Bird flu: It's here. But will it spread to humans?

The first sign that anything was wrong came on Tuesday morning. On the Bernard Matthews farm at Holton in Suffolk, 159,000 young turkeys were being fattened for the plate, living permanently under the low roofs of 22 giant sheds. But in among the burbling eight-week-old poults that morning were 71 dead birds.

Some birds will always die, however careful the biggest producer in Europe is to raise his trademark "bootiful" turkeys, but this was serious enough to be noticed. The next day there were 186 deaths, but it was on Thursday that the crisis became critical, with 860 bodies lying among the flock.

By the time vets from the Department for the Environment, Food and Rural Affairs made it up to the farm near Southwold on the Suffolk coast that night, 2,600 birds were dead. This was very serious, the experts knew. If the H5N1 strain of avian flu were to blame they were facing the first outbreak on a British farm. Anyone coming into contact with the faeces or skin of the birds in those crowded sheds would be at risk of dying, like the 164 people who have been killed since H5N1 began to wipe out flocks of birds in Asia in 2003.

In Turkey, children caught bird flu and died after playing with the severed heads of sick chickens. Bernard Matthews runs his 57 British farms in a very different way to that: the company says it "meets and exceeds" official standards for protecting against bird flu. But Defra was still worried enough to begin monitoring the health of the workers at Holton.

The risk to the general public is "negligible", it insists. But the big dread every time a new outbreak of H5N1 is announced anywhere in the world is that this will be the time it mutates. That means the virus incubating inside a human body and developing a new, far more virulent strain that is not resistant to any known drugs. Experts say the resulting epidemic could kill millions of people.

"It's not just a national problem; people in Holland, France, and Germany will all be rather quaking in their shoes as well," said John Oxford, a professor of virology at the Queen Mary School of Medicine, yesterday. And even if nobody dies there are likely to be dire consequences for the poultry industry as a result of the scare. A year ago a wild swan was found with H5N1 in Fife, Scotland, having died at sea and been washed ashore; in May more than 50,000 chickens were culled in Norfolk when some had the H7N3 strain. The National Farmers' Union believes these two stories cost £58m in sales of chicken and turkey meat and related products, as people played safe.

"There is no risk to consumers at all," insisted Bart Dalla Mura, commercial director for Bernard Matthews, yesterday. The birds had lived in the shed all their short lives and never left; they had not entered the food chain. But still, however irrational it may seem to the scientists, the company must have known as it waited for the test results that H5N1 would mean its sales taking a hammering.

Blood from the corpses was sent down to the Veterinary Laboratories Agency in Weybridge, Surrey, one of the leading labs for such work in the world. It was here that the Turkish birds were tested, on behalf of the European Commission. On Friday night the laboratory confirmed that it was avian flu, but did not specify the strain. Then just after 11am yesterday the news that everyone feared was confirmed in an email from the EC: it was H5N1.

"There is absolutely no need for panic or hysteria," said Conservative MEP and bird flu expert Neil Parish. But people living close to the farm, which is in open countryside a few miles from the market town of Halesworth, were shocked. "I am worried for people who work at the factory," said Lillian Foreman, 43, of Holton. "What will happen to them? If turkeys started dying on the Tuesday why wasn't Defra notified then?"

Yesterday Defra had stopped all but essential movement in and out of the farm, and visitors were being disinfected. Vets were making preparations for the slaughter of all 159,000 birds, and taking very careful precautions, such as the use of masks and protective suits. Anyone with poultry within 10km (six-miles) of the farm was told to keep them inside and out of the way of wild birds, ensuring water was not shared with them. Birdwatchers at a wetlands area close to the farm were replaced by Defra officials keeping a close eye out for signs of avian flu.

Villagers were not confined to their homes, however, as they had feared. "People are coming and going as they please," said Melanie Shenton, landlady of the Lord Nelson Inn, the only pub in the village. "We've seen no officials, only loads of press."

Suffolk County Council was confident, saying: "This is an animal disease. There is a negligible effect on human health." But Marilyn Hatcher, 53, of Holton said: "It is frightening for this to have happened on our doorstep. Local people will be asking questions."

The rest of the country wanted answers, too. Reassurances from Defra that the virus had probably been contained inside the farm and would be eradicated by the slaughter did little to prevent fears that it may yet spread. The most pressing question, which scientists were still attempting to answer last night, was where did the virus come from and how did it get inside the turkey shed.

So far H5N1 has spread through two main routes: other birds and the international agricultural industry. Most experts blame it on the migration of wild flocks. They say the key moment was the death of thousands of wildfowl from the virus in the summer of 2005 at a remote body of water in north-west China called Qinghai Lake.

