The results of this epidemiologic investigation indicated that the outbreak of acute gastroenteritis in the factory under investigation was caused by contamination of a secondary water supply system by norovirus. Noroviruses and total coliforms were detected in the underground reservoir water and the water tanks on top of the buildings, which in turn supplied the DDWDs. Employees in these buildings experienced significantly higher rates of acute gastroenteritis than those living in buildings that supplied bottled water.
To confirm the source of the contaminants in the underground reservoir, we investigated the surrounding environment and found eight access holes in the lid covering the reservoir. Furthermore, the lid was at a similar level to the ground around it, making it possible for contaminated substances to enter the underground reservoir through these access holes. Following the outbreak, a 1.2-m high cover has been erected over the reservoir to prevent contamination via the access holes. No new waterborne acute gastroenteritis cases were reported in this factory within the subsequent 2.5 years.
Although total coliforms in the secondary water supply system were above the upper limit required according to drinking water standards , no pathogenic bacteria were detected. Norovirus is a robust, chlorine-resistant virus, and 20 ppm chlorine causes no significant reduction in human norovirus infectivity [5, 13, 14]. Moreover, norovirus in groundwater remains infectious even after storage at room temperature in the dark for 61 days . Meanwhile, norovirus has a high prevalence in groundwater and surface water [16, 17]. Overall, these features of norovirus may explain why it is a more common causative agent of waterborne outbreaks of acute gastroenteritis than other viruses. Norovirus is the predominant cause of viral, waterborne outbreaks of acute gastroenteritis, especially in viral drinking-water outbreaks . Secondary water supply systems are common in China. Although many surveillance systems are in place to detect indicator microorganisms in secondary water supply systems every 3 months, surveillance does not cover all such systems, and the detection interval is too long. Sporadic cases and outbreaks of acute gastroenteritis caused by norovirus contamination of drinking water are common in China [19–22], which suggests that norovirus contamination of secondary water supply systems may play an important role in the occurrence of these outbreaks.
There was a notable limitation of the current study. Contact with patients represents one of the major transmission routes for norovirus, but the number of non-responses to this question was high. We were therefore unable to fully investigate the role of transmission by contact in this outbreak.
To our knowledge, this study represents the first report of an outbreak of acute gastroenteritis caused by norovirus contamination of a secondary water supply system in China. Such water systems are common in China, and cases of norovirus-associated acute gastroenteritis are also frequent. However, the contamination sources responsible for most of these norovirus gastroenteritis cases have remained unidentified. The current results highlight the risk of contamination of secondary water supplies by robust norovirus.