A series of food poisonings resulting from eating mussels and scallops harvested in the Tohoku District occurred in late June of both 1976 and 1977. A total of 164 persons were officially documented to have suffiered severe vomiting and diarrhea. Toxicity studies using mouse assay on the left-over foods and on shellfishes from the areas where the causative specimens were harvested revealed the presence of an unknown fat-soluble toxin in the hepatopancreas of the shellfishes.
The dominant symptoms in human cases were diarrhea, nausea, vomiting, and abdominal pain. The onset time of these symptoms ranged from 30 minutes to a few hours after ingestion and the victims recovered after three days without any ill effects. Epidemiological data indicated that as little as 12 MU of toxin was enough to induce a mild form of poisoning in humans. In 1976 this remarkable toxicity of shellfishes was recognized only in the areas around Motoyoshi-cho, Miyagi Prefecture. In 1977, however, the toxic region spread along a 100 km stretch of Sanriku Coast from Onagawa Bay, Miyagi Prefecture, to Okirai Bay, Iwate Prefecture. In addition, shellfishes in Mutsu Bay, Aomori Prefecture and in Funka Bay, Hokkaido, also bore the toxin.