Following Britain’s first outbreak of H5N1 virus on a commerical poultry farm, explains what causes the disease, how it is spread and whether it could become a serious global threat to human health.

Why are public health experts so concerned about bird flu?

The most lethal human epidemics - including the three great flu pandemics of the 20th century - have all involved germs crossing the species barrier from birds and animals to people. Many microbiologists say a combination of four factors makes influenza potentially the most dangerous of all known viruses: it crosses the species barrier readily, it can be very virulent, killing a high proportion of those infected, it is highly contagious, spreading rapidly between people, and it mutates fast into more dangerous strains.

When did this particular strain of bird flu first affect people?

The current cause of concern is the highly potent H5N1 strain, harmful to humans, which emerged in 1997 in Hong Kong. It went on to kill more than 50 people across south-east Asia, in an outbreak which reached its height in late 2003/early 2004. The latest human death from avian flu was reported in Vietnam in August this year.

The sudden appearance of virulent bird flu on a Suffolk turkey farm seems to have taken everyone by surprise. How worried should we be?

The diagnosis of H5N1 avian flu on one of Europe’s largest turkey farms is deeply disturbing for the commercial poultry industry, after a year in which European birds had largely escaped infection by the virus. It reappeared on a Hungarian goose farm two weeks ago and has now leapt 1,500km north-west to East Anglia. This means that birds anywhere in Europe are potentially vulnerable. But for the general public the latest outbreak poses virtually no health risk.

How can you be so sure that the risk to human health is so small?

H5NI in its present form has evolved to spread easily between birds but not between people. The virus has infected many millions of birds in Asia and, to a lesser extent, Africa but according to the World Health Organisation there have only been 271 human cases, 165 of whom have died. Most of the victims have come into very close contact with infected birds in cultures where poultry and people live together – much closer than any worker on a European poultry farm.

Why is everyone saying reassuringly that it is safe to carry on eating poultry, despite the latest scare?

In the first place, no turkeys from the infected Bernard Matthews site have entered the food chain. But anyway, flu virus dies at temperatures above 70 deg C so cooking removes any remote risk of H5NI infection from eating birds or eggs.

But doesn’t this latest outbreak make it more likely that a new human flu pandemic will start?

Hardly. Of course the Suffolk outbreak is big news in Britain and Europe, but on the global scale it makes little difference. If H5N1 is to undergo the genetic changes that everyone dreads, so that it passes readily from person to person, this will almost certainly happen first in Asia or Africa, where people and poultry live in close proximity. We may indeed suffer a terrible flu pandemic that kills tens of millions of people worldwide but Britain is most unlikely to be the starting point.

How would the H5N1 virus have got into the Bernard Matthews turkey shed?

Veterinary scientists would love to know, so that they can improve biosecurity for the poultry industry. As David Catlow, president of the British Veterinary Association, says, many experts thought that the greatest risk of infection was in small-scale back-yard poultry operations, where birds are more exposed to flu virus from wild birds – not in a modern, enclosed factory farm.

Wild birds are the most likely source of infection. Their droppings contain large amounts of virus, which can survive for weeks in dust and soil. A small bird carrying the virus might somehow got into the building. More probably, the virus originated in some of the migratory waterfowl that frequent the East Anglian coast in winter – and was carried in by a contaminated worker, feed or equipment. But it is also possible that the virus came from another commercial poultry farm.

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