The dead swan that tested positive for the virulent H5N1 form of bird flu migrated to Britain, Scotland's senior vet said today.
DNA testing identified the bird - whose decaying corpse was found on the coast in Cellardyke, Fife, on March 29 - as a whooper swan, a migratory species that spends the winter in Britain.
Charles Milne, the Scottish chief veterinary officer, said it was impossible to be sure whether the bird had caught the disease in another country or contracted it in the UK. The remarks left open the possibility that other birds in the area could be infected.
Earlier, the dead bird had been tentatively identified as a mute swan, a species that is native in Britain and only migrates in exceptional circumstances.
Scientists struggled to confirm its species because the carcass was badly decomposed and its head was missing, meaning beak markings could not be noted.
Mr Milne said many whooper swans had been tested over the past few months, and all had been negative. He said the birds originated from outside Britain but around 7,500 migrate to UK shores during the winter months.
"We are working on the assumption that the bird migrated to this country, but it's impossible to say precisely where it died," he added.
He said the implications of the DNA test results needed to be considered before it was considered whether any further restrictions should be brought in.
"There's some significance in the fact that whooper swans are long distance migrants," he added. "They do potentially move around more than mute swans. But we need to discuss these issues to get a full understanding of what we are dealing with."
A 3km protection zone and 10km surveillance zone have been set up around where the swan was discovered, and a 2,500km sq wild bird risk area has also been established along the coastline.
The Department for Environment, Food and Rural Affairs said it would normally expect whooper swans to be leaving Britain for their summer breeding grounds at this time of year.
"The Cellardyke swan is the first highly pathogenic H5N1 avian influenza case detected in a wild bird in the British Isles, and our current hypothesis is that the swan originated outside Great Britain," Defra said in a statement.
"We know already that movement of swans associated with cold weather and on migration has been a feature of recent developments in Europe."
No other cases of bird flu have been discovered in Britain from tests on 3,397 birds in recent months, with nthat figure including 428 swans tested between February 1 and April 1, Defra said.
Professor Hugh Pennington, a microbiologist from Aberdeen University, said it was positive news that the swan was migrant rather than native.
"This whooper swan probably came from Germany and may have died off the Scottish coast as it struggled across the sea," he said. "This raises the likelihood that it had no contact with any native birds and that this case of H5N1 on our shores was a one-off."
A spokesman for RSPB Scotland said: "It possibly became ill halfway across the sea, died and was washed up, so hopefully had zero interaction with native birds.
"In terms of the unequivocal announcement last week that this was a mute swan, what this underlines is the importance of getting the species right.
"We hope the authorities will learn from this mistake, as species is vital in nailing down where the virus is travelling to and from."
Samples of the H5N1 virus found on the swan closely matched those found in dead swans in the Ruegen Islands, off the German coast.
There are two main breeding populations of whooper swans, in Iceland and in Scandinavia and northern Russia. They winter in the UK and parts of continental Europe.
Earlier today, it was reported that scientists were looking into the possibility that the swan had died abroad and floated across the North Sea to Scotland.
Citing Whitehall sources, the BBC's Today programme reported it was believed the bird "was a non-resident mute swan" that died somewhere else and "floated in".
Prof Pennington said the H5N1 virus could survive in dead animals for at least a number of days, particularly in cold conditions, but doubted the bird could have drifted from the Baltic.
"For what it's worth, I do not think it was washed all the way from the islands in the Baltic," he said. "You only have to look at the map. However, it may have been infected elsewhere on the continent and washed across the North Sea."
Mute swans breeding in the UK are largely sedentary - few movements within Britain have exceeded 30 miles.
Only in very severe winters do birds migrate from or to mainland Europe, with the last record of such movement happening during an extensive period of cold weather in Britain in 1962 and 1963.
Meanwhile, it was also reported today that the government is considering imposing targets for the time it takes to test suspected cases of bird flu.
The move follows criticism that it took eight days for the Scottish outbreak to be confirmed.
A Defra spokesman said no targets were in place but that the department had been "working on developing specific aims and a common understanding of the hierarchy and prioritisation of the work".
