Genoa's mayor Giuseppe Pericu told reporters scientists "have ascertained that all the parameters for swimming are normal" but monitoring would continue.

Nearly 200 bathers were treated in hospitals for fever, nausea and irritation to eyes and nose over a period of a few days from last Sunday having come into contact with the algae, "Ostreopsis ovata," while swimming or lying on local beaches.

Pericu ordered the beaches closed on Tuesday.

"The poisoning, which was very slight in all cases, was not contagious but limited to single individuals, for that reason we haven't extended the swimming ban," the mayor told a press conference in the northern port city to the northwest of the affected stretch of coastline.

"The phenomenon remains present however because the algae lives along the Italian, French and Spanish coasts," he said. "The algae is innocuous but can bloom and create problems anywhere, not only in the Ligurian sea, when concomitant factors are present such as calm seas, strong sunlight or warm currents."