A fourth swan from the Abbotsbury swannery in Dorset is believed to have been infected with the deadly H5N1 avian flu virus. The result is to be confirmed today by the Department for Environment, Food and Rural Affairs (Defra).

The mute swan was found dead on Friday and sent for testing to the Veterinary Laboratories Agency in Weybridge.

Government vets are not surprised by the further case and believe it is likely that other swans at the important reserve near Chesil Beach may be infected.

It is not known precisely how the virus arrived in the area, though the most plausible explanation is that a duck or similar species arrived from across the Channel during the cold snap before Christmas. The birds at the swannery are fairly static and do not migrate.

Among the brids that migrate to the UK when there are cold spells on the Continent are mallard, teal, widgeon and pochard as well as gulls.

John Houston, general manager at Abbotsbury Tourism, said that the latest case delayed a return to normal business. “It puts us back to square one in terms of waiting for a 21-day clearance before we are out of the woods,” he said.

“It's disappointing but not totally unexpected this early in the outbreak. It would be wonderful if it disappeared immediately but that's unrealistic. It's going to take a while for it to be contained and weeded out.”

The all-clear will be given only after 21 days have passed without any new bird infected with the virus at the swannery.

Abbotsbury is one of the most popular tourist attractions on the south coast and is due to reopen to the public on March 15 after the winter.

It has been owned by the Ilchester Estate since the 1540s though the swannery was established by Benedictine monks who built a monastery at the site in the 1040s.

There is some relief that there is no evidence of the virus being endemic in wild birds in the area. Similarly, the virus has not been found in any chick on a commerical poultry farm or backyard flock in the area.

The existing control and monitoring zones are to remain in place. There are some 32 premises within the zone, of which 19 are free-range operations, involving a total of 24,588 birds.

In the wider monitoring area there are 34 farms, of which 17 are free-range, with a total of 111,488 chickens.

Fred Landeg, the acting Chief Veterinary Officer, is appealing to anyone in the area who might own a small number of chickens to come forward so that their birds and premises can be checked.

All birds inside the control zone - which extends 15 miles southeast of Abbotsbury and includes Weymouth, Chesil Beach and Portland Bill - must be kept indoors. Owners are also advised to keep poultry away from wild birds in the larger monitoring area, which extends to some 20 miles and includes the town of Dorchester.

Under current government regulations only owners with more than 50 birds must enlist to the official poultry register, but others are encouraged to do so voluntarily.

France has raised the level of alert for a lethal avian flu outbreak as a result of the British outbreak and ordered all birds to be locked indoors in areas near lakes, ponds and the coast. Defra said there were no such plans in Britain.

The new outbreak has renewed calls for the routine vaccination of all free-range, organic and hobby birds against the deadly flu virus.

The Elm Farm Organic Research Centre has been campaigning for such a preventive strategy for more than two years. Defra has a stock of 10 million does of vaccine but so far they have been offered only for use to save rare birds and collections in zoos.

Richard Sanders, senior policy researcher at the centre, said the latest outbreak at Abbotsbury was now “a trigger point” and gave a definite indication the virus was circulating in wild birds.

He urged Defra to release its stocks of H5N1 vaccine to start an orderly programme of preventive treatment.The alternative to vaccination was to lock up all birds a move that was “unacceptable, impractical and with some species such as geese, impossible”, he said.

“When the national mood, as voiced so loudly by Jamie Oliver and Hugh Fearnley-Whittingstall, is for quality, high-welfare poultry production, then we must do everything in our power to protect and grow the sector.”

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