Today, NL illness is the number one cause of foodborne illness in the United States (10) and perhaps throughout much of the world. As an enteric virus, NLVs are transmitted by the fecal-to-oral route. The principal means of transmission are through contaminated food and water and by person-to-person spread. Norwalk-like viruses can be introduced into foods by improper hygiene of food handlers, through irrigation or fertilization of crops with contaminated water or sewage sludge, and from the use of contaminated water during food processing, sanitation, or ice preparation (43). Field workers can contaminate fruits and vegetables during harvest as a result of inadequate bathroom facilities or poor hygiene while working in the fields. Among the foods most commonly cited as contributing to viral illness from handling by infected workers are salads, raw fruits and vegetables (44-48), and bakery products (49,50)—foods that are generally served raw or that may be adulterated after cooking.

Consumption of raw or undercooked shellfish harvested from contaminated estuaries is a common cause of NLV outbreaks (51,52). Bivalve mollusks such as oysters, clams, and mussels are especially prone to transmit NLVs when raised in water that is subject to fecal contamination. Shellfish effectively bioconcentrate these viruses from the water through their normal filter feeding activities (53).

As a group, the nonenveloped enteric viruses are relatively resistant to physical and chemical inactivation and consequently can persist for extended periods in aquatic environments. It has been suggested that enteric viruses adsorbed to colloidal clays and debris have enhanced stability in water (54-56). Using feline calicivirus (FCV) as a surrogate for Norwalk virus in long-term stability studies, a 1095-50% tissue culture infectious dose (TCID50) of FCV was reduced by one order of magnitude over 60, 20, and 10 days at 4, 25, and 37°C, respectively (57). However, when dried, the virus was unstable at 37°C and did not survive more than 24 hours (57). At 60°C, the virus remained viable for as long as 30 minutes.

Norwalk virus may be especially thermally stable in the context of contaminated oysters. Inadequately cooked shellfish can readily transmit NLVs, since the shells and the high protein content of oysters afford some degree of thermal protection. Even if "properly cooked," shellfish may not be absolutely free of infectious Norwalk virus, since some recent outbreaks have been associated with cooked oysters (58-60). Product contamination after cooking may also attribute to some cases of NL illness.

Early evidence suggested that Norwalk virus may be resistant to chlorination (61); however, further studies are needed. It is unclear whether properly functioning sewage treatment plants can discharge viable Norwalk virus. Less than optimal chlorination of drinking water has clearly been implicated in Norwalk virus outbreaks (62). The use of contaminated water in food processing or sanitation has led to food contamination (63-65).

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