A worker in protective clothing starts the slaughter of birds at a poultry market in Shanghai

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It was thought that the H7N9 virus couldn't be passed between animal species and could only be contracted by exposure to infected poultry. But health officials in Shanghai, who have studied its genetic sequence, believe it has now mutated into a different strain, is spreading much more easily between different animals and may have entered the wider food chain. That means the virus is much more likely to be transported beyond China and could eventually mutate further into a form which can be passed directly between humans. Airline stocks plunged around the world amid fears that the virus would wreak havoc on international travel, as the swine flu virus did when it sparked a human pandemic four years ago.

A child with the flu receives treatment at a hospital in Hefei

The death toll from a new strain of bird flu mounted to six today, spreading concern overseas

British Airways owner International Airlines Group fell 6% or 14.69p to 237.61p, Thomson owner TUI Travel plunged 14.12p or 5% to 299.08p, and easyJet shedding 6% or 68.75p to 1028.25p. It came as authorities ordered the slaughter of all poultry at a Shanghai market where the virus was first detected. Animal health officials in protective overalls and masks worked through the night at the market, taking notes as they stood over piles of poultry carcasses in plastic bags. The area was guarded by police and cordoned off with plastic tape.

A vendor puts on protective suit before walking into a wholesale market from which H7N9 was found

Authorities in Shanghai closed all poultry markets and began culling birds

The city of Shanghai also announced a suspension of the sale of live poultry starting Saturday, city spokesman Xu Wei said at a news conference. David Hui, an infectious diseases expert at the Chinese University of Hong Kong, said: "In the past usually you would see chickens dying before any infections occurred in humans, but this time we've seen that many species of poultry actually have no apparent problems, so that makes it difficult because you lose this natural warning sign."

Slaughtered birds are thrown into rubbish sacks in China today

An emergency response plan has been activated following two deaths of the H7N9 strain of bird flu

He added that the disease, which initially appeared in pigeons - a popular meat in China - had now probably spread to other poultry in the human food chain.



Scientists are alarmed that H7N9 is mutating because it was proven last year that it takes just five genetic mutations for a potentially pandemic strain of bird flu - one that can be passed between humans - to evolve. As well as the deaths, 16 people have been left seriously ill in China since the first cases were announced on Sunday, two of which only emerged today.

A patient with fever is accompanied by a family member while receiving treatment at the hospital

Staff from the animal disease prevention and control center inject a chicken with the H5N1 vaccine

In the past usually you would see chickens dying before any infections occurred in humans, but this time we've seen that many species of poultry actually have no apparent problems David Hui, an infectious diseases expert at the Chinese University of Hong Kong

The latest death was a 64-year-old farmer in the eastern city of Huzhou. Authorities said Thursday the virus also killed a 48-year-old man who transported poultry for a living and a 52-year-old woman, both in Shanghai.



Several among the infected are believed to have had direct contact with fowl.

Chinese authorities slaughtered over 20,000 birds at a poultry market in Shanghai

A vendor pushes a cart loaded with slaughtered chicken at Huannan Market in Taipei