The findings confirmed what had been observed in the cats in Thailand.

First, Dr. Kuiken's team introduced the Vietnamese virus into the airways of three European shorthair cats, the breed generally used in animal experiments.

All three became sick beginning the next day, and one died on the sixth day of illness.

In comparison, none of three cats infected with the most common type of human influenza virus became ill.

In the second experiment, three cats were fed infected chicken.

Examination of their tissues under a microscope showed that all three had developed severe lung damage similar to that seen among birds and humans. (People are not vulnerable to infection by eating chicken that is cooked, but the person who cooks it may be at risk from handling it, health officials say.)

In the third experiment, the researchers put two healthy cats in the same cage two days after infecting a third cat.

The healthy cats also became ill.

Dr. Kuiken said in telephone interviews that he did not know whether these two cats had caught the infection by licking, through droplets or through the air.

His study, he said, was not devised to determine how the cats spread the virus.

Additional research is needed because of the small size and scope of the Dutch study, experts said.

But the Food and Agriculture Organization "is not set up to conduct this type of research," Dr. Lubroth said, adding that scientists at universities and other research institutes would have to do much of it, though with technical advice from his agency.

One avenue of research will be to test whether cats that are susceptible to other strains of influenza virus can spread those strains as well.

In addition, Dr. Kuiken said his team planned to test whether the original A(H5N1) virus, from the 1997 Hong Kong case, could infect cats, or whether only the later, mutated form could do so.

At the same time, Dr. Lubroth said, agricultural workers need to educate farmers about good practices like not raising swine with chickens.

Another reform will be teaching farmers to keep cats away from poultry, although that step, Dr. Lubroth said, "may be as difficult as herding wild cats.''