Incidence of Norwalk virus infections during a prospective epidemiological study of drinking water-related gastrointestinal illness.

To determine the seroprevalence of Norwalk virus and whether Norwalk virus contributed to an observed increase in illness in tap water drinkers participating in a prospective epidemiological study, sera collected during the study were examined for changes in Norwalk virus antibody titer, using a specific enzyme immunoassay. Antibodies to Norwalk virus were measured in sera collected in March, June and September 1988 and in June 1989, and antibodies were found in 79% of the individuals. Seroprevalence increased with age, being 55% (ages 9-19), 79% (20-39), 87% (40-49), 84% (50-59), and 100% (60 and older). Norwalk infections occurred in 33% of the individuals during the course of the study. The highest rate of infection (expressed as a monthly rate) was observed during the summer of 1988. These results confirm that a large number of infections owing to Norwalk viruses occur throughout the year. A previous seroconversion or a high serum titer were not always protective. Finally, there was no detectable difference in infection rate between consumers of tap water and consumers of water treated by reverse-osmosis units, suggesting that Norwalk virus infections were not responsible for the excess of gastrointestinal illness observed in tap water drinkers during this epidemiological study.