Switzerland has identified a third person infected with a rare strain of the E. coli bacteria that has killed 18 people in Europe, authorities said Friday.

The federal public health office said on its website that 28 cases of enterohaemorrhagic E. coli (EHEC) have been identified in 2011, including 15 since the start of May.

In three of these cases, lab results confirmed that the bacteria "is the same strain of bacteria as that identified in the north of Germany," the online statement said.

The public health office confirmed that the three people had visited Germany, where all but one of the EHEC-related fatalities has occurred.

The three, who are in stable condition, have not suffered from the more serious symptoms like bloody diarrhoea and potentially life-threatening conditions such as haemolytic uraemic syndrome (HUS), a kidney disease.

Swiss public health officials said it is possible that more cases of the rare strain will be identified.

Authorities announced the discovery of the first case last Friday.

At least 499 people, 470 of them in Germany, have HUS, according to the World Health Organisation, with 10 other European countries plus the United States reporting HUS or EHEC infections.

Authorities are still in the dark as to the origin of the outbreak but health officials say there are currently no indications that the bug can be transmitted from person to person.

It is primarily spread by consuming raw vegetables.