The world should be on high alert for the return of avian influenza amid signs that a mutant strain of the deadly virus is spreading in Asia, the United Nations has warned.

The variant virus, known as H5N1–, has appeared in Vietnam and China and is able to sidestep the defences provided by vaccines.

Virus circulation in Vietnam poses a direct threat to Cambodia, Thailand and Malaysia as well as endangering Korea and Japan. Wild bird migration can also spread the virus to other continents, the United Nations’ Food and Agriculture Organisation chief veterinary officer Juan Lubroth has said.

“Wild birds may introduce the virus, but peoples’ actions in poultry production and marketing spread it. Preparedness and surveillance remain essential. This is no time for complacency. No one can let their guard down with H5N1.”

The H5N1 virus has infected 565 people since it first appeared in 2003, killing, according to World Health Organization.

The latest death occurred earlier this month in Cambodia, which has registered eight cases of human infection this year – all of them fatal.

Since 2003, H5N1 has killed or forced the culling of more than 400 million domestic poultry and caused an estimated $20 billion of economic damage across the globe before it was eliminated from most of the 63 countries infected at its peak in 2006.