GPs advised how to spot bird flu as virus continues to mutate at an alarming rate Virus has taken first steps towards becoming a global threat to humans Has two of five genetic changes thought necessary for human transmission British experts warning against complacency as virus a real threat to the UK Is 'definitely something we need to be concerned about,' says leading expert GPs have been sent letters advising what action to take if they spot a case A deadly bird flu virus sweeping through China has taken the first steps towards becoming a global threat to humans, experts have revealed. In the space of one month, the avian strain known as H7N9 has spread through all 31 Chinese provinces and claimed 125 victims, killing a fifth of those infected. Scientists say it is mutating rapidly and already has two of five genetic changes believed to be necessary for human-to-human transmission. Experts speaking in London today said there was no room for complacency over H7N9, and warned against the mistake of assuming it was a far-away foreign problem. A Chinese tourist wears a face mask in front of a portrait of leader Sun Yat-sen at Tiananmen Square in Beijing. Scientists say it is mutating rapidly and becoming more of a threat to humans GPs have been sent letters advising them on how to identify cases and what action to take if one is suspected. So far the the virus has been found in chickens and only affected people who have had close contact with the birds, often at live markets. If it were to become fully adapted to human hosts it could result in a serious worldwide pandemic claiming millions of lives. The 1918 Spanish Flu pandemic - the worst flu pandemic in modern history - killed up to 80 million people worldwide and is believed to have originated in birds. Research is ongoing into how the virus is behaving and what makes it so virulent. The virus has infected people of all age groups, from two to 81, suggesting that humans have no natural immunity to it. So far 20 per cent of victims have died, 20 per cent are recovering and the rest remain ill. In fatal cases, the virus has triggered sepsis - a massive inflammatory response - leading to multiple organ failure. Leading British expert Professor Peter Openshaw, director of the Centre for Respiratory Infection at Imperial College London, said: 'This is a very, very serious disease in those who have been infected. So if this were to become more widespread it would be an extraordinarily devastating outbreak. The bird flu virus is mutating rapidly and already has two of five genetic changes believed to be necessary for human-to-human transmission 'It's very unusual to see more than 100 new cases in a very short time period. I think it's definitely something we need to be concerned about.' Scientists have learned lessons from the H5N1 bird flu strain which also emerged in China, and since 2003 has led to 628 confirmed cases in 15 countries and 374 deaths. With a 60 per cent mortality rate, H5N1 is even more deadly than the new strain. Luckily it has not yet shown any sign of making the jump to human-to-human transmission. Although there is no reason to think that H7N9 will acquire this ability any time soon, it already displays some degree of human adaptation. In H5N1, five key genetic changes were identified that scientists believe are needed for the virus to become a potential pandemic threat. Laboratory studies have revealed two of these mutations in H7N9. One of them is integral to the 'H7' part of its structure and enables the virus to latch onto certain receptor molecules in the human respiratory tract. It is seen even when the virus is confined to chickens. British experts today warned against the mistake of assuming it was a far-away foreign problem The other has only been acquired after human infection and helps the virus to replicate inside cells. How easy or difficult it might be for the other three changes to occur, and to what extent H7N9 needs the same mutations as H5N1 to pass between humans, is unknown. 'There are hallmarks of human adaptation in the virus,' said Professor Wendy Barclay, chairwoman of influenza virology at Imperial College London. What that means is that the virus is mutating rapidly still and acquiring selective adaptive mutations when it crosses into people. Although H7N9 readily infects chickens, and probably wild birds, it does not kill them - unlike H5N1. This makes it extremely difficult to track and hinders attempts to reduce the chances of human infection. One previous outbreak of an 'H7' flu strain was recorded in the Netherlands in 2003. However this was a very different sub-type of virus, since it caused non-fatal eye infections. A teacher guides a student on how to use disinfectant in a junior high school in Taipei, Taiwan Dr John Watson, head of respiratory diseases at Public Health England, the body that has taken over from the Health Protection Agency, said it was important not to assume there was no threat to the UK. 'It might be tempting to feel, fine, let's just sit back and wait,' he said. 'We feel it's important nevertheless to be taking this very seriously, because of the illness that it's caused, because of the novelty, and because of the possibilities there are about what it does next.' An ever present possibility is someone importing the virus into the UK from China. If a case were confirmed, the patient would have to be isolated and treated quickly. Details of his or her history and contacts with other people would also have to be obtained. The experts pointed out that the virus was sensitive to antiviral drugs such as Tamiflu, stocks of which remained in the UK after the swine flu pandemic of 2009-10. Currently there is no readily available vaccine against the strain.