Fears of a resurgence of the deadly bird flu virus have been raised by the United Nations, saying wild bird migrations had brought it back to previously virus-free countries.

A mutant strain of the disease is said to be spreading through Asia and at least eight people have died in Cambodia this year of bird flu.

The new strain of the H5N1 virus is spreading in China and Vietnam and can apparently sidestep defences of existing vaccines, the U.N. Food and Agriculture Organisation said in a statement yesterday.

It urged greater surveillance to ensure that any outbreaks are contained, although it is still uncertain though whether the mutant virus can be passed on to humans.

Last week, the World Health Organization reported that a six-year-old Cambodian girl had died on August 14 from bird flu, the eighth person to die from H5N1 avian influenza in the country this year.

Vietnam suspended its springtime poultry vaccination this year, FAO said.

Most of the northern and central parts of the country where the virus is endemic have been invaded by the new strain.

Elsewhere, FAO says bird migrations over the past two years have brought H5N1 to countries that had been virus-free for several years, including Israel, the Palestinian territories, Bulgaria, Romania, Nepal and Mongolia.

‘Wild birds may introduce the virus, but people’s actions in poultry production and marketing spread it,’ said FAO’s chief veterinary office Juan Lubroth in urging greater preparedness and surveillance.

WHO says globally there have been 331 human deaths from 565 confirmed bird flu cases since 2003 when it was first detected.

The virus was eliminated from most of the 63 countries infected at its peak in 2006, but it remained endemic in six countries: Bangladesh, China, Egypt, India, Indonesia and Vietnam.

The number of outbreaks in poultry and wild bird populations shrank from a high of 4000 to 302 in mid-2008, but outbreaks have risen progressively since, with almost 800 cases reported in 2010-2011, FAO said.

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

But leading microbiologist, Professor Hugh Pennington, says that while there is no evidence that the flu is yet mutating to spread from person to person it is a ‘reasonable speculation’ that it has become resistant to Tamiflu – the main drug used in the UK to fight the virus.

The British and Scottish Governments spent over £100m on over 50 million doses of Tamiflu – enough for 80 per cent of the population

The emeritus professor of bacteriology at Aberdeen University had warned last year that deadly flu pandemics with special antiviral drugs could create an even more dangerous, untreatable ‘superflu’ after the UK expanded anti-viral stocks from 35 million to 50 million.

The Scottish government’s effort to combat a possible pandemic is also heavily reliant on Tamiflu, an oral drug produced by the Swiss manufacturer Roche, despite warnings that it may be ineffective.

“Tamiflu should only be given to high risk cases. I think it is a reasonable speculation to make – given the new reports – that Tamiflu has been handed out inappropriately and the virus has developed a resistance to it. If that is the case that would mean that the UK’s stocks of the drug would be largely ineffective,’ said Professor Pennington.

‘The idea that Tamiflu was a cure-all was plainly wrong.

‘There is no need to panic yet because there is no evidence that the virus is spreading person to person and there are other anti-virals – such as Relenza – that do have an effect.

‘There is also no evidence that the virus is becoming vaccine resistant. A new vaccine will have to be made, but that is to be expected. Flu is always changing and it would be staggering if this flu didn’t. Vaccines have to be constantly updated.’

However, the UK Government has stockpiled only about ten million doses of Relenza and it is more difficult to take. While Tamiflu can be swallowed as a pill at home, Relenza is a powder that must be administered with an inhaler, normally under medical supervision.

But Professor Pennington warned that stocks of Relenza had to be increased. ‘We have to get as much Relenza as possible because we don’t know how effective Tamiflu will be,’ he said.

‘With flu the best advice is to prepare for the worst and hope for the best.’

Prof. Pennington, a past president of the General Society for Microbiology, said it was also ‘impossible’ to predict when this year’s flu would strike.

Dr Steve Gamblin, the joint head of molecular structure at the National Institute for Medical Research, has also previously warned that the mutation renders Tamiflu about 250 times less effective than it should be.

A spokesperson for the Health Protection Agency, told the Mail Online: ‘H5N1, like other influenza viruses, is constantly changing so that over a period of time new subtypes will emerge.

‘New strains may not be sensitive to the vaccine currently used to protect poultry in these areas but there is currently no indication of the transmission of this new virus to humans.

‘The HPA is monitoring the situation carefully with colleagues from the WHO and others.

‘In England the HPA has in place a network of laboratories capable of testing for H5N1.

‘Instructions and advice are available on the HPA’s website to help clinicians in dealing with travellers who are suspected of having H5N1.

‘The agency also has agreed plans with Defra for dealing with the human and public health issues that may arise from a possible outbreak of avian influenza in birds in the UK (including H5N1).’