The Scottish Executive ordered poultry to be kept indoors throughout a 965 sq mile (2,500 sq km) high-risk zone stretching north of Dundee and Perth, as it emerged that a further 14 wild birds were being tested for the deadly virus.

Scientists said that it was highly unlikely that the swan that died of bird flu more than a week ago in the fishing village of Cellardyke was an isolated case. It was likely to be part of a cluster infected by migrating wildfowl.

The bird is understood to be a mute swan, a native species that does not normally migrate long distances, suggesting that it contracted the virus from another bird arriving from the Continent.

Charles Milne, the Chief Veterinary Officer for Scotland, said that the wild bird high-risk zone would cover 3.1 million domestic birds kept on about 175 poultry farms.

The owners of 48 free-range farms will have to bring their 260,000 birds indoors, or take measures to ensure that they have can have no contact with wild birds. The directive will also prohibit bird markets and shows in the area, and provides for enhanced disease surveillance of wild birds.

The protection zone will stretch from the Forth Road Bridge, north of Edinburgh, to Stonehaven in the north east and Perth in the north west. It greatly expands the 3km (1.8 mile) radius protection zone and 10km (6.2 mile) surveillance zone established around Cellardyke on Wednesday night when preliminary tests showed that the swan had died of an H5 virus.

Ross Finnie, the Scottish Environment and Rural Affairs Minister, said that further restrictions might follow if there were more outbreaks.

The move was announced after tests at the Veterinary Laboratories Agency (VLA) in Weybridge, Surrey, confirmed that the Cellardyke swan had the H5N1 strain of the influenza virus, the deadly form that has devastated poultry and wild birds across Asia and caused 192 human cases and 109 deaths.

The dead bird was first reported on Wednesday, March 29, and was collected the next day by animal health officers. Samples were received by the VLA on Friday, March 31, but testing did not start until Monday, April 3.

Dr Milne defended the delay, saying that there was no reason to have expedited testing, given that more than 1,000 birds had been investigated without finding the H5N1 virus.

“We have to remember that the dead bird was found as part of routine surveillance,” he said. “The bird was reported after five o’clock on March 29. In line with standing instructions it was collected the next day and dispatched to the VLA, where it was received on March 31, again in line with standing instructions.

“Laboratory investigations started on Monday. We have to allow for the proper laboratory investigations of these cases. We have to be sure that these results are robust. Results were received yesterday afternoon and it was at that point it was announced that we had suspicion of a highly pathogenic H5 avian influenza.”

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