovirus, provisionally called Farmington Hills strain. This strain is similar to a common type strain that circulated worldwide in 1995-1997, and it may have characteristics that increase person-to-person transmissibility.
Sources: Centers for Disease Control and Prevention. Outbreaks of gastroenteritis associated with noroviruses on cruise ships—United States, 2002. MMWR Morb Mortal Wkly Rep. 2002;51:1112-1115; Centers for Disease Control and Prevention. Norovirus activity—United States, 2002. MMWR Morb Mortal Wkly Rep. 2003;52:41-45.
The CDC’s Vessel Sanitation Program (VSP) conducts surveillance for acute gastroenteritis (AGE) on cruise ships with foreign itineraries that sail into United States ports. AGE is defined as 3 or more loose stools in a 24-hour period or vomiting with 1 additional symptom such as abdominal pain, cramps, headache, myalgia, or fever. An outbreak is defined as AGE occurring among either 3% of passengers or 3% of crew.
From January 1 to December 2, 2002, 21 outbreaks of AGE on 17 cruise ships were confirmed. Nine were caused by noroviruses, 3 were due to bacterial agents, and 9 were of unknown etiology. Five of the 9 norovirus outbreaks on 5 different cruise ships are described. The percentages of passengers who were ill ranged from 5% to 41% and the rates of crew illness ranged from 1.5% to 5.3%. Three of the 5 ships voluntarily withdrew from service to aggressively disinfect and sanitize their ships.
On land, an increase of possible norovirus outbreaks, particularly in settings such as nursing homes, was reported as well. Clark County, Washington, received reports of 10 outbreaks of AGE during November to December 2002, affecting a total of 354 patients. Outbreaks in 4 long-term care facilities (LTCFs) accounted for 327 (92%) of the cases. In 2002, New Hampshire received reports of 35 outbreaks of AGE clinically consistent with norovirus, affecting a total of 2312 persons. Eighty percent of these outbreaks were reported from LTCFs. Finally, New York City reported 66 outbreaks of AGE epidemiologically consistent with norovirus infection from November 6, 2002, to January 13, 2003. Fifty-one of these outbreaks were in LTCFs or rehabilitation facilities and affected 1700 persons. Specimens from 6 of these outbreaks tested positive for norovirus.
Comment by Mary-Louise Scully, MD
Noroviruses were previously called "Norwalk-like viruses" until the summer of 2002 when the International Committee on Taxonomy of Viruses (ICTV) approved the new terminology, which should be used from now on. The genus Norovirus is in the family Caliciviridae (see Table). To date, all attempts to culture noroviruses have been unsuccessful. Caliciviruses, when viewed under the electron microscope, often have a surface structure with cup-shaped indentations or hollows (Latin calyx, cup, hence calicivirus), which appear to form a 6-pointed star.
It is estimated that 23 million cases of AGE are caused by noroviruses each year. After an incubation period of 12-48 hours, norovirus infection typically lasts 12-60 hours and is characterized by sudden onset of nausea, vomiting, and watery, nonbloody diarrhea. Patients can also experience vomiting alone, a condition first identified as winter vomiting disease.1 Dehydration is the most common complication in the young and the elderly.
Transmission of noroviruses is by the fecal-oral route, direct person-to-person contact, consumption of contaminated water or food, or contact with contaminated environmental surfaces. Aerosolized vomitus has also been implicated as a mode of transmission, especially during outbreaks in confined settings. Shellfish such as oysters and clams can concentrate noroviruses in their tissues, and even cooking (ie, steaming) may not inactivate these viruses.2
Prolonged viral shedding of norovirus for up to 7 days has been demonstrated by viral antigen detection in stools of both asymptomatic and symptomatic infected volunteers.3 Further research is needed to determine if this finding represents infectious virus or just soluble antigen. Also, when human volunteers were experimentally infected with norovirus, then re-challenged 27-42 months later with the same strain, all became ill again. Thus it would appear that no long-term protection following infection with norovirus occurs, though short-term immunity for up to 14 weeks has been demonstrated.4
Use of the more sensitive and specific reverse transcriptase-polymerase chain reaction (RT-PCR) test is enabling increased norovirus identification. In addition, nucleotide sequencing of detected strains has helped the CDC determine that 41% of the norovirus outbreaks since July 2002 were due to a single, newly identified strain, provisionally named Farmington Hills strain (genotype II, cluster 4), after Farmington Hills, Michigan, where the first cases with this strain were identified earlier in 2002. This strain is similar to the global "common strain" of 1995-1997 that circulated widely and accounted for 55% of the norovirus outbreaks in the United States during that time. From preliminary data, it appears that the strains currently circulating in Europe are also quite similar to the Farmington Hills strain, suggesting that, as in 1995-1997, the predominant strain in the United States is also distributed worldwide. These strains may have characteristics that increase person-to-person transmissibility, such as increased prevalence of vomiting.5
The low infectious dose (as few as 10 viral particles), the ability of the virus to survive relatively high doses of chlorine and varying temperatures (from freezing to 60 degrees) and their relative stability in the environment, make control of norovirus outbreaks difficult. However, control measures such as frequent hand washing with soap and water (lathering of hands for greater than 10 seconds then rinsing), disinfection of soiled surfaces with at least a 1:50 solution of domestic bleach and, if possible, and 72-hour isolation of infected patients, passengers, or crew, can improve outbreak control.
For those planning a cruise, an interesting web site is CDC’s Vessel Sanitation Program site, www.cdc.gov/nceh/vsp where the results of a ship’s most recent inspection can be viewed. A score of 86 or higher at the time of inspection indicates that the ship is providing an acceptable standard of sanitation. With the distinctive characteristics of noroviruses mentioned above, this by no means ensures smooth sailing. For example, 2 cruise ships, later involved in norovirus outbreaks, scored 99 and 96 respectively when inspected in June of 2002.
Health care providers are urged to submit appropriate samples for norovirus testing in suspected outbreaks of viral AGE. For assistance in testing, local and state health departments should contact CDC’s Viral Gastroenteritis Section, telephone 404-639-3577 or e-mail [email protected].
References
1. Adler JL, Zicki R. Winter vomiting disease. J Infect Dis. 1969;119:668-673.
2. Centers for Disease Control and Prevention. Norwalk-like Viruses: Public health consequences and outbreak management. MMWR Morb Mortal Wkly Rep. 2001; 50(No. RR-9):1-17.
3. Graham DY, et al. Norwalk virus infection of volunteers: new insights based on improved assays. J Infect Dis. 1994;170:34-43.
4. Parrino TA, et al. Clinical immunity in acute gastroenteritis caused by Norwalk agent. N Engl J Med. 1977; 297:86-89.
5. Fankhauser RL, et al. Molecular epidemiology of "Norwalk-like viruses" in outbreaks of gastroenteritis in the United States. J Infect Dis. 1998;178:1571-1578.
Recent outbreaks of acute gastroenteritis on land and at sea have occurred and are due to a predominant strain of norovirus, provisionally called Farmington Hills strain. This strain is similar to a common type strain that circulated worldwide in 1995-1997, and it may have characteristics that increase person-to-person transmissibility.
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