Economists put the cost of a worldwide bird flu pandemic at $800 billion today as health experts described the "relentless" spread of the virus at a World Health Organisation meeting in Geneva.

Basing his estimate on the Sars outbreak of 2003, Milan Brahmbhatt, the World Bank’s lead economist for the East Asia and Pacific region, said the world would suffer a drop of 2 per cent in global GDP if bird flu takes hold across the planet, costing $800 billion.

Mr Brahmbhatt, who was speaking at the start of a three-day meeting of 600 health experts in Geneva, said most economic losses would be caused by "panic and disruption" as people tried to avoid infection by staying at home, refusing to work and shop.

Mr Brahmbhatt's forecast was accompanied by a second estimate, based on the expected impact of a bird flu pandemic in the world's richest countries. A World Bank report based on the deaths of between 100,000 and 200,000 people in America calculated a cost of $550 billion to the world's industrialised nations alone.

The bank said it could not estimate reliable costs for the word's poorer nations because their health systems were too haphazard and mortality rates were likely to be much higher.

Mr Brahmbhatt's warning was followed by a grim forecast from Lee Jong-wook, the director-general of the World Health Organization, who said that there were signs that the H5N1 virus is about to mutate to a form of the disease that can be easily transmitted by humans.

"We have been experiencing a relentless spread of avian flu; among migratory birds and domestic poultry," said Mr Lee. "It is only a matter of time before an avian flu virus - most likely H5N1 - acquires the ability to be transmitted from human to human, sparking the outbreak of human pandemic influenza."

"The signs are clear that it is coming," he said.

The start of today's conference, the first attempt to devise a global strategy for a pandemic, coincided with the announcement that the makers of Tamiflu, the most promising anti-viral treatment for the H5N1 strain of bird flu, is in talks with third parties to produce the drug.

Roche, a Swiss pharmaceutical company, said it would increase production of Tamiflu to 300 million treatments from 2007 and that it was in talks with eight pharmaceutical companies and at least two governments to find ways to mass produce the drug.

"It means a tenfold increase over the capacity in 2004 when the decision was taken to increase production," said a statement from the company, which confirmed that it is negotiating with the governments of Taiwan and Vietnam to manufacture Tamiflu.

Roche, which has been under pressure from developing countries and the US government to grant licences to make generic versions of Tamiflu, said it had been approached by 150 possible partners. Tamiflu, also known as oseltamivir, is one of a class of antiviral drugs recommended by the World Health Organisation to stockpile in case of a bird flu pandemic.

Meanwhile, in Vietnam, where 41 of the 63 known victims of the H5N1 strain of bird flu have died, health officials today pleaded with farmers to stop dumping chicken extrement into rivers to feed fish.

State media reported that large quantities of chicken excrement in Vietnam's rivers increased the risk of possible mutation and human exposure to the H5N1 virus, which can survive for up to a month in cool and wet conditions, although slightly less in water.

"Dropping chicken excrement into Tri An lake during the period when bird flu is evolving into a pandemic is extremely dangerous," said Le Hoang Sang, deputy director of Ho Chi Minh City’s Pasteur Institute, referring to a lake upstream of the Vietnamese capital where farmers drop an estimated 100 tonnes of chicken waste every day.

About seven million people in Ho Chi Minh City use water purified from the Dong Nai river, Nguyen Van Phu, director of the Saigon Water Supply Co, told the state-run Lao Dong newspaper.

Local government officials told the state-run Lao Dong newspaper that farmers had been told to stop putting chicken excrement in Vietnam's waterways at the beginning of this year's rainy season but that there was evidence that they were continuing to do so under cover of darkness.

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