It was predicted at the time that any birds surviving there would spread the virus as they returned to their wintering grounds. Sure enough, that autumn H5N1 spread progressively westward, first through Russia and then to Turkey. It reached Romania, then turned up in Egypt and Nigeria. A year ago, cold snaps drove wild birds carrying H5N1 across Europe, as far as Germany and France. Soon after this the dead swan infected with the virus was found in Fife.

The contrary theory is that bird flu is mainly spread by the global trade in chickens, eggs, and other "poultry products" such as manure, and that it flourishes in factory farms. This view has been promoted by bird charities, which have an obvious interest in minimising the culpability of their feathered friends - but it also has some independent support.

A year ago research published in the official journal of the US National Academy of Sciences suggested that the trade was mainly to blame for the virus spreading from China to Vietnam and Indonesia, where it has killed the most people.

At around the same time a report by an international agricultural pressure group - Genetic Research Action International - concluded: "The deadly H5N1 strain of bird flu is essentially a problem of industrial poultry practices. Its epicentre is the factory farms of China and Southeast Asia ... and its main vector is the transnational poultry industry, which sends the products and wastes of its farms around the world."

The report did not deny that wild birds carry the disease, but said that they have it only in a mild form before its gets into the factory farms, which are "then ideal breeding ground" for deadly strains. In the past 30 years, battery chicken farming has multiplied eightfold in Thailand, Vietnam and Indonesia. In China, where H5N1 is thought to have originated, battery farming tripled during the 1990s alone.

Qinghai Lake was surrounded by many intensive poultry farms and is near a fish farm, where chicken faeces may have been used as food. The report adds that the outbreak in eastern Turkey occurred only two weeks after a visit by a truck selling off old chickens from a factory farm.

The protection zone extended around the Bernard Matthews farm by Defra included some of the reedbeds of Walberswick where wild birds flourish. But the sheltered life of the infected birds at the farm lessens the likelihood they could have been infected by migrating outsiders. Another theory is that the virus could have been carried into the shed though poor bio-security - perhaps on a worker's boots after stepping in an infected wild bird dropping. But the company's pride in its bio-security argues against that.

The Suffolk outbreak casts grave doubt on the Government's repeated claims to lead the world in its defences. Last year Professor Sir David King, the Government's chief scientist, boasted that "we are probably better prepared than any other nation". The UK strategy is based on "early detection" of the disease, through monitoring wild birds and "prompt action" when it is found. But if the disease was brought by wild birds, as officials seem to believe is most likely, the monitoring failed to pick this up. Professor Hugh Pennington of Aberdeen University - the immediate past president of the Society for General Microbiology and one of the country's foremost experts on the disease - says that the Government's much-vaunted "surveillance programme" is "just scratching the surface".

Though the turkeys started dying on Tuesday - and over 2,600 had perished by Thursday night - it was not until yesterday that the Government began to take action. And the measures taken seemed at first to be too little, as well as too late.

Last year, as the virus began to spread through Europe, other countries, from Sweden to Austria and France, brought their poultry indoors to stop them mixing with wild birds, usually before the virus reached them.

British ministers refused even to consider following suit until the virus reached this country. But yesterday, when it had indisputably arrived, they did so first only in a zone of a 10-kilometre radius around the infected farm, and then in a 2,090 sq km "restricted zone" in east Suffolk and south-east Norfolk.

They could have argued that, since the turkeys had become infected in an enclosed shed, this was unnecessary. Sensibly, they did not. But once they had decided to take the measure it seems illogical to have confined it to such a restricted area.

Birds fly around freely, not restricting themselves to the zones that governments draw up. As it is unlikely that the turkeys were so unlucky as to be infected by the only diseased bird in Britain, there may be a lot of them with the virus out there. If poultry are to be brought indoors at all, it would make sense to do it nationwide.

Whatever answers come out of the labs and investigations as to how the turkeys became infected, there will clearly be more questions to follow.

Health: 'This is not an issue of food safety'

The six big supermarkets reassured customers yesterday that poultry products are safe:

Sainsbury

"Bird flu is not a food safety hazard. If there is any risk to poultry we will remove it. We would advise against any inappropriate hysteria."

Waitrose

"High standards of biosecurity are in place at our farms. We will continue to monitor the situation. But bird flu is not a food safety issue."

Tesco

"Our poultry and egg products are safe. The disease can only be spread by direct contact with birds, not through poultry products."

Morrisons

"The affected birds have not entered the food chain and the outbreak was contained to the farm. Defra says there is no reason for concern."

Asda

"We do not stock turkeys from the farm, which is one where chicks are fattened up. None of the birds has entered the food chain."

Marks & Spencer

"None of our suppliers is within a three-mile radius of the farm. We are keeping an eye on the situation and following Defra's advice."