Invertebrates
Molluscs
Mollusc poisoning or ‘shellfish poisoning’ is caused by the consumption of bivalve molluscs that accumulate toxins of protozoal or algal origin. Toxins from algal sources are also referred to as phycotoxins, analogous to the mycotoxins from fungal sources . Mollusc poisoning caused by algal toxins is usually classified according to the symptoms they cause in humans:
• paralytic shellfish poisoning (saxitoxins)
• diarrhoetic shellfish poisoning (okadaic acid) • neurotoxic shellfish poisoning (brevetoxin)
• amnestic shellfish poisoning (domoic acid)
• intoxication with venoms (the conotoxins) from snails belonging to the genus Conus is another form of mollusc poisoning.
Paralytic shellfish poisoning (PSP) is caused by saxitoxins (STX; Fig. 1) produced by marine ‘red tide’ dinoflagellates and freshwater blue–green algae such as Alexandrium spp., Gymnodinium catenatu and Pyrodinium bahamense. STX are potent agents that can block sodium channels in nerves and muscles at the extracellular side of the channel, which leads to conductivity disturbances and paralysis. About 1600 cases of poisoning are estimated to occur every year. In severe cases, the neurological symptoms spread to the extremities and respiratory muscles and, without ventilatory support, patients die between 2 and 12 hours after ingestion. Lawrence et al. (1996) described a fast and reliable analytical method to detect STX in molluscs, based on HPLC with fluorescence detection after pre-chromatographic oxidation. LC-MS methods are now available.
The major causative agent of diarrhoetic shell fish poisoning (DSP) is okadaic acid (Fig. 2), which is produced primarily by ‘red tide’ dinoflagellates belonging to the genera Dinophysis and Prorocentrum. DSP toxins are lipophilic and accumulate in the digestive gland of mussels. Okadaic acid is a potent inhibitor of protein phosphatases 1 and 2A. In humans, consumption of contaminated molluscs leads almost exclusively to gastrointestinal symptoms: diarrhoea, nausea, vomiting and abdominal pain, which appear between 30 minutes and a few hours after the meal and can be caused by as little as 40 lg of toxin. Treatment is supportive and recovery is complete after a few days. An LC MS/MS method has been developed for the detection and quantification of okadaic acid in mussels (McNabb et al. 2005). The method is applicable for the detection of other algal toxins including domoic acid.
Figure 1 Saxitoxin.
Figure 2 Okadaic acid.
Neurotoxic shellfish poisoning (NSP) is caused by a toxin produced by another ‘red tide’ dinoflagellate, Gymnodinium breve, which has been observed on the west coast of Florida, in the Gulf of Mexico, Japan and New Zealand. The active principle is the lipid-soluble polyether breve toxin (Fig. 3), which has a molecular weight of around 900 and is one of the most potent neurotoxins known. In humans, ingestion of brevetoxin-contaminated shellfish can result in gastroenteritis with neurological symptoms. Within 3 hours, nausea and vomiting, paraes thesias, reversal of hot/cold sensation, throat tightness and ataxia may occur. There is no paralysis. There is complete recovery from these symptoms within 2 days without specific treat ment. No human deaths have been reported with brevetoxin poisoning. ELISA methods have been developed to detect brevetoxins in shellfish (Quilliam 1999, Naar et al. 2002) and the toxins may also be detected with the HPLC-MS/MS method for ciguatoxin (CTX) described by Lewis et al. (1999) and the LC-MS method of Nozawa et al. (2003). Amnesic shellfish poisoning (ASP) was identified as a marine toxin disease in 1987 in Canada, when more than 150 people were affected by the consumption of cultured blue mussels, which resulted in the deaths of three patients. The toxin responsible is the tricarboxylic acid, domoic acid, which is formed by certain species of the diatom genus Pseudonitzschia. Domoic acid (Fig. 4) is a neurotoxic agent. Acute symptoms include vomiting and diarrhoea and, in some cases, are followed by confusion, memory loss, disorientation, coma or death. A large number of different analytical methods have been described for domoic acid, probably because of its relatively straightforward chemical structure. Methods for analysis in shellfish include TLC and HPLC, including a fluorimetric HPLC method for the determination of domoic acid in seafood and marine phytoplankton with derivatisation using 4-fluoro-7-nitro-2,1,3 benzoxadiazole (NBD-F). Immunoassay methods are also available as are capillary electrophoresis methods with UV detection. An LC-ESI-MS method has been developed for the determination of domoic acid in serum and urine, and applied to the investigation of poisoning in animals (Tor et al. 2003). LC-MS methods are also available for the determination of domoic acid in shellfish (see method of McNabb et al. (2005) for okadaic acid above).
Figure 3 Brevetoxin.
Figure 4
