The United Nations Food and Agricultural Organisation, or FAO, is warning that the bird flu virus is spreading again and some strains have mutated and are immune to known vaccinations.

The FAO says that outbreaks of the highly infectious strain of Avian Influenza are beginning to rise again – with 800 new outbreaks of H5N1 recorded in 2010 and 2011.

The FAO’s head veterinarian, Juan Lubroth, says the bird flu has been returning to countries that have been virus free for several years.  

It has recently been detected outside of its traditional Asian home in Palestine, Bulgaria, Romania, Nepal and Mongolia.

Meanwhile in China and Vietnam a new mutant strain has evolved which Lubroth says cannot be controlled by existing vaccines.

Since 2003 the H5N1 virus has led to the culling of more than 400 million domestic poultry and caused $20 billion in financial losses.

It has also jumped species and infected 565 humans – killing 331 of them.

Eight Cambodians have died in 2011 after being infected with the bird flu.