The highly infectious H5N1 strain of the disease is responsible for the deaths of hundreds of thousands of wild birds and millions of domestic ones.

The H5N1 strain is deadly and can spread through entire flocks of domestic birds within a matter of days, through birds' droppings and saliva, or through contaminated feed and water.

"160 million domestic birds worldwide have been killed by this virus, or have had to be culled by farmers to contain it," says Professor Munir Iqbal of the UK's Pirbright Institute, which specialises in animal welfare.

More wild birds than ever before have been killed by bird flu this year - with sea birds being especially hard hit.

The current virus has affected 80 different bird species," says Professor Iqbal.

Dr Nancy Beerens, bird flu expert at Wageningen Bioveterinary Research in the Netherlands, which analyses suspected bird flu samples, says the virus may now be ubiquitous in wild birds.

"As the virus now has infected many wild bird species, it becomes unlikely that it will disappear again from the bird population," she says.

Farmers in the UK and France have also been told to bring free range poultry flocks indoors, to stop them being infected by wild birds.

In some instances, humans have caught bird flu when they have come into close contact with infected birds.

However, he says: "We need proper surveillance of how far it is spreading, by monitoring wild birds and getting reports from vets dealing with domestic animals."?