Ice-associated norovirus outbreak predominantly caused by GII.17 in Taiwan, 2015

Norovirus is the leading global cause of acute diarrheal disease as well as gastroenteritis outbreaks [1, 2]. Previously, the most common genotype in norovirus outbreaks worldwide and in Taiwan was GII.4, although new variants emerge every 2–4 years [3, 4]. The GII.17 genotype has historically been an infrequent cause of sporadic norovirus cases, but the emergence of a new GII.17 variant was recognized in several east Asian countries, including China, Japan, Korea, and Taiwan since 2014 [5, 6] and speculated as potentially causing the next norovirus epidemic [7, 8]. In Taiwan, norovirus GII.17 has been identified in one major gastroenteritis outbreak in February 2015 [9].

On 5 March 2015, Taiwan Centers for Disease Control (Taiwan CDC) was notified of more than 200 patients of acute gastroenteritis among students of a senior high school who stayed and ate breakfast at a resort on 4 March and 5 March during a graduation trip in Kenting, a famous tourist attraction in southern Taiwan. Norovirus was suspected because of the predominance of vomiting in clinical presentations. Because another student group from a different school staying and dining at the same resort also reported cluster of gastroenteritis simultaneously on 5 March, we conducted an outbreak investigation to identify the source and possible vehicle of the pathogen.

A total of 275 (96%) of 285 students completed the survey. Eight students were excluded from analysis because of missing data or illness onset before the implicated breakfast on 4 March. Of 267 students included in the analysis, 144 (54%) met our case definition. Median age of cases was 17 years (range 16–26) and male-female ratio was 1.29. The epidemic curve showed a single wave pattern and median incubation period using the breakfast on 4 March as the start point was 32 h (range 4–47) (Fig. 1). Most common symptoms were nausea (78%), vomiting (69%), dizziness (64%), abdominal pain (60%), weakness (54%), fever (47%), and diarrhea (42%).

The following statistical analysis was focused on the breakfast on 4 March based on the onset time of students’ illness, estimated incubation period of norovirus and less than a half of students consumed 5 March breakfast because 22 students had already developed symptoms. In bivariate analysis, only those who consumed iced tea, including lemon tea or milk tea, had increased risk for gastroenteritis (risk ratio [RR] 1.54, 95% confidence interval [CI] 1.22–1.98) (Table 1). Regression analysis revealed similar findings in students who consumed iced tea (RR 1.48, 95% CI 1.17–1.84) (Table 2).

Based on epidemiologic, laboratory, and environmental evidence, this outbreak of acute gastroenteritis in Kenting was caused by norovirus, which was spread through waterborne transmission. Epidemiological analysis suggested the iced tea was the most likely vehicle for transmission. Sequence analysis showed both tap water and ground water were mainly contaminated by norovirus GII.17, which was clustered into the same genotypes with the human strain.

The epidemiological curve illustrated a point-source outbreak indicating possible foodborne or waterborne transmission. With application of molecular epidemiology in our investigation, we can confirm the source of the pathogen and provide solid laboratory evidence. Regarding both food-borne and water-borne norovirus outbreaks can be caused by multiple genotypes of norovirus [17] and the common existence of norovirus in the natural environment such as ground water [18, 19], it is often difficult to figure out whether norovirus found in the environment is the causative pathogen of illness in patients. By using phylogenic analysis, we confirmed that the norovirus found from ice and water belonged to the same clusters of genotypes as those found from human samples. This evidence combined with the findings of our epidemiologic investigation provided a stronger evidence for identification of the source and transmission route, which is important for health authorities to implement effective outbreak interventions.

Although the restaurant had a filtration system for the ice machines, our investigation discovered that it was not effective against norovirus because the filtration system had pore-size of 5 μm, and norovirus with same genotypes were found from ice and water before and after filtration. According to the recommendations of the US CDC [20], only nanoflitration or reverse osmosis has high effectiveness in removing norovirus. Adequate water treatment in these ways should be recommended to prevent waterborne outbreaks especially when ice is made from un-boiled water.

