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Multiple Norovirus Outbreaks Due to Shredded, Dried, Laver Seaweed in Japan
To the Editor—Seaweed has been an important traditional Japanese food, and various types of seaweed are widely used in soup stock, seasonings, and sushi dishes. The history of seaweed consumption is long and deeply rooted in Japanese tradition; remains of marine algae were even found in some prehistoric relics in the Japanese archipelago. Interestingly, comparative metagenome analyses of gut microbiota have suggested the presence of a unique microbiome derived from marine bacteria related to seaweed consumption in Japanese individuals.1 Among seaweeds, shredded, dried, laver seaweed is commonly used as a condiment; previously, it has rarely been considered to cause food poisoning. Here, we report unprecedented multiple norovirus outbreaks due to shredded, dried, laver seaweed packaged for long-term preservation in Japan. Beginning on February 17, 2017, dozens of patients,
identified among cooking staff or in the kitchen facilities, the Tokyo metropolitan office identified the strain of norovirus that affected the outbreak patients in several packs of shred- ded, dried, laver seaweed used as condiments for the school lunches.2,3 The seaweed had been harvested in western Japan, Hyogo Prefecture. After being processed in a facility in Osaka
mainly primary-school students, were treated in our clinic in Tachikawa City in Tokyo (Figure 1). A norovirus outbreak was responsible for illness in this unusually large cohort of patients.2,3 Ultimately, this outbreak included ~1,100 students and teachers from 7 primary schools in Tachikawa. All schools receive school lunches from the same centralized school lunch kitchen. Although neither norovirus nor other pathogens were
figure 1. Geographical locations related to the multiple norovirus outbreaks due to shredded, dried, laver seaweed in Japan.
Prefecture, it was shipped nationwide with a December 2017 expiry date. The company stated that the seaweed was heat treated using a
conveyor-typemachine for ~7 seconds at 240 °C after boiling for 2 hours at 90 °C. An infected subcontractor had handled the seaweed with bare hands during the subsequent cutting and packaging process in the factory. In addition, 2 similar outbreaks had occurred in January 2017 involving >800 and >100 school children, respectively, in geographically distant cities, Wakayama Prefecture and Kodaira City in Tokyo. In both outbreaks, sea- weed from the same company contaminated with the same norovirus strain was provided in the school lunches.4 Noroviruses are surprisingly resistant to harsh environ-
mental conditions, and they are widely recognized as the leading cause of viral gastroenteritis due to foodborne infec- tion across all age groups.5–7 The diagnosis of norovirus gas- troenteritis cannot be made based on clinical features alone, and the widespread availability of molecular diagnostic tests has resulted in an increased awareness of norovirus-associated acute gastroenteritis worldwide. Because no vaccines or specific antiviral agents are currently available for the preven- tion or treatment of norovirus infection, rigorous hygiene (eg, hand washing) and heat treatment of foods and water remain the main prevention measures to combat norovirus
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