A total of 49 people who ate at two London restaurants suffered acute nausea, vomiting, diarrhoea, abdominal pain, and feverishness lasting eight hours.

All had been served dishes of mussels originating from the UK, including one who was made ill by mussel soup.

Doctors diagnosed Diarrhetic Shellfish Poisoning (DSP) arising from poisons called phycotoxins produced by small marine organisms.

UK shellfish contamination

Although imported mussels caused one previous occurrence of DSP in 1994, the outbreak in June last year represented "the first incident for 30 years caused by phycotoxin contamination of UK shellfish".

Writing in the Lancet medical journal, health experts in London said tests on mussels from both restaurants yielded high concentrations of phycotoxins.

When filter-feeding molluscs like mussels ingest the organisms, dinoflagellates, that produce the toxins the poisons accumulate within them.

The particular phycotoxin involved in this incident, okadaic acid, had not been seen in a food poisoning outbreak before.

Threat in Europe

Dr Anne Scoging, from the Public Health Laboratory Service, and Dr M Bahl from Camden and Islington Health Authority, said DSP was one of three types of shellfish poisoning that pose a threat in Europe.

The others were amnesic and paralytic shellfish poisoning. There had been no cases of amnesic poisoning in the UK and the last report of paralytic poisoning was in 1968.

Poisons in shellfish were monitored by the Ministry of Agriculture, Fisheries and Food, which regularly tested shellfish harvested in the UK.

DSP toxins were first detected in UK shellfish - a batch of cockles from the Thames estuary - in June 1991.