Paralytic shellfish poisoning (PSP) in humans is caused by ingestion of shellfish containing PSP toxins. These PSP toxins are accumulated by shellfish grazing on algae producing these toxins. Symptoms of human PSP intoxication vary from a slight tingling or numbness to complete respiratory paralysis. In fatal cases, respiratory paralysis occurs within 2 to 12 hours of consumption of the PSP contaminated food.
The PSP toxins are a group of 21 closely related tetrahydropurines (see Figure 2.1). The first PSP toxin chemically characterized was saxitoxin (STX). The various PSP toxins significantly differ in toxicity with STX being the most toxic. The PSP toxins are produced mainly by dinoflagellates belonging to the genus Alexandrium, which may occur both in the tropical and moderate climate zones. Shellfish grazing on these algae can accumulate the toxins but the shellfish itself is rather resistant to the harmful effects of these toxins. During the last 20 years, there seems to have been an increase in intoxications caused by PSP. However, as yet it is unclear whether the increase is real, whether it could be a consequence of improved identification, detection and medical registration, or whether it is due to expanded shellfish culture and consumption. A few dozen countries have regulations for PSP toxins. Most regulations are set for PSP toxins as a group.