Rome ? Bird flu has reached Western Europe, with Italy and Greece announcing Saturday they had detected the H5N1 strain of the virus in dead swans.

The announcement that the disease was detected in five swans in southern Italy came a day after the opening of the Winter Games in Turin, several hundred miles to the north. Italian officials said the virus had only affected wild birds and posed no immediate risk to people.

The European Union said the deadly strain, which has infected at least 166 people and killed 88, most in Asia, also had been confirmed in swans in Bulgaria.

No human infections were reported in the three countries, but the outbreak raised concerns that the spread of the disease could increase chances for it to mutate into a form easily transmissible among humans, who generally catch the disease from domestic poultry.

“It’s a relatively safe situation for human health; less so for animal health,” Italian Health Minister Francesco Storace said.

Also Saturday, authorities in Nigeria said they were investigating whether the deadly strain, which was discovered in the country last week, had spread to humans after at least two children were reported ill.

Swans are seen Saturday at the port town of Stavros, Greece. Greek authorities appealed for calm after a British laboratory confirmed samples from three wild swans in Greece had tested positive for the deadly H5N1 strain of bird flu.

The U.N.’s chief bird flu expert said the spread of bird flu, which has been ravaging poultry stocks across Asia since 2003, increased the chance that the virus would mutate into a form transmitted between humans and set off a pandemic. Most human deaths from the disease so far have been linked to contact with infected birds.

“We have got bird flu now in southeast Asia, central Asia, eastern Europe, and west Africa,” Dr. David Nabarro said, before the Greek and Italian announcements. “Compared with eight months ago, this is a major extension of the avian influenza epidemic.”

Experts said they were reassured by the fact that the virus has been detected in wild birds in Western Europe instead of on farms.

“The risk to humans is less if the disease is in wildlife than if it is in poultry,” said Juan Lubroth, a senior animal health officer at the Rome-based U.N. Food and Agriculture Organization.

The virus was found in five swans in the southern Italian regions of Puglia, Calabria and Sicily, Storace said. The birds had arrived from the Balkans, he said, likely pushed south by cold weather.

The European Union said Italy and Greece had agreed to create six miles of protection and surveillance zones around each outbreak area, where birds will be isolated to avoid infecting pet birds, tested for the virus and killed if they are infected.

Hunting wild birds will be banned in the zones, and poultry will not be allowed outside them, according to the Italian Health Ministry.

Greek authorities said health experts were checking poultry on farms and homes in the region where infected swans were found outside the northern Greek city of Thessaloniki, Greece’s second-largest city.

The 25-nation EU said that H5N1 had been found in wild swans in the Bulgarian wetland region of Vidin, close to the Romanian border.