Regulatory Implications of Diarrhetic Shellfish Poisoning (DSP) Toxins in U.S. Shellfish

Jonathan Deeds, Ph.D.

U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition, Office of Regulatory Science, College Park, MD 20740

Diarrhetic Shellfish Poisoning Toxins (DSTs) [okadaic acid (OA), dinophysistoxins (DTX), and a series of fatty acid diol-esters of these toxins] are produced by the marine dinoflagellates Dinophysis spp. and Prorocentrum spp. OA and the DTXs accumulate in commercial shellfish and can be further converted to a series of fatty acid acyl-esters. Consumption of DSTs results in acute diarrhea, nausea, vomiting, and abdominal pain. The additional non-diarrhetic marine toxins yessotoxin (YTX) and pectenotoxin (PTX) can co-occur in shellfish with DSTs in some areas, complicating analysis.  YTX, produced by different organisms than those that produce DST’s, are regulated separately in the EU at 100 μg/100g shellfish tissue.  Because they co-extract, are lethal in traditional in-vivo assays, and are produced by some of the same organisms that produce DSTs, PTXs are currently included in the 16 μg OA equivalents/100 g EU guidance level for DSTs. FDA currently makes no recommendations for controls for either YTX or PTX for shellfish harvested in the U.S., but has set a regulatory guidance level of 16 μg OA equivalents/100g total (free plus esterified) OA and DTXs in shellfish.

In 2008, fatty acid acyl-esters of OA were found in oysters (Crassostrea virginica) from the Texas Gulf coast in excess of the FDA guidance level, resulting in the first shellfish harvesting closures due to toxin presence in the U.S. In 2011, outbreaks of Diarrhetic Shellfish Poisoning (DSP) occurred in both British Columbia (Canada) and in Washington State (U.S.).  DTX 1 was found in excess of FDA guidance levels in mussels (Mytilus edulis) in both regions resulting in shellfish harvesting closures. In U.S. Mid-Atlantic (Maryland and New York) and Atlantic Northeast (Massachusetts) waters, DST and PTX producing Dinophysis sp. have been observed and assessments are underway to determine the prevalence of toxins both in source waters and in shellfish harvested for human consumption in these regions.

Around the world, in-vivo assays have historically been the preferred method for regulatory analysis for DSTs, but there is currently an EU mandate to develop and validate acceptable non-animal alternatives. In the U.S., the National Shellfish Sanitation Program (NSSP) currently has no specific guidance for acceptable testing methods for DSTs. At present, LC/MS, immunoassays, and in-vitro functional assays based on phosphatase inhibition are all in various stages of evaluation. New FDA guidance levels for DSTs are for total toxins (free plus esterified), meaning that any acceptable method must account for all of these toxins. At FDA, efforts are underway to assess available methodologies for DST analysis and provide appropriate guidance on this issue to our state regulatory partners for shellfish management.

Dr. Deeds holds a Masters of Science Degree in Environmental Toxicology from the University of Louisiana at Lafayette, and a Ph.D. in Marine, Estuarine, and Environmental Science from the University of Maryland. He is currently a research biologist in the Office of Regulatory Science at the US Food and Drug Administration’s (FDA) Center for Food Safety and Applied Nutrition in College Park, Maryland, USA. His research, particularly on the sources, fate, and methods of detection for marine biotoxins, supports FDA regulation and policy efforts for seafood safety and labeling.