Zinc accumulation in oysters giving mouse deaths in paralytic shellfish poisoning bioassay

During the last 2 years, extracts from cultured oysters (Ostrea edulis and Crassostrea gigas) intended for screening for paralytic shellfish poisoning (PSP) toxins have on several occasions been lethal to mice upon extended o bservation periods. The prolonged observation time in the PSP toxins assay was toxins responsible for amnesic shellfish poisoning (ASP). The symptoms observed in the mice deviated from those of both PSP and ASP. Based on lack of knowledge of the toxic principle, and the obligation to protect public health, oyster marketing has been stopped on such occasions. The probable s olution of the problem was suggested to us by oyster farmers who, via inter national contacts, brought to our attention similar Canadian experiences. T hey found that occasionally oysters accumulate large amounts of zinc which are lethal to mice when amounts injected intraperitoneally exceed about 450 mu g in a 20 g mouse. When analyzing zinc levels in the suspect Norwegian oyster extracts, correlation with the Canadian data was very convincing, in dicating the same "false positive" result in our analyses. Zinc at this lev el presents no health threat to humans with normal consumption of oysters. This article is written as a caution to other regulatory personnel involved in oyster monitoring against installing unnecessary restraints on oyster f armers before considering the possibility of zinc giving "false positive" r esults.