Three dead swans that were found in a bird sanctuary in Dorset had the deadly avian flu virus H5N1, it was confirmed yesterday.

The virus had been circulating in British birds for more than two weeks and may already be endemic in the wild bird population, experts have said. Tests are under way at the Veterinary Laboratories Agency in Weybridge, Surrey, to establish just how long the virus has been present in the mute swans from Abbotsbury.

The Times has learnt that one of the birds that was submitted to the agency by the swannery for routine testing was found dead on December 27, just over two weeks ago. The bird was put into cold storage. A second bird was found injured on December 30 and was also placed in storage. A third swan was found on January 4, but it was not until Monday that they were sent for testing.

Health chiefs appealed for calm and reminded the public that it was extremely difficult for humans to contract the virus, but 12 staff at the swannery have been given Tamiflu, the antiviral treatment.

The swannery, next to Chesil Beach, a tourist attraction, has more than a thousand birds: it is not uncommon for staff to find a dead one. No alarm was raised because large numbers were not affected. At least one of those infected was found with a broken leg. It was humanely killed, but showed no obvious symptoms of any virus.

Staff at the swannery were astonished to learn that the birds were infected with the avian flu virus. January is not a migration period and there has been no cold snap on the Continent to force birds to cross the Channel. The birds at Abbotsbury do not migrate.

Test results are expected today on two more dead swans found near Abbotsbury. Currently there are no plans to cull the other swans at the conservation site. Wild birds are being tested and monitored in a control zone that extends 15 miles southeast of Abbotsbury, in an area includng Weymouth, Chesil Beach and Portland Bill.

Every poultry farm in the area will be inspected but there is no evidence that the virus has infected chickens or turkeys. A ban on shooting wild and game birds is also in place.

Government vets want to establish how long the virus has been in the country and how far it may have spread in wild birds. They are also attempting to discover if there is any link with the flu strain found in turkeys on the Norfolk-Suffolk border in November. One theory for that outbreak was that wild birds, which gathered at an ornamental lake next to an organic farm, had infected the outdoor turkeys.

It is clear that the virus has been in some birds but, until yesterday, had escaped routine surveillance. Sampling of dead and injured birds has been going on for months without positive results. Ornithologists suggest that this means that any infected birds have a low level of the virus and are able to carry it without showing signs of infection.

If experts find that the disease has been circulating in wild birds for nearly three months, owners of free-range and organic poultry farms may be forced to review their business, especially if their birds are reared near where wild birds congregate.

Fred Landeg, the acting Chief Veterinary Officer, said: “While this is unwelcome news, we have always said that Britain is at a constant low-level of risk of introduction of avian influenza. Our message to all bird keepers is that they must be vigilant, report any signs of disease immediately and practise the highest levels of biosecurity.”

Mark Avery, of the Royal Society for the Protection of Birds, said: “If H5N1 was going to arrive in the country then Abbotsbury is a prime location for it to manifest itself. The swans are particularly vulnerable to disease.”

The public have been asked to report any sightings of dead, dying or sick, wild birds to the helpline of the Department for Environment, Food and Rural Affairs, on 08549-335577.

This is the second highly pathogenic H5N1 case detected in a wild bird in the British Isles. The previous case was in a swan found in Fife in April 2006.

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