Number 54      February 1995

MAFF UK - PARALYTIC AND DIARRHETIC POISONING TOXINS IN ENGLISH AND WELSH SHELLFISHMAFF UK

Index to MAFF UK Food Surveillance Information Sheets, 1995

see also:
31: MAFF UK- Paralytic and Diarrhetic Poisoning Toxins in English Shellfish

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 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. Since 1994, summaries of these results have been published in the Food Safety Information Bulletin. Surveillance has shown that occurrence of PSP in England and Wales is limited to the NE coast of England where there is no commercial mussel fishing. Routine monitoring for DSP began in 1992 after DSP toxins were first detected in English shellfish during 1991. Limits for both PSP and DSP toxins in shellfish for human consumption are laid down under 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 mollusc must not exceed 80mg/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. Local food authorities are notified weekly of monitoring results by the Department of Health through the Food Hazard Warning System. As a precautionary measure, local food authorities in the NE of England respond to high PSP levels by posting 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 80mg/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, two consecutive positive bioassays will also trigger follow up action. If there is a commercial fishery in the area where the samples are 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.

Parallel surveillance of PSP and DSP toxins is also undertaken for the Scottish Office, but these results are not reported here.

Results

The results of surveillance for DSP toxins in English and Welsh shellfish collected during 1994 are given in Table 1. Those for PSP toxins are given in Table 2 . A list of sampling sites is also provided on a subsequent page.

As shown, 3 of the 225 samples (just over 1 percent) of mussels analysed contained detectable levels of DSP toxins. Contaminated samples were all collected from the NE coast of England. No positives were found in any species other than mussels.

For PSP toxins, 9 percent of 234 mussel samples contained concentrations in excess of 80mg/100g of flesh. These contaminated samples were all collected during May and June and were from the NE coast of England. Other areas of England and Wales were not affected with the exception of one sample from south Devon which did not exceed the statutory limit. With the exception of one lobster head sample, found off the NE coast of England in June, all the other shellfish species did not exceed the regulatory limit for PSP toxins, although lower levels of PSP toxins were found in the other samples.

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 figures for 1993 and 1994 are 2 percent and 1 percent respectively. At present, however, there is not a sufficiently large body of data available to show whether the apparent reduction over the past two years falls within the 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, 2 percent in 1992, and 16 percent in 1993 contained concentrations of PSP toxins greater than the limit. This compares with 9 percent in 1994. The maximum level of 1796mg/100g shellfish flesh recorded in 1994 exceeds those of 1588mg/100g flesh in 1993, 150mg/100g flesh in 1992 and 521mg/100g flesh in 1991. However, the maximum PSP concentration measured in 1994 is considerably less than that of 4000mg/100g flesh found in 1990 which then remains the worst year for PSP toxin contamination in shellfish since 1968 when routine surveillance began.

The Department of Health notified the food authorities in the NE of England of the results of PSP and DSP monitoring through the Food Hazard Warning System. As a result, warning notices against casual gathering of shellfish were put up along the NE coast as a precautionary measure.

The toxicological significance of the presence of DSP and PSP toxins in food have been considered by the Committee on the Toxicity of Chemicals in Food, Consumer Products and the Environment. Its full opinion was published in Food Surveillance Paper No. 42 (Ministry of Agriculture, Fisheries and Food (1994) Naturally Occurring Toxicants in Food Food Surveillance paper No. 42. HMSO. ISBN 0-11-242971-8.). The Committee welcomed the ongoing surveillance programme but did not indicate any toxicological concerns about the concentrations of toxins routinely found.

Contact Point

Further information can be obtained from:

Ms. E. Mortby
MAFF, Food Safety and Science Group
Food Contaminants Division
Room 210, Ergon House, c/o Nobel House
17 Smith Square
London SW1P 3JR

Tel: +44 (0) 171 238 6222
Fax: +44 (0) 171 238 6591

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