Scientists warn that Tamiflu use could devastate wildlife and trigger a second, deadlier pandemic.

Britain faces an ecological catastrophe that could wreak havoc on wildlife populations when the first outbreak of Asian flu hits the country.

Scientists say they fear that tons of the anti-viral agent Tamiflu - taken by Britons trying to combat the disease - would be flushed down sewers into rivers and lakes.

Natural populations of microbes would be killed off by a deluge of water polluted with concentrated amounts of the anti-viral drug. As a result, birds, fish and other creatures that rely on these bacteria and viruses for their survival could be devastated.

In addition, waters containing Tamiflu would provide ideal conditions for the evolution of drug-resistant strains of bird flu virus. These strains would then infect wildfowl and ultimately human beings, triggering a second outbreak of the disease - although this time Tamiflu would provide no protection against the virus.

'Anti-viral drugs are quite new and no one has ever planned to use them in the vast quantities that are now being considered,' said Dr Andrew Singer, of the Centre for Ecology and Hydrology in Oxford. 'However, there are some very alarming environmental implications about giving out millions of doses of Tamiflu in order to combat an outbreak of Asian bird flu. These have not been considered by health authorities. This is unknown terrirory.'

The prospect of a pandemic of bird flu sweeping the world is a growing worry for scientists, doctors and health officials. They fear that the deadly flu strain H5N1, which is now established in poultry in many areas of the Far East, could soon mutate so that it infects human beings. A pandemic that would affect hundreds of millions of people could spread rapidly around the world as a result.

A vaccine against such a strain could take up to a year to develop and, as a result, most countries are relying on Tamiflu to provide the necessary protection for their citizens. The drug should alleviate symptoms and also limit the spread of the disease from person to person.

In Britain, health officials have persuaded the government to stockpile Tamiflu tablets. It is estimated that about 15.6 million doses are now in storage, ready for use. 'The aim is that people will take the drug as soon as the first outbreak is reported,' explained Singer.

'It is estimated that more than 100 million tablets, each containing 75mg of Tamiflu, could be consumed in the first weeks of a flu outbreak. Most of that will pass through people and be released in their urine. It will then be flushed away. Several tons of a powerful drug whose ecological behaviour is unknown will therefore be swept into our waterways, where it will remain active for several weeks in the country's sewers, rivers and lakes.'

Computer models developed by Singer and his colleagues at the CEH have shown that large areas of polluted water would quickly be established in many areas, probably within a few weeks of the flu cases first being reported. 'Tamiflu is an anti-viral agent, but is also known to attack bacteria - and that poses real problems for the country,' Singer said. 'Animal species have symbiotic relationships with many kinds of microbes. Human digestion relies on bacteria in our gut, for example. If these bugs are killed off by the tons of Tamiflu being dumped in our waters, all kinds of devastation could be triggered. Fish in our rivers, birds that feed off them and various kinds of river plant life could be seriously affected.'
In addition, there will also be serious implications for bird flu. A build-up of Tamiflu in our rivers is likely to cause the avian flu virus - which can infect ducks, swans and other wildfowl - to mutate so that it becomes resistant to the drug. The result will be the triggering of a second wave of the disease, though this time it will be resistant to the only form of defence we will have developed to counter it. The consequences could be extremely serious.

Singer - whose work is funded by the Natural Environment Research Council - believes health officials urgently need to tackle the problems raised by his research. 'There are things that we can think about doing,' he said. 'One course of action would be to develop chemicals that could break down Tamiflu once it has passed through a person's body but before it is flushed away. You could simply pour these into toilets to deactivate the drug before it is released into the environment. Certainly, we badly need to do a lot more research into this issue.'