By Michael Kahn

LONDON, Oct 24 (Reuters) - Controls will probably prevent the large-scale spread of bird flu from one farm to another in Britain but there is still a significant chance that the virus will escape, British researchers said on Wednesday.

Using data collected on poultry farms, feed mills and slaughter houses, the researchers ran millions of computer simulations to calculate the potential impact of the H5N1 avian influenza virus striking Britain"e;s poultry industry.

The detailed model showed that with current control strategies about 73 percent of the infections would not spread beyond the originally infected farm.

But this also meant there was a 27 percent chance a random infection would spread, though probably only to a small number of sites because of safeguards in place, said Kieran Sharkey, a mathematician at Liverpool University, who worked on the study.

"We find that although the majority of randomly seeded incursions do not spread beyond the initial infected premises, there is significant potential for widespread infection," Sharkey"e;s team wrote in the Proceedings of the Royal Society.

Even a single H5N1 outbreak is devastating because other nations often shut off poultry imports from affected nations.

Outbreaks of the virus across Europe the past year have caused poultry sales to plunge in many countries. Bird flu has killed or prompted the slaughter of tens of millions of birds worldwide since 2003.

In February, Britain confirmed it had found the H5N1 strain on a commercial turkey farm and eventually culled some 160,000 birds.

The computer model indicated the highest risk for a widespread outbreak in Britain stemmed from ducks, which are silent carriers of the H5N1 virus that has infected flocks across the world and represents the main candidate to cause a human flu pandemic.

This means they may carry the H5N1 virus but show no symptoms for some time. Many species of ducks never get ill from H5N1, although the virus kills chickens quickly.

A large outbreak starting in ducks could be spread quickly through feed or slaughter trucks, humans visiting different farms or wildlife carrying the virus to nearby farm, Sharkey said.

The study, funded by the Department for Environment Food and Rural Affairs, showed a 37 percent chance of the virus spreading to more than 50 sites if it originated in ducks on a large, operation comprising many meat-producing farms, he added.

However, this risk was all but eliminated if health officials immediately started blood testing ducks for the virus on nearby farms in control zones set up to halt the spread of infections, the researchers said.

The model, which included data from nearly 12,000 poultry farms, 174 slaughter houses and 86 feed mills, also showed the areas in Britain at greatest risk were in East Anglia, Lincolnshire and South Yorkshire.

Some of the reasons for this include the large number of duck farms in the region and the fact that many of them are located close to each other, Sharkey added.