ROME – Heightened readiness and surveillance against a possible major resurgence of the H5N1 Highly Pathogenic Avian Influenza is being urged by the Food and Agriculture Organization of the United Nations (FAO) following signs a mutant strain of the deadly Bird Flu virus is spreading in Asia with unpredictable risks to human health.

Recently affected countries are Israel and the Palestinian Territories, Bulgaria, Romania, Nepal and Mongolia.

Since the H5N1 virus first appeared in 2003, it has infected 565 people killing 331 of them, according to World Health Organization figures. 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 throughout the world before it was eliminated from most of the 63 countries infected at its peak in 2006.

The virus, however, remained endemic in six nations, although the number of outbreaks in domestic poultry and wild bird populations shrank steadily from an annual peak of 4000 to just 302 in mid 2008. But outbreaks have risen progressively since, with almost 800 cases recorded in 2010-2011.

The beginning of renewed geographic expansion of the H5N1 virus both in poultry and wild birds was marked in 2008. According to FAO Chief Veterinary Officer Juan Lubroth, the advance appears to be associated with migratory bird movements. He added migrations help the virus travel over long distances, so that H5N1 has in the past 24 months shown up in poultry or wild birds in countries that had been virus-free for several years.

"Wild birds may introduce the virus, but peoples' actions in poultry production and marketing spread it," Lubroth charged.

There appears to be a variant virus apparently able to sidestep the defenses provided by existing vaccines in China and Vietnam, he added. Vietnam suspended its springtime poultry vaccination campaign this year. But most of the northern and central parts of the country – where H5N1 is endemic – have been invaded by the new virus strain, known as H5N1 -

As a result, Vietnam's veterinary services are on high alert and considering a novel, targeted vaccination campaign this fall. Virus circulation in Vietnam poses a direct threat to Cambodia, Thailand and Malaysia. It also endangers the Korean peninsula and Japan further afield. Wild bird migration can also spread the virus to other continents.

"The general departure from the progressive decline observed in 2004-2008 could mean there will be a flare-up of H5N1 this fall and winter, with people unexpectedly finding the virus in their backyard," Lubroth said.

Countries where H5N1 is still firmly entrenched – Bangladesh, China, Egypt, India, Indonesia and Vietnam – are likely to face the biggest problems, but no country can consider itself safe, he said.