Victims had come into either direct contact with the algae while swimming or inhaled it because of a windblown "aerosol affect", doctors at Genoa's Galliera hospital said. All were discharged within a few hours, after being treated for fever, nausea and irritation to eyes and nose.

The toxic algae, known by its scientific name "Ostreopsis ovata", first appeared on Sunday. Genoa mayor Giuseppe Pericu ordered the beaches closed to bathers on Tuesday.

And while the azure Ligurian sea looks inviting in the summer heat, fines of 50 euros will be imposed on anyone defying the ban.

Giacomo Zappa, director of the Galliera hospital, said the poisonous micro-organism could travel up the food chain and authorities warned against consumption of local fish or shellfish until further notice.

"We know that the toxins in this algae are capable of causing food poisoning in people who eat contaminated fish or molluscs," said food hygienist Gaetano Maria Fara at Rome's Sapienza university.

"It doesn't matter if they are raw or cooked, because the toxin cannot be destroyed by heat."

Italian experts see the algae, common in warmer climates, as another manifestation of the "tropicalisation" of local waters, with barracuda and other tropical fish increasingly common in recent years.

"This could be due to global warming or simply the globalisation of transport. Some species arrive in the ballast of ships," said Fara.

Environmental agency Arpat has warned that the appearance of such tropical algal blooms will become increasingly common along Italian coasts. An algal bloom appeared along the Adriatic coast near Bari in 2003 and 2004, which scientists believe is linked to an outbreak of respiratory illness in 28 people living nearby.

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