The findings, Dr. Lubroth and the study's authors said, underscore a need to investigate the possible role of cats and an array of other animals in the spread of avian influenza among poultry farms and to humans.

An estimated 200 million birds have either died of A(H5N1) or been slaughtered to control the outbreak since last winter, when the strain simultaneously appeared in eight Asian countries.

United Nations officials have described the scale of the epidemic - geographically and economically - as unprecedented for an avian flu outbreak.

The strain has also been particularly lethal for humans, killing 25 of the 35 people infected.

Many influenza experts and health officials fear a worst-case occurrence in which a person becomes infected with both an avian influenza virus and a human one.

Under such a circumstance, the viruses might swap genes, creating a new virus that could cause an epidemic all over the planet much like that of the so-called Spanish flu of 1918-19, which killed 675,000 people in the United States alone and more than 20 million around the world.

The laboratory in Rotterdam that reported the new findings has conducted research on A(H5N1) since 1997, when its scientists detected the strain in a child who had died of the disease in Hong Kong.

The Hong Kong case was a scientific bombshell, because it was the first in which a new avian influenza virus had been transmitted from birds to humans without first mixing with mammalian influenza strains in pigs.

Since then, the A(H5N1) virus has mutated to become more virulent.

Last January a clouded leopard died, apparently of avian influenza, at a zoo in Thailand after eating virus-infected chickens, Thai health officials recalled in recent interviews in Bangkok.

A month later, scientists identified the A(H5N1) virus in three dead cats, and in a white tiger that recovered after becoming ill in the same zoo where the leopard died.

The cats belonged to a Thai woman who had 15 in all, 14 of which apparently died of avian flu, although the remains of only those 3 could be found for testing.

The woman did not develop bird flu.

Tests showed that the molecular makeup of the viruses isolated from the cats and the tiger was the same as that of the virus found in chickens.

After learning about those infections, the Rotterdam team, led by Dr. Thijs Kuiken, conducted three laboratory experiments by using the A(H5N1) virus isolated from a Vietnamese patient who had died of it.