DEFRA officials have been unable to identify the source of the H5N1 virus in the recent Dorset mute swan outbreak, according to its epidemiology report which was published yesterday, the same day as officials confirmed a seventh positive swan.

While it has not been possible to conclusively identify the source of the infection, state vets suspect the most likely hypothesis is that it was introduced by an infected migratory wild bird.

However, the investigation did establish that the strain of the virus is similar to those isolated in continental Europe in the latter part of 2007.

Based on testing and surveillance, there is currently no evidence to suggest widespread disease in the wild bird population, but poultry keepers in the area are still being reminded to remain vigilant and report any signs of disease immediately.

The current outbreak has been confined to seven birds found at the Abbotsbury Swannery, which DEFRA stresses represents a very low level of infection in the wild bird population on the site.

Targeted surveillance for avian flu in wild birds is in place throughout the UK and focuses on species that are most likely to spread the disease and on high risk areas where there is a greater abundance of poultry and waterfowl.

There have been over 2000 patrols undertaken since the start of this migration period in September/October at over 200 sites.