About 2,500 turkeys died of the virus on the farm owned by Bernard Matthews PLC, Europe's largest turkey producer. It was the first time H5N1 has been found on a British farm.
As scientists worked to trace the source of the outbreak, Bernard Matthews said the first farm birds had died Tuesday. The State Veterinary Service said the cause initially was diagnosed as E. coli poisoning, but after more turkeys died, authorities were called Thursday and began tests for bird flu.
The Department for Environment, Food and Rural Affairs, or DEFRA, said the virus found on the farm was the highly pathogenic Asian strain, similar to that found January in geese in Hungary — the first known bird flu outbreak within the European Union since August 2006.
Bird flu has killed or prompted the culling of millions of birds worldwide since late 2003, when it began ravaging Asian poultry stocks. It has killed at least 164 people worldwide, but remains difficult for humans to catch.
Professor Pat Troop, chief executive of Britain's Health Protection Agency, told the British Broadcasting Corp. that the public had little to fear as the virus "doesn't pass easily from bird to human."
She said people who had been affected by the disease had lived and worked in very close proximity to birds. Workers involved in the culling and all those who could have come into contact with sick birds were being given an anti-viral drug as a precaution, authorities said.
Health Secretary Patricia Hewitt said the government took seriously the risk the disease could mutate into a form that was easily passed between humans.
"We are preparing very, very seriously and thoroughly for the possibility of a pandemic flu," she said. "It is a very remote risk, but if it did happen it could be very serious indeed."
Government vets worked through the day to gas all 159,000 turkeys on the farm in Suffolk county, about 130 miles northeast of London. The carcasses were being transported in sealed trucks to a site in central England for incineration.
DEFRA announced new measures to control the movement of birds in areas surrounding the farm. A 806-square-mile restriction zone was set up in which all commercial birds must be isolated from wild birds and all movement of poultry must be licensed.
Closer to the farm, authorities implemented a 1.5-mile protection zone, to which access is restricted, and a 6-mile surveillance zone.
The restrictions, though "difficult for poultry farmers, will stop this disease from spreading," said Jill Korwin, assistant head of Trading Standards for Suffolk. "Poultry owners must keep their birds under cover."
The outbreak is the first known instance of H5N1 in Britain since an infected wild swan was found in Scotland in March. Turkeys and chickens are more susceptible to H5N1 than wild birds, who can carry the virus over long distances without showing symptoms.
The European Commission said EU food and animal health experts would discuss the outbreak on Tuesday and review British measures to contain the disease.
Dr. David Nabarro, senior U.N. system coordinator for bird flu and human flu, told The Associated Press that bird flu outbreaks would increase around the world, but said the virus was unlikely to spread from the affected farm in Britain.
"From what I picked up during the last three days, the U.K. government has instituted the proper precautions... This should mean that (it) won't spread out into other parts in the vicinity," he said in an interview in Indonesia, the country worst hit by bird flu.
Last year, H5N1 was discovered in countries in Africa, the Middle East, Central Asia and Europe. The World Health Organization has warned that a repeat is possible this year, encouraging countries to remain on high alert.