Scientists in the Netherlands have developed a new strain of bird flu, that, if released, could kill millions.

H5N1 - better known as bird or avian flu - can kill humans, but has not gone pandemic because it cannot spread easily among us, according to New Scientist.

But, this could change - five mutations in just two genes have allowed the virus to spread between mammals in the lab. Despite the mutations, the virus remains just as lethal.

"The virus is transmitted as efficiently as seasonal flu," Ron Fouchier of the Erasmus Medical Centre in Rotterdam told New Scientist.


H5N1 evolved in poultry in east Asia and has spread across Eurasia since 2004. In that time 565 people are known to have caught it and 331 have died. During that time, no strain that spreads easily among mammals has developed, despite millions of infected birds, and infections in people, cats and pigs.

Efforts to create such a virus in the lab have failed, and some virologists think H5N1 simply cannot do it.

Until now.

Fouchier's team first gave H5N1 three mutations known to adapt bird flu to mammals. This version of the virus killed ferrets, which react to flu viruses in a similar way to humans. However, the virus did not transmit between them.

Then the researchers gave the virus from the sick ferrets to more ferrets - a standard technique for making pathogens adapt to an animal. They repeated this 10 times, using stringent containment. The tenth round of ferrets shed an H5N1 strain that spread to ferrets in separate cages - and killed them.

The process yielded viruses with many new mutations, but two were in all of them. Those plus the three added deliberately "suggest that as few as five [mutations] are required to make the virus airborne", says Fouchier.

"That it has not adapted doesn't mean it cannot," said Jeffery Taubenberger of the US National Institutes of Health in Bethesda, Maryland, who studies how a bird flu became the deadly pandemic of 1918.

"It simply means that so far it has not - luckily for us."