The sick cats were among 170 living at an animal shelter where the disease was detected in chickens last month, authorities said.
The World Health Organization called bird flu a greater global challenge than any previous infectious disease, costing global agriculture more than $10 billion and affecting the livelihoods of 300 million farmers.
Meanwhile, U.S. health officials announced plans for a second vaccine to protect people from bird flu because the virus that is spreading among birds in Asia and Europe has changed significantly in the past year.
The government has several million doses of an earlier bird flu vaccine, but it was based on a sample of virus taken from Vietnam in 2004.
The germ is believed to have mutated enough since then that the form now circulating in Africa and Europe may be different, health officials said.
U.S. Health and Human Services Secretary Mike Leavitt said Monday he had authorized the National Institutes of Health and the federal Centers for Disease Control and Prevention to begin working on a second vaccine for humans.
In other developments:
Poland reported its first outbreak of the disease, saying Monday that laboratory tests confirmed that two wild swans had died of the lethal strain.
Dr. Margaret Chan, who is spearheading WHO's efforts against bird flu, told disease experts meeting in Geneva to discuss bird flu preparations that the organization's top priority was to keep the deadly strain from mutating into a form easily passed between humans. That could trigger a global pandemic.
Since February, the virus has spread to birds in 17 additional countries in Africa, Asia, Europe and the Middle East, she said.
"We truly feel that this present threat and any other threat like it is likely to stretch our global systems to the point of collapse," said Dr. Mike Ryan, WHO's director of epidemic and pandemic alert and response.
WHO spokeswoman Maria Cheng said experts hope to isolate outbreaks and establish agreements allowing international health authorities to respond quickly, testing viruses and putting in place measures to contain the disease.
In Austria, all the cats from the affected shelter have been moved to a location where they will remain under observation. The shelter has been closed, Health Minister Maria Rauch-Kallat told reporters in Vienna.
"We have decided to put all the cats in quarantine," Rauch-Kallat said. "Here they will be observed by veterinarians and experts in the coming days and weeks."
German authorities last month confirmed that a cat on the Baltic Sea island of Ruegen had succumbed to the deadly virus, which it is believed to have caught by eating an infected bird.
That would be consistent with a pattern of disease transmission seen in wild cats in Asia.
German officials have warned pet owners to keep their cats indoors and dogs on a leash in areas where the disease has been detected
Austria confirmed the nation's first case of H5N1 in a wild bird last month and has since detected several dozen cases in birds, including 29 in Styria.
According to WHO, several tigers and snow leopards in a zoo and several house cats were infected with H5N1 during outbreaks in Asia in 2003 and 2004.
Poland announced that the infected swans were found dead last Thursday in Torun, about 120 miles northwest of Warsaw. Samples were being sent to Britain for further tests.