Ice-associated norovirus outbreaks have been reported several times because ice was usually made by un-boiled water and the risk of contamination was easily neglected [21, 22]. Current regulation for ice in Taiwan only requested that the source water must meet the drinking water quality standards. However, the microbiological indicators used in the standards, such as total bacterial count and coliform levels in water, have been proven non-predictive for norovirus [23]. On the other hand, routine viral tests for water sources are not feasible because of its high cost, although monitoring of norovirus has been proposed in the third Unregulated Contaminant Monitoring Rule in US [24]. Chlorination is generally considered effective for norovirus disinfection [25], but it is not obligatorily used for treatment of ground water in Taiwan. In dual public water systems using both treated tap and ground water, it might be safer to adapt the regulation of small water treatment facilities and to use chlorination for ground water because contamination can not be completely avoided.

The laboratory examination in our investigation showed the outbreak was caused by multiple variants of norovirus, including GI.3, GI4, and predominantly GII.17. This finding is not uncommon in waterborne norovirus outbreak, especially when strong sewage contamination was noticed [26]. Although norovirus GI was more predominant in previously reports of waterborne outbreaks, our investigation revealed the potential of GII.17 to cause large waterborne outbreak. We also find one waiter tested positive for only norovirus GI.4. However, his symptoms had subsided one week before this outbreak, suggesting his infectiousness was low and he denied any contacts with the students as well. Therefore, we considered he was another patient accidentally infected via contaminated water one week before rather than being the index case.

Previously, GII.4 was the most important genotype of norovirus because new variants caused cyclic epidemics every 2 to 4 years. Norovirus GII.17 was only sporadically reported in ground water before 2014. However, recent studies reported high frequency of GII.17 in ground water in Kenya [27] and in sewage or costal water in Asian countries including Korea, China and Japan [7, 18, 28]. Outbreaks caused by a new variant of GII.17 have also increased since late 2014 [18, 29, 30]. A survey conducted in Korea revealed GII.17 was the most prevalent strain of norovirus in asymptomatic food handlers [6] and the proportion of GII.17 detected in human norovirus cases was also increasing in China and Ethiopia [31, 32]. Our investigation suggests that this new variant GII.17 can cause large waterborne outbreaks. Public health professionals should maintain a high index of suspicion of water and ice as the source of large norovirus outbreaks because water-borne outbreaks usually result in larger size and higher attack rate than those spread through direct person-to-person contact. We recommend ongoing collaboration between human, food, and environmental health authorities during investigations and re-evaluating drinking water standards in response to the challenges raised by waterborne norovirus outbreaks.

There are some limitations in our investigation. First, recall bias is difficult to avoid since our investigation used a self-reported questionnaire. However, the response rate to the questionnaire was high and the elapsed time from the implicated breakfast to interviews was short. Thus, the recall bias could be minimized. Second, it is difficult to determine exactly where the contamination occurred in the ground water and the treated water system in the resort because we did not trace out pipelines used in the two systems. Thus, the local health authorities used chlorinating tablets in the whole water system to prevent further transmission of norovirus until no norovirus was detected. Last, not all students who consumed contaminated iced drinks developed gastroenteritis. The reason may be the cross-protection from prior norovirus exposure or other host factors but our investigation could not provide this information. We could not conclude the clinical characteristics and the tendency on age because this outbreak occurred in a close student tourists group and no other tourists visit the resort hotel restaurant at the same time as well. Further investigations may be worthwhile to delineate host factors that predispose these students to GII.17 infection and the clinical characteristics.

By using molecular methods, we confirmed a norovirus gastroenteritis outbreak predominantly caused by GII.17. The source of infection was norovirus-contaminated ice made from un-boiled water. Adequately filtered or boiled water should be used to make ice, especially when dual treated and ground water supply system was established in the facilities.