Outbreak of viral gastroenteritis due to drinking water contaminated by Norwalk-like viruses

Heinavesi, a Finnish municipality with a population of 4860 inhabitants, ha d an outbreak of gastroenteritis in March 1998, On the basis of an epidemio logic survey, an estimated 1700-3000 cases of acute gastroenteritis occurre d during the outbreak, Municipal water consumption was found to be associat ed with illness (risk ratio [RR] = 3.5, 95% confidence interval, 3.11 > RR > 3.96), Norwalk-like virus (NLV) genogroup II (GGII) was identified in unt reated water, treated water, and 4 tap water samples by use of reverse tran scription-polymerase chain reaction. This was the first time NLVs had been detected in municipal tap water. Fifteen of 27 patient stool samples had NL V GGII, with an identical amplification product to that found in the water samples, indicating that the outbreak was caused by this virus. In some pat ients, NLV genogroup I was also encountered. This virus, however, could not be detected in the water samples. Inadequate chlorination contributed to t he survival of the virus in the water.