Outbreak investigators in Japan have identified contamination in a seaweed production plant as the cause of seven foodborne illness outbreaks that sickened almost 2,100 people in 2017.

The seven foodborne norovirus outbreaks were all attributable to dried, shredded seaweed called nori, according to a report from researchers in Japan and published in recent days by the U.S. Centers for Disease control and Prevention.

The outbreaks hit four remote areas as well as Tokyo. Norovirus GII.P17-GII.17 detected in stool specimens from patients. The strain is a novel variant that emerged in Asia during 2014–15, according to researchers.

Food survey results showed the seaweed was served before all outbreaks. Norovirus was also detected in nori-containing dishes. An investigation revealed that the nori served in each instance was manufactured by the same food processing company.

Outbreak 1 was reported Jan. 26, 2017, in four kindergartens, six elementary schools and five junior high schools in the prefecture of Wakayama, Japan. Of 1,943 children and 119 school staff members who had eaten lunch the day before, 678 (34.9 %) children and 85 (71.4 %) school staff members had acute gastroenteritis. Norovirus was also detected in the stool specimens from 10 of the 27 food handlers in the central kitchen who had eaten the same lunch. Food survey results indicated that the boiled vegetables mixed with shredded nori was responsible for this outbreak.

Outbreak 2 was reported Feb. 17, 2017, in seven elementary schools in the city of Tachikawa, part of metropolitan Tokyo. A total of 1,084 (35.2 %) of the 3,078 people who ate the lunch served the day before had gastrointestinal symptoms. The lunch included shredded nori as a topping on cooked rice.

Outbreaks 3 and 4 were reported in separate, self-catered school lunch settings on Feb. 22 and 25, 2017, in the city of Kodaira, also part of metropolitan Tokyo. The numbers of patients with gastrointestinal symptoms were 26 (5.6 % attack rate) in outbreak 3 and 81 (12.6 % attack rate) in outbreak 4. Shredded nori was served as a topping on cooked rice in both outbreaks.

Outbreak 5 was reported in Western Tama, also in metropolitan Tokyo, on Feb. 27, 2017. A central kitchen served school lunch to 19 people, and two of them had gastrointestinal symptoms. Shredded nori was served with boiled vegetables.

Outbreak 6 was reported March 9, 2017. After conducting retrospective surveillance, the Kurume city government in Fukuoka prefecture announced that a foodborne outbreak occurred in a business office on Jan. 25 that was attributable to a nori product with the same expiration date as the nori implicated in outbreaks 1 through 5. A total of 39 (92%) of the 42 employees who had eaten at the office’s café had gastrointestinal symptoms. Shredded nori had been served as a salad topping.

Outbreak 7, announced by the Osaka prefectural government, caused illness in 99 people Feb. 18-24, 2017, including four food handlers who consumed a bento box (a single-portion take-out or home-packed meal common in Japanese cuisine).

The Japanese study concluded that the seven foodborne norovirus outbreaks were attributable to a shredded nori product that was most likely contaminated during manufacturing.

Norovirus is the most common cause of acute gastroenteritis, or infection of the stomach and intestines, due to food poisoning. Highly contagious and active at low temperatures, norovirus causes stomach pain, nausea, diarrhea and vomiting. People become infected by eating or drinking contaminated substances, touching contaminated surfaces and coming into contact with an infected individual.

With a 24- to 48-hour incubation period, the virus can also cause muscle aches, headaches and fever. In severe cases, patients require hydration therapy. There is no available vaccine or treatment options for the virus. Symptoms usually subside within one to three days.