Colorado State University Extension
SafeFood Rapid Response Network
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Update on Noroviruses
Edited by: Mary Schroeder, M.S., R.D. & Pat Kendall, Ph.D., R.D., Colorado State - Summer 2003
Norwalk-like viruses, also known as noroviruses, are common causes of viral enteritis, accounting for over 66% of the estimated cases of foodborne illnesses in the United States each year. Gastroenteritis outbreaks in the U.S. between the years 1996-2000 have been caused by noroviruses in 39% of restaurant and catered events outbreaks, 30% of outbreaks in nursing homes, 12% of those in schools and day care centers, 10% of vacation outbreaks, and 9% miscellaneous outbreaks. Its implication among cruise ship outbreaks has brought this more obscure pathogen into the limelight in recent months.
Symptoms
Noroviruses are called such because there is not just one, but rather a series of four, small RNA-viruses that are implicated in the transmission of disease. They are environmentally stable and will survive water chlorination and a wide temperature range, from freezing and heating to 140º F (60ºC). Onset of illness occurs within 12-48 hours and lasts approximately 12-60 hours. Symptoms include nausea, vomiting, abdominal cramps, and diarrhea.
Transmission-Foodborne
Food contamination by infectious food handlers is the most common cause of norovirus-related gastroenteritis outbreaks. Transmission usually occurs from exposure to fecally contaminated food or water resulting from failure to wash hands properly after using the restroom. Shellfish, in particular oysters and clams, have been implicated due to the ability of noroviruses to concentrate in their tissues or to contaminate waters where the shellfish are harvested. Of particular concern is transmission through ready-to-eat foods, which do not require cooking, such as salads and deli sandwiches. Because only a very low exposure is needed to result in a substantial outbreak, attention must be given to preventive actions. These include emphasis on frequent handwashing, exclusion of ill foodworkers from the workplace, properly cleaning and disinfecting surfaces and limiting possible contamination of ready-to-eat foods by either customers or foodhandlers.
Transmission-Person-to-Person
Person-to-person spread of noroviruses occurs by direct fecal-oral and airborne transmission. This has been a factor in institutional settings such as nursing homes, day care centers and on cruise ships. Wearing masks can be effective in protecting individuals, such as hospital or nursing home staff, who clean areas contaminated by feces or vomitus. For hospital and nursing home staff, protective measures include properly disinfecting surfaces of known contamination, taking special care in laundering soiled linens, and wearing of masks by staff that clean areas contaminated with feces or vomitus.
Transmission-Water
Although infrequent, gastroenteritis outbreaks have been associated with fecal-contaminated municipal water, well water, stream water, commercial ice, lake water and swimming pool water. In such instances, high level chlorination might be required for adequate disinfection.
Bottom Line
Although it is impossible to completely eliminate possible exposure to noroviruses in our environment, we can minimize our risk by taking the following actions:
- Wash your hands frequently, especially after using the toilet and changing diapers.
- Drink only potable water.
- Avoid consuming raw shellfish, especially from contaminated waters.
- Carefully wash fresh fruits and vegetables before consuming.
- Be cautious about exposure to persons who have the "flu."
If you or your family comes down with a norovirus infection or the "flu":
- Flush or discard any vomitus and/or stool in the toilet and make sure that the surrounding area is kept clean.
- Thoroughly clean and disinfect contaminated surfaces immediately after an episode of illness by using a bleach-based household cleaner.
- Immediately remove and wash clothing or linens that may be contaminated with noroviruses using hot water and soap.
Sources:
1) "Norwalk-Like Viruses" Public Health Consequences and Outbreak Management. U.S. Department of Health and Human Services. Centers for Disease Control and Prevention. Morbidity and Mortality Weekly Report (CDC/MMWR). Recommendations and Reports. June 1, 2001/Vol. 50/No. RR-9.
2) Epidemiology of Noroviruses. by Craig Hedberg; Environmental and Occupational Health, School of Public Health, University of Minnesota. Rocky Mountain Food Safety Conference. Boulder, CO. May 13, 2003.
3) Norovirus Q & A. Centers for Disease Control and Prevention. Last reviewed January 21, 2003. available at www.cdc.gov/ncidod/dvrd/revb/gastro/norovirus-qa.htm
