Number 31      June 1994

MAFF UK - PARALYTIC AND DIARRHETIC POISONING TOXINS IN ENGLISH SHELLFISH

Index to MAFF UK Food Surveillance Information Sheets, 1994

see also:
54: MAFF UK - Paralytic and Diarrhetic Poisoning Toxins in English and Welsh Shellfish (February 1995)

Background

Paralytic and diarrhetic shellfish poisoning toxins are produced by algae and taken up into the food chain by filter feeding bivalve molluscs. Within the shellfish, the toxins are accumulated in the hepatopancreas and sufficient toxins can accumulate to cause poisoning in humans if the shellfish are eaten. Paralytic shellfish poisoning (PSP) toxins can cause symptoms ranging from tingling of the tongue to loss of consciousness and respiratory failure. Diarrhetic shellfish poisoning toxins (DSP) have less severe effects, causing diarrhoea, nausea and vomiting.

In the UK, the last outbreak of PSP occurred in 1968 when 78 people suffered illness after consuming mussels taken from sites off the North East (NE) coast of England. Following this incident, the Ministry began a yearly surveillance programme and the results of this programme have been regularly reported, initially in the Aquatic Environment Report series and latterly in the Food Surveillance Paper series. Surveillance has shown that occurrence of PSP in England is limited to the NE coast where there is no commercial fishery. Routine monitoring for DSP began in 1992 following the first finding of DSP toxins in English shellfish during 1991. Concentrations of both PSP and DSP toxins are now controlled via The Food Safety (Live Bivalve Molluscs) Regulations 1992 (S.I. [1992] No. 3164) which implement into national law the requirements of EC Directive 91/492/EEC.

Under the regulations, the 'total concentration of PSP toxins in the edible parts of the molluscs must not exceed 80 µg/100g of mollusc flesh'. If any molluscs collected as part of the routine surveillance programme are found to contain PSP toxins approaching or exceeding this level, sampling is increased in and around the affected area and high levels in mussels triggers sampling of other species, including crustaceans. During the PSP season, local authorities in the NE of England post notices along the coast warning against beach collection of shellfish by the public. If necessary, commercial fisheries are closed until the concentration of the toxins falls below 80 µg/100g shellfish flesh.

In the case of DSP, the regulations state that 'customary biological methods must not give a positive result to the presence of DSP in the edible parts of the mollusc'. In the UK, positives in the bioassay are generally confirmed by HPLC which has a limit of determination of 20 µg/100g of hepatopancreas. Alternatively, two consecutive positive bioassays will also trigger follow up action. If there is a commercial fishery in the area where the samples were taken, all fishermen are advised that a voluntary ban should be put in place until such a time as two further samples have both given negative results. As for PSP, public warnings are posted to warn casual gatherers of shellfish of the hazard for as long as elevated concentrations of toxin persist.

Results

The results of surveillance for DSP toxins in English shellfish collected during 1993 are given in Table 1. Those for PSP toxins are given in Table 2.

For DSP, samples testing positive by the bioassay are usually then tested by HPLC for confirmation purposes. It can be seen from Table 1 that although five samples of mussels were positive by bioassay only three were subsequently confirmed. No positives were found in species other than mussels.

For PSP, samples of mussels containing concentrations of toxins in excess of 80 µg/100g of flesh were taken during May and June from the NE coast of England (Table 2). No samples taken from other sites breached the limits. In addition, no samples of shellfish other than mussels contained PSP toxins in excess of 80 µg/100g flesh.

Interpretation

In 1992, the first year for which surveillance data are available for DSP, 11 percent of mussel samples tested positive for DSP toxins. The corresponding figure for 1993 is 2 percent. However, at present there is not a sufficiently large body of data available to show whether this apparent reduction falls within normal variability of the monitoring data.

The finding of the highest concentrations of PSP toxins in May and June is in line with data from previous years. For mussels, 11 percent of samples in 1990, 9 percent in 1991 and less than 2 percent in 1992 contained concentrations of PSP toxins greater than the limit. This compares with 16 percent in 1993. However, although the frequency of occurrence was greater in 1993, the maximum concentration detected in 1993 was only 1588 µg/100g flesh compared with 4000µg/100g flesh in 1990. Maximum levels in 1991 and 1992 were 520 and 150 µg/100g flesh respectively. Thus, 1990 remains the worst

year for PSP since 1968 when routine surveillance began.

The toxicological significance of the presence of DSP and PSP toxins in food has recently been considered by the Committee on the Toxicity of Chemicals in Food, Consumer Products and the Environment (COT). Its full opinion can be found in Food Surveillance Paper Number 42: Naturally Occurring Toxicants in Food. In brief, the committee welcomed the on-going surveillance programme but did not indicate any toxicological concerns about the concentrations of toxins routinely found.

Contact Point

Further information can be obtained from:
Dr Julie Norman
MAFF, Food Safety and Science Group
Food Contaminants Division
Room 208, Ergon House,
c/o Nobel House
17 Smith Square
London SW1P 3JR

Tel: +44 (0)171 238 6198

Fax: +44 (0)171 238 6591

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