German officials warned cat owners yesterday not to sleep accompanied by their pets, and to keep them indoors, following confirmation that a cat has died of the H5N1 avian flu virus.

The cat was found at the weekend on the Baltic island of Rügen, near to where most of Germany's 121 cases of H5N1-infected wild birds have been found. Tests carried out on the animal by scientists at Germany's Friedrich-Loeffler institute confirmed H5N1, probably from having eaten infected birds.

Thomas Mettenleiter, director of the laboratory, said it was well established that when cats eat infected birds they can themselves become infected. There had been no confirmed cases of the virus moving from cats to humans, he said. "An infection of humans, which theoretically cannot be ruled out, could probably only occur with very intimate contact to infected animals," he said.

The H5N1 virus was first discovered in swans and wild birds in Germany two weeks ago. It appears to be spreading across western Europe.

Bird flu was detected yesterday for the first time in Bavaria in southern Germany, bringing to five the number of German states where the disease has been confirmed. Sweden also reported its first case yesterday.

The World Organisation for Animal Health said: "The spread of the infection to domestic poultry in other European and neighbouring countries is highly likely, and may even be made worse by the arrival in Europe of possibly infected birds from Africa and the Middle East next spring."

Germany's agriculture ministry said last night there was no reason to panic but warned that cat owners in affected areas should keep their pets indoors. Pet owners were advised to keep their cats at arm's length, and not to allow them on to their bed at night.

"It isn't easy for a cat to become infected. This must have happened in very unusual circumstances. Probably the cat ate a highly infectious animal," said Michael Schmidt, a virologist at Berlin's Free University. "It is very rare for an infected animal to infect humans. There have only been about 160 cases so far. Nevertheless, it's best if cat owners avoid taking their cats into their beds. They should keep a distance."

The news caused concern for the cat population in the UK. The British Veterinary Association urged cat owners to remain calm, pointing out that the animals were as yet not at risk, because the virus has not reached the UK.

Freda Scott-Park, president of the BVA, said that the risk to the cat family had been known of since tigers in Thailand had got HSN1 two to three years ago after being fed infected chickens, but vets had not talked about it openly for fear of a backlash against pet cats.

"We have got to be alert to the dangers to our domestic cats. We have been very well aware of it, but we haven't advertised the risks," she said."Already vets are having birds brought in by people saying please put them down in case they get avian flu. There is potentially a welfare problem of enormous proportions looming."

There is no vaccine to protect cats against flu, and unlikely to be one, because expensive clinical trials would have to be done to establish its effectiveness.

Avian flu does not transmit easily to humans. Those people who have caught H5N1 have handled sick birds. It has long been thought that, for the feared pandemic to occur, there would have to be a mixing of the avian flu virus with a human flu virus. That might well take place in another mammal.

A number of mammals are already known to get flu - the mouse family, rabbits, hares, ferrets, pigs, cats and macaque monkeys - as well as humans. But, said Dr Scott-Park, "current strains of the virus appear to be inefficient at infecting non-human species".

Of all the mammals that get flu, the most likely "mixing vessel" is not the cat, she said, but the pig. "Pigs are a superb mixing vessel, because they manufacture virus at a rate of knots," she said. "Pigs are a virus factory." They are also biologically similar to humans.

It was not clear last night whether the dead cat succumbed to the same strain of H5N1 which has already devastated poultry stocks across Asia and Turkey and has killed more than 160 people.