Tests are continuing on birds around Chesil Beach in Dorset, as vets try to contain the virulent form of bird flu found in swans at a sanctuary.
Workers at the swannery have been given flu jabs as a precaution
Three mute swans found dead at Abbotsbury Swannery tested positive for the H5N1 strain of bird flu.
So far culling has been ruled out, but there are restrictions on movements of captive birds nearby.
Two more dead swans were found along the Fleet on Thursday night and will be collected by Defra later for testing.
However, John Houston of the swannery - which holds 600 swans - said there was nothing out of the ordinary about two swans being found dead in winter and the tests on them at the department's laboratory in Weybridge, Surrey, were routine.
The results are expected to be known in about two days.
He said: "It's not unusual for birds to die in the winter of natural causes. In fact, there are less dying at the moment than normal because it's quite warm.
"Until I hear otherwise I am going to assume the best."
All commercial poultry premises in the area are to be inspected by government officials for signs of the disease.
Acting Chief Veterinary Officer Fred Landeg urged all bird keepers to remain vigilant.
The outbreak was picked up by routine testing.
The Health Protection Agency is now monitoring about 12 staff at the reserve for signs of the disease, though the risk of infection is said to be low. The workers have begun a course of Tamiflu tablets as a precaution.
There are two restricted areas in place - a wild bird control area and a larger wild bird monitoring area.
The control area extends about 25km (15 miles) to the south east of the swannery, and includes the town of Weymouth, Chesil Beach and the Portland Bill headland.
The larger monitoring area of some 20 miles also covers the town of Dorchester.
Bird owners in the zones must house their flocks where possible, to separate them from wild birds.
They are permitted to move their flocks only under special licence.
Bird gatherings such as auctions are banned in the area, as is hunting wild birds.
Ornithologists say mute swans rarely migrate, so it is most likely the virus got into the colony from a passing wild bird.
This is not a major migration season, but wintry weather could have prompted some birds to fly to Britain from Europe in search of food.
Mr Houston said staff at the swannery were "very concerned", but had been encouraged by Defra's comments about the situation of outbreaks in wild ducks, where immunity had built up quickly.
He said staff entering the site were wearing protective suits, and removing them and disinfecting themselves on leaving.
Prime Minister Gordon Brown said the government would do everything it could to stop the spread of infection.
He said: "We have had to deal with this issue before and the important thing is that people know we have placed protection zones around the affected area."
Conservative MP Oliver Letwin, whose West Dorset constituency includes Abbotsbury, said he was glad to see the outbreak being "treated with the seriousness which it deserves".
"I very much hope that we will get through this with the swannery intact because it is a remarkable national institution of real beauty and real ecological significance," he said.
Dorset is not an area of extensive poultry production, and farmers' leaders say that, apart from those producers who are close enough to the outbreak to be under movement restrictions, there will be no impact on poultry supplies in the supermarkets.
The swannery is one of the area's most popular tourist attractions
One neighbouring poultry keeper expressed concern that he had not yet been given any advice by Defra on what precautions he needed to take following the bird flu outbreak.
Graham Hutchings keeps 70 chickens at his home, just 500m from the swannery.
He said: "It's a nightmare. I'm just waiting for Defra to be in touch. They are supposed to be coming to see my birds.
"Every time one sneezes or coughs, I just panic. It's all fingers crossed for the swannery and for my birds."
The discovery in Dorset is the latest in a series of bird flu cases in the UK.
In November 2007, around 5,000 birds were slaughtered after the H5 strain of avian flu was confirmed in turkeys at Redgrave Park Farm, Suffolk.
Previously, a strain was found in chickens at a Norfolk farm in April 2006 and the month before that the deadly H5N1 strain was found in a dead swan on the Fife coast.
Experts say cross-infection to humans is still relatively rare and usually occurs where people have been in close contact with infected birds.
But they say if the H5N1 strain mutates so it can be passed between humans, it could become a global pandemic.
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