Norwalk-like viruses

Norwalk viruses (and related caliciviruses) are important causes of sporadic and epidemic gastrointestinal disease in the United States. An estimated 181,000 cases occur annually. Viral gastroenteritis due to caliciviruses has been associated with eating contaminated oysters. Oysters can become contaminated before harvest or during preparation. Water and ice are other sources of infection.

Symptoms of Norwalk virus infection include:

• Nausea
• Vomiting
• Diarrhea
• Abdominal cramps

Headache and low-grade fever may also occur. Persons with this infection usually recover within 2-3 days without serious or long-term health effects. Shellfish become contaminated via infected fecal matter from sick food handlers, or by the practice of some harvesters and/or recreational boaters of dumping raw sewage overboard. (Note: the only source of calicivirus is feces from sick persons.)

Since 1993, three oyster-related gastroenteritis outbreaks, attributed to calicivirus, have been documented in Louisiana. In 1993, 73 persons in Louisiana and about 130 others in the United States who ate oysters from Louisiana became ill. A malfunctioning sewage system was the cause of an outbreak in 1996. The current outbreak implicates sewage from oyster harvesting vessels as the probable source.

Oyster-related outbreaks probably will continue to occur until seafood regulators and the oyster industry develop and enforce standards for proper disposal of human sewage. Food handlers with symptoms of Norwalk-like illness should limit contact with other persons and be excluded from food handling and preparation.