Dr Lynn Gratton, of Maryland University, discusses the symptoms

The single-celled creature was first identified in Maryland, USA, but has also been found off Britain's coast in the North Sea last November.

It can cause dementia-like symptoms with sufferers experiencing memory loss and difficulties with concentration, attention span and learning abilities.

People suffer a stinging or burning sensation to the skin on contact with water containing the organism. Headaches and stomach problems follow.

The severity of the symptoms appears to be proportional to the amount of exposure to the toxin excreted by the microbe.

Most sufferers have been repeatedly exposed to the threat over a period of weeks, however one water skier fell ill after just a few hours exposure.

Fishermen were the first to suffer

The first evidence that the microscopic creature, a dinoflagellate, could kill came from US fishermen who noted that fish in the Pocomoke River and nearby estuaries were swimming erratically and had strange lesion marks on their skin.

At about the same time some local fishermen began experiencing memory problems and other symptoms including headaches, diarrhoea,weight loss and skin irritation.

Investigators discovered dinoflagellates living in the water, and tests showed that the creatures caused a frightening array of symptoms.

'Acute confusion'

Researchers, writing in the Lancet, said victims suffered "acute confusion, episodes of disorientation, new or increasing forgetfulness, or difficulties concentrating".

Problems included motorists setting out in their cars and then being unable to remember where they were going, and people forgetting they had mailed packages.

Some individuals were unable to recall numbers, and fishermen forgot to bring routine equipment on to their boats.

The symptoms amounted to a "new clinical syndrome", said the researchers.

"The most consistent objective finding among exposed individuals is a pattern of deficits in new learning and selective and divided attention," they added.

However, the symptoms were reversible. After three to six months, people who had previously suffered symptoms were back to normal.

Hugely toxic

About 1,100 species of dinoflagellates are known to science, a few of which produce some of the most toxic substances known.

They inhabit cool and temperate waters and in large numbers can produce "red tide" blooms.

Some species are responsible for shellfish poisoning, caused by people eating shellfish that have ingested the organism.

The type thought to be affecting fishermen in Maryland is the species Pfiesteria piscicida. It is found in brackish water usually thought unsuitable for recreation.

Pfiesteria is almost indestructible. When pfiesteria cysts are put into sulphuric acid they survive for 30 minutes. A human body thrown into sulphuric acid would quickly dissolve.

Surveillance operation

A spokesman for the Public Health Laboratory Service said: "To date we have not seen any cases of human illness associated with this organism in England and Wales.