Northwest Fisheries Science Center


Staff Directory

Diarrhetic Shellfish Poison (DSP)

Map of Puget Sound

On June 25, 2011, three people became ill with diarrhetic shellfish poisoning (DSP) after eating mussels from Sequim Bay in northern Puget Sound. On Salt Spring Island, Canada, there were an estimated 60 illnesses from DSP in August 2011. This illness is caused through ingestion of shellfish that have accumulated toxins from the harmful algal bloom organism Dinophysis, which produces a suite of neurotoxins including dinophysistoxins and okadaic acid (OA). Approximately 2000 lbs of shellfish have been recalled due to this poisoning event. Dinophysis abundance in Sequim Bay reached 53,000 cells/L and the primary species was D. acuminata. Toxin concentrations in blue mussels from Sequim Bay reached 160 micrograms OA equivalents/100g shellfish (the regulatory limit is 16 micrograms/100g) by liquid chromatography/mass spectroscopy (LC/MS). The primary toxin isomer was dinophysistoxin-1.

The Northwest Fisheries Science Center’s Marine Biotoxin Program has been working with the Washington State Department of Health and the US Food and Drug Administration (FDA) to analyze ecosystem indicators that can provide early warning of these harmful algal bloom events. The SoundToxins partnership, a collaboration of the NWFSC, shellfish growers, tribes, environmental learning centers, and private citizens, routinely monitors phytoplankton and environmental factors in Puget Sound. SoundToxins partners have alerted the Washington State Department of Health to the high numbers of Dinophysis at locations around Puget Sound. The NWFSC has screened toxins in shellfish and seawater samples to provide a rapid assessment of shellfish toxicity. The Washington State Department of Health is collaborating with the NWFSC Marine Chemistry group and the FDA to implement analytical methods for DSP toxins.

Microscopic and SEM images of Dinophysis Dinophysis sp.

In Europe, the consumption of shellfish that have filtered cells of the dinoflagellate genus Dinophysis have led to cases of intestinal upset. Two varieties appear responsible for DSP: Dinophysis acuta and D. acuminata. Because of these symptoms, this syndrome has been designated as Diarrhetic Shellfish Poisoning or DSP. DSP is a significant problem in northern Spain, Ireland, and the Mediterranean/Adriatic Sea. The toxin has been detected in shellfish in Eastern Canada. While no cases of DSP have ever been reported along the West coast of the U.S., the organisms that cause this poisoning are commonly found in British Columbia and Puget Sound in Washington State. In 2003, the toxins were detected in shellfish from British Columbia and are more than likely to be found wherever the organisms are present.

The toxin in DSP appears to be okadaic acid and some related compounds, the dinophysistoxins. The toxins are fat soluble, have high molecular weights, and belong to a class of compounds called polycyclic ethers. Interestingly, these compounds (see chemical structures below) are somewhat similar in chemical structure to the rather exotic marine toxins that are associated with Ciguatera Fish Poisoning or Tropical Fish Poisoning in that the repeating polyether moieties are present in these molecules.

While at first glance, diarrhea would appear to be a relatively minor ailment compared to symptoms of PSP and domoic acid; the DSP toxins have been reported to be tumor promoting agents. Due to the relatively minor and generic symptoms associated with DSP, it is difficult to diagnose whether outbreaks of this particular poisoning have occurred in the U.S.

Okadaic acid (OA)
Dinophysistoxin-1 (DTX-1)
Dinophysistoxin-2 (DTX-2)
7-O-Acyl Derivates

{Structure after E.P. Carmody, K.J. James, S.S. Kelly, and K Thomas in Harmful Algal Blooms (P. Lassus, G Arzul, E. Erard, P. Gentien, C. Marcaillou Editors) 1995. page 273.}

Risk Management of DSP

Up until recently, DSP was managed by mouse bioassay and/or monitoring shellfish growing waters for the presence of Dinophysis organisms. Within the last decade or so, chromatographic tecniques using HPLC have been developed that permit both quantification and identification of the toxins. These techniques, while accurate, require well equipped laboratories with expensive equipment (i.e., HPLCs and perhaps mass spectrometers). Recently, molecular biological methods based on ELISA have been developed. For the future, these procedures, offer the potential for much lower costs, rapid assessment, and also field testing.

Because they have very large shellfish industries, Ireland and Spain have implemented extensive monitoring programs for DSP. Nevertheless, if monitoring costs are to be controlled, we need a far better understanding of the life cycle of Dinophysis and toxin production.

A full understanding of the production of the toxins by Dinophysis has proved difficult because the laboratory culturing of this dinoflagellate has proved very challenging. For reasons that are not understood, the organism can only be grown through one or two generations under laboratory conditions. Until the organism can be cultured in sufficient quantities, studies on the toxins will be hampered.