Poultry producers worldwide have been urged to step up their vigilance for avian influenza following the emergence of a new strain of the disease in the Far East.

A statement issued this week by the Food and Agriculture Organisation in Rome warned that a new variant virus, “apparently able to sidestep the defences provided by existing vaccines”, had emerged in China and Vietnam.

Known as H5N1-2.3.2.1, it had invaded most of the northern and central parts of Vietnam, after the country suspended its spring-time poultry vaccination campaign. The FAO warned that the mutant strain was spreading in and beyond Asia, “with unpredictable risks to human health”.

FAO chief veterinary officer Juan Lubroth said the spread was associated with wild bird migrations, which helped the disease travel over long distances. “Wild birds may introduce the virus, but people’s actions in poultry production and marketing spread it,” he said.

The countries where H5N1 is still firmly entrenched – Bangladesh, China, Egypt, India, Indonesia and Vietnam – were likely to face the biggest problems, but no country could consider itself safe.

According to World Health Organisation figures, the H5N1 virus has infected 565 people worldwide and killed 331 of them since it first appeared in 2003.

It has also forced the culling of more than 400 million domestic poultry and caused an estimated $20 billion of economic damage across the globe.

The virus was eliminated from most of the 63 countries infected at its peak in 2006, but has remained endemic in six nations, although the number of outbreaks shrank steadily from an annual peak of 4,000 to just 302 in mid-2008.

But outbreaks have risen progressively since then, with almost 800 cases recorded in 2010-2011. “The general departure from the progressive decline could mean that there will be a flare-up of H5N1 this fall and winter,” said Mr Lubroth.

Professor Robert Booy from the Centre for Immunisation Research and Surveillance in Sydney acknowledged that the H5N1 vaccine used for poultry had become less effective due to genetic change increasing the virus’s resistance.

But there was no evidence that the newly mutated virus recently found in Vietnam could pass between humans more easily.

• A new study by the University of Wageningen in Holland has concluded that culling all birds within a 3km radius of an outbreak of AI is more effective than vaccinating them. While vaccination is cheaper, culling brings AI under control more quickly.