Cats are significantly more likely to catch and pass on bird flu than has generally been thought and could help the virus to mutate to cause a human pandemic, scientists said today.

The pets’ role in the spread of the H5N1 virus, and the potential risk they pose to their owners, have been underestimated by public and animal health bodies, according to a team of leading virologists from the Netherlands.

Research at the Erasmus Medical Centre in Rotterdam has shown that the cats catch bird flu reasonably easily, either by close contact with infected birds or by eating them, and that they can transmit the virus to other cats.

This could give the H5N1 virus new opportunities to adapt to mammals, including humans, making the emergence of a pandemic strain that spreads easily from person to person more likely, the scientists said in the journal Nature.

Albert Osterhaus, who led the work, said that the findings make it important for cats to be confined indoors in areas where the disease is endemic, to limit their contact with infected wild birds or poultry.

Affected regions of Germany and France have already ordered that cats be kept indoors, but the Department of the Environment, Food and Rural Affairs (Defra) has not yet said whether it would make similar provisions should bird flu reach Britain.

Dr Osterhaus said that while none of the 191 human cases of bird flu has been contracted from a cat, it is impossible to rule out such transmission. Though infected cats shed much less of the virus than do birds, the very close contact that many owners have with their pets could potentially put them at risk.

"The point we are trying to make is that as soon as the virus becomes endemic in wild birds or poultry, it would be wise to realise that cats are susceptible animals," he said. "As soon as you have birds that become sick cats are very effective a catching and eating them. Our advice is that in endemic areas you should keep cats indoors and dogs on the lead.

"There is also a public health concern. Although the risk is not large because the level of virus excretion is lower than birds, there is a concern because people tend to take good care of sick pets. It is unlikely that people will get too close to a chicken, but many people do with cats. You would not want an infected cat in a household situation. There is still no need to panic, but if you’re in an endemic area you should keep your cats indoors."

The first cases of H5N1 among cats were reported in Thailand in 2004, when 14 out of 15 cats in a household near Bangkok became ill and died after eating an infected chicken carcass. Infections were then identified among leopards and tigers fed on poultry at Thai zoos. In Europe, cases have recently been confirmed among domestic cats in Germany and Austria.

Dr Osterhaus said it was also clear that infection is prevalent among domestic cats in countries where bird flu is endemic such as in Thailand, Iraq and Indonesia.

In the last of these, the feline version of the disease has its own name: "It is sufficiently well known to have been given an onomatopoeic name in the local Javanese dialect — ‘aargh-plop’," he said.

Experiments in Rotterdam have shown that cats can be infected via the respiratory tract, by eating infected meat, or by contact with other infected cats. While the overall risk to humans is difficult to assess, it is impossible to rule out, Dr Osterhaus said.

"Apart from the role that cats may play in H5N1 virus transmission to other species, they also may be involved in helping the virus to adapt to efficient human-to-human transmission," he said.

"Cats may provide the virus with an opportunity to adapt to efficient transmission within and among mammalian species, including humans, thereby increasing the risk of a human influenza pandemic."

"We believe that the potential role of cats should be considered in official guidelines for controlling the spread of H5N1 virus infection. Most international guidelines currently lack such considerations."

"The possible role of cats in the epidemiology of H5N1 virus infection has been largely overlooked by the human- and animal-health communities."

The warning was issued as Defra began a two-day exercise to simulate an outbreak of bird flu in Britain. Hundreds of people are taking part in Exercise Hawthorn, a scenario in which a highly pathogenic form of the virus is identified on a free-range farm in Norfolk before spreading to the North of England and South Wales.

• The highly pathogenic H5N1 strain of bird flu was found in domestic poultry in Germany today, in only the second case to be identified in farmed birds in Europe. France was the first EU country to report an H5N1 outbreak on a farm in February.

The virus was brought to Germany by wild, migrating birds in February but had not infected any farmed poultry. More than 16,000 turkeys, chickens and geese will be culled in the farm in Wermsdorf, near Leipzig, health officials said.

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