BATHING was banned along 15 kilometres of the Italian Riviera yesterday when more than a hundred holidaymakers were taken to hospital after being infected by toxic tropical algae.

Doctors at Genoa, where most of the victims were treated, said that the poisonous micro-organisms were almost invisible. Ostreopsis ovata are single-cell algae normally found in tropical waters but they have migrated to the Mediterranean because of rising sea temperatures.

Doctors at the San Martino Hospital in Genoa said that the symptoms suffered by swimmers — children and adults — included nausea, breathing difficulties, high fever, stomach cramps, irritation of the eyes, vomiting and diarrhoea.

La Stampa newspaper said: “The sea looks inviting during the heat wave. No one suspected killer algae were lurking in it. It was like one of those 1950s science fiction films about an invasion of deadly germs from outer space.”

Paolo Moscatelli, head of the regional health authority, said that the infected area had been identified and the situation was under control. But neighbouring resorts are on alert.

Giuseppe Pericu, the Mayor of Genoa, said that he had imposed a ban on swimming along a 15km (9ml) stretch of beach on the Ligurian coast, from Genoa to Bogliasco. Police had been instructed to impose fines of €50 (£34.50) on anyone defying the ban and entering the sea.

Silvia Uslenghi, a holidaymaker from Milan, said that people on the beach suddenly developed headaches and coughing and felt their eyes burning. Sunbathers who had not gone swimming were affected, suggesting that the poison was carried in the air.

Bianca Bobbio, 60, who swims regularly from the beaches near the port city, said that she had first noticed something wrong last Friday, when she went for a swim and emerged, feeling nauseous, to find her eyes puffed up. “Then it got worse,” she said.

The beaches affected include Portofino, which, with its yacht-filled harbour and ranks of hotels set above a sparkling bay, has become one of Europe’s most exclusive and expensive resorts. It is also where Silvio Berlusconi, the Italian Prime Minister, maintains a lavish holiday home. Also closed were the beaches of the Cinque Terre, the five isolated clifftop villages along the coast from Genoa where motor traffic is banned. The villages are considered among the most unspoilt spots in Italy, if not the whole of Europe.

Regional health officials, police chiefs and local authorities had at first suspected a bioterrorist attack, La Repubblica newspaper reported. Tests established, however, that the culprit was Ostreopsis ovata.

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