A leading bird flu expert said there was no evidence that the virus was showing any signs of mutating into a form that would be more infectious in humans.

But Ian Brown, head of avian influenza at Britain's Veterinary Laboratories Agency, an official lab that tests for the virus, also said that it was extremely difficult to track bird flu as it mutated.

Although hard for humans to catch, scientists fear bird flu could mutate into a form that can pass easily between humans, causing a pandemic.

Brown was asked if the bird flu virus is showing any signs of mutating into a form that would be more infectious in humans.

"There is no evidence at the moment," he said. "We know that this virus will mutate under normal circumstances, but we have to stress that we don't understand fully what would make this virus more dangerous."

Brown was speaking as tests showed that a wild swan found dead in Scotland had the lethal H5N1 strain of bird flu. Britain becomes the 14th country in the European Union to find on its territory a disease that has been blamed for 109 human deaths elsewhere since 2003.

Bird flu remains essentially an animal disease, but can infect people who come into direct contact with infected birds.

Speaking after a symposium on bird flu attended by veterinary scientists from around the world, the symposium's co-chair Illaria Capua, based at a laboratory in Padua, Italy, was asked about the possible threat of infections of bird flu in cats.

"The infection of cats is in fact a limited occurrence, which is due to the spillover of infection from birds," Capua said. Brown said there was little research into the threat of the spread of bird flu from cats.

"It would be highly speculative without any scientific data to make a statement on what threat that poses for the future," he said.

"Where you don't have infection in poultry or wild bird populations extensively, the risk of infection in cats is low and that has been demonstrated," he said.

"There has been no evidence of human infection as a result of exposure to infected cats."