Swansong - dead swan a foreigner and one and only so far

The dead swan found in Fife in Scotland on March 29th has now been identified by DNA tests as one of a migratory species, quelling fears that it was the harbinger of the first outbreak in the UK.

Scotland's chief veterinary officers say they have no idea where it had contracted the virus but believe the swan died at sea and washed up on the Scottish coast.

The news that the infected swan may be an isolated case raises hope that the virus may not have become established in domestic wild birds in Britain.

It was initially thought the bird found dead in Cellardyke harbour was a Mute swan, a species which does not migrate.

The swan was found to have the lethal H5N1 strain of bird flu and ignited fears that the disease had spread to native bird populations.

The tests have identified the bird as a Whooper swan which is common in Iceland, Scandinavia and northern Russia and spends the winter in Britain and parts of Europe.

The dead swan is the only wild bird in Britain to date found to have the H5N1 virus. The virus has led to the death and culling of some 200 million birds since late 2003.

Scientists fear bird flu could become highly dangerous to humans if the virus mutates into a form easily passed on from one person to another. It has so far killed 109 people since late 2003, most of the victims in Asia.

Scotland's chief medical officer has said the risk of the virus passing into humans is extremely low, nevertheless Scottish officials have taken steps to prevent the spread of the disease to domestic poultry farms.

Poultry owners within wild bird risk area have been instructed to keep birds indoors or else ensure they are kept away from wild birds, the transport of birds within 6 mile (10km) surveillance zone has been restricted, and poultry within 1.8 mile (3km) protection zone must be kept indoors and will be tested.

A wild bird risk area has also been established which includes 175 registered poultry premises, containing 3.1 million birds, 260,000 of which are free-range.

The government is apparently considering introducing targets to regulate the time between reporting a dead bird and tests being completed.

No other birds have tested positive for H5N1 since the discovery of the swan in Cellardyke.

The H5N1 virus cannot pass easily from one person to another and at present does not pose a large-scale threat to humans.

Experts, however, have long feared the virus could gain this ability if it mutates, triggering a flu pandemic with the potential of killing millions of humans.

Bird flu viruses apparently have 16 H subtypes and nine N subtypes and 4 types of the virus are known to infect humans - H5N1, H7N3, H7N7 and H9N2.

Most lead to minor symptoms, apart from H5N1 which has caused more than 100 deaths in Cambodia, China, Indonesia, Thailand, Turkey and Vietnam.

According to the World Health Organization not all H5 or H7 strains are severe, but their ability to mutate means their presence is always a worry.