VETS and researchers have launched an investigation to find the source of the Suffolk avian flu outbreak with early suspicions pointing to wild birds.

Waterfowl are among the most likely carriers because they migrate over the kinds of distances needed to move the virus around the world.

Such species have brought H5N1 to Britain before. In March 2006, a wild swan found dead in Cellardyke, Fife, had the H5N1 strain. The virus behind the outbreak at the Bernard Matthews factory in Suffolk was also confirmed last night as the highly pathogenic Asian strain, similar to that found in an outbreak in Hungary last month.

Some experts believed this pointed towards a migratory bird having brought the virus into the farm. Another line of inquiry focuses on Matthews’s plants in Hungary, although this is considered a less likely source as the outbreak was in a different part of the country on a goose farm.

John Oxford, a virologist at the London Queen Mary’s School of Medicine and Dentistry, believes the outbreak may have started with a small bird contracting the disease and getting into the turkey shed via a ventilation shaft.

However, others see this as improbable. A recent three-year study by the environment department, in which it tested thousands of wild birds for flu viruses, found only a handful were infected. All were carrying a relatively harmless strain.

Perhaps the most likely means of infection involves a combination of routes — with a worker picking up the virus from the faeces of an infected bird and carrying it into the turkey sheds.

Peter Bradnock, chief executive of the British Poultry Council, said: “Faeces on the concrete outside could have been walked in by a worker.”

Such infections are nothing new. In May last year, more than 50,000 chickens were culled after an outbreak of H7 bird flu in farms in Norfolk. An outbreak of H7 on a Dutch farm in 2003 led to 80 people being infected and the death of a vet. The Dutch government ordered the slaughter of more than 30m birds.

Worldwide the H5N1 strain has infected 270 people, and killed at least 164 — mostly in southeast Asia — since 2003. However, in all these cases the victims had been in close proximity with poultry.

So far there have been no confirmed cases of transmission between humans. However, the number of outbreaks of H5N1 among poultry has risen sharply in recent months. Thailand, Vietnam and South Korea, which had suppressed earlier infections, are among those to have reported outbreaks.

What worries health workers is the possibility of H5N1 jumping between humans — and each human infection makes this a little more likely. This is because once inside a human body, H5N1 can swap genes with human flu viruses — gaining the power to jump between people.

A study published in Science magazine last week showed how easily this could happen. Terence Tumpey, of the US Centers for Disease Control, looked at the Spanish flu virus of 1918-19 that killed more people than the first world war.

He found the key feature of the virus was the proteins that made up its outer coating. The avian precursor of the virus was coated in proteins that helped it infect cells lining the respiratory tracts of birds. Somehow it acquired new genes that changed its outer coating, enabling it to both infect people and, fatally, to jump between them.

Could this happen with H5N1? So far, no one knows. However, since 1997 three avian flu virus strains, including H5N1, have acquired the ability to infect humans from poultry.

It means, say experts, that a new flu pandemic is only a matter of time.

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