Epidemiology

Norwalk-like illness was originally described in 1929 as "epidemic winter vomiting disease" syndrome (66). Although it occurs with some increased frequency in the winter months, NL infections readily occur year-round (67). Identification of Norwalk virus as the etiological agent for acute gastroenteritis was demonstrated after administration of bacteria-free stool filtrates from a previous outbreak to healthy volunteers. Among these volunteers, rising antibody titers against Norwalk virus correlated with illness, while persons who remained healthy did not show a substantial rise in antibody titers (68).

While the reporting of NL gastroenteritis has increased in recent years, NL infections have been grossly underreported. There are several reasons for the underreporting. Norwalk-like illness may be very mild and not of sufficient consequence to warrant a visit to the doctor. Only in severe cases, especially when involving young children, the elderly, or persons with various other medical problems, is medical care generally sought. Reporting of NL illness is not required in most countries. In the United States, there are no national reporting requirements for NL illness by health care providers. In most instances, laboratory diagnostic tests are not routinely performed to confirm the etiological agent involved in the illness.

Consistent with a large number of estimated annual NL illnesses, serological evidence suggests that many adults secrete NLV-specific antibodies (69). This confirms that infection with genogroup I or II viruses is quite common and that antibodies readily circulate among different worldwide communities (69,70). In fact, multiple genotypes are occasionally identified during investigations of outbreak sources (62,71,72). This background level of community infection is probably unnoticed until an infected individual contaminates a common food or water source or facilitates rapid person-to-person spread via close contact with individuals in a closed or institutional-type setting. In these settings, direct person-to-person transmission can occur via contact with soiled linens, vomitus, feces, aerosols, or fomites (48,73). Outbreaks of NL illness have occurred from exposure at banquets, geriatric facilities, psychiatric wards, emergency rooms, cafeterias, recreational lakes, swimming pools, dormitories, campgrounds, hotels, schools, restaurants, and cruise and military ships (9,73-75).

Two recent outbreaks that have been extensively studied reveal how readily Norwalk virus is transmitted and how long it may persist within aquatic settings. In a Finnish town, more than 1600 cases of NL gastroenteritis resulted from the ingestion of improperly chlorinated tap water from a lake (62). Based on reverse transcription-polymerase chain reaction (RT-PCR) and DNA sequence analyses, it was determined that this virus was identical to a Norwalk-like strain causing a foodborne outbreak four months previously in a town 70 km upstream. The clear implication is that this virus was discharged upstream and remained viable for 4 months in ice-covered lakes and streams while eventually drifting downstream to the town's water intake.

Another well-documented outbreak of NL illness involved Louisiana oysters from Mississippi Sound. This outbreak, which affected 70 people within Louisiana and 120 from five other states, illustrates the manner in which Norwalk viruses can be spread via shellfish consumption (76,77).

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.

In November 1993, the Louisiana public health office received reports of persons becoming ill after eating raw oysters. The outbreak was subsequently tracked to a particular harvesting area. Daily harvest from this area was 1 million oysters per day, with approximately 60% of this harvest subsequently shipped beyond Louisiana borders. Fecal coliform testing was minimal in this area because the shellfish bed was believed to be quite distant from potential sources of sewage contamination; the reef was approximately 10 miles from the nearest sewage outlet and was separated from shore by open water. Results of subsequent coliform testing of water from a monitoring station closest to the shellfish bed one week prior to shellfish harvest and from the shellfish bed 2 days after harvest were within acceptable limits. The investigation of potential viral contamination sources revealed that boats routinely discharged sewage into harvest waters, despite regulations against the practice. Six harvesters reported diarrhea or vomiting during November 1993; two between November 7 and 10, when contamination was believed to have occurred. Both men reported discarding vomitus and feces overboard.

Although shellfish are well known for their ability to transmit NL illness, other foods have contributed substantially to NL infection. Raw fruits and salads are commonly seen as vehicles of virus transmission. Contamination can occur as a result of surface contamination of fruits and vegetables from irrigation or fertilization practices, from the harvester or transporter, or from contamination during food preparation. Sick food handlers have contributed greatly to the incidence of NL illness, either by contaminating foods with unsanitized hands or exposing products to unclean surfaces. One outbreak of NL illness was attributed to the contamination of potato salad by mixing it in a sink that a restaurant worker had vomited in a day earlier (48). Epidemiological investigations demonstrate that many outbreaks of NL illness result from a general lack of knowledge among some food handlers due to inadequate training in sanitation and hygiene.

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