The avian influenza virus that spread widely among poultry and other birds in Southeast Asia has just crossed another species barrier previously thought impossible: it's jumped to domestic cats, formerly thought resistant to influenza A virus like the avian strain.

In an even more frightening development, researchers put two healthy cats in the same cage as an infected third cat, and the healthy cats became ill with the virus.

Thus, it has now been demonstrated to be transmissible within the cat population.

The avian influenza virus has proved extraordinarily virulent in humans, killing 25 of the 35 people who were infected.

Many influenza experts fear a worst-case scenario in which a person becomes infected with both an avian influenza virus - as has happened a lot in recent years - and a human one.

Under such circumstances, the two viruses might swap genes, creating a new virus variant that could cause an epidemic planet-wide, much like that of the so-called Spanish flu of 1918-19, which killed more than 675,000 people in the U.S. alone and over 20 million world-wide.

Here's Lawrence K. Altman's excellent piece from last Friday's New York Times.
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Study Finds Bird-Flu Virus Can Spread Among Cats

The avian influenza virus that has spread widely among poultry and other birds in Southeast Asia and infected some people there has also crossed another species barrier to infect cats, and can be spread among them as well, Dutch scientists have found.

The finding is "extraordinary because domestic cats are generally considered to be resistant to disease from influenza A virus infection," like that of the avian strain, the researchers are reporting in today's issue of the journal Science.

In the Dutch study, some cats with the infection died of it, while others survived. A few did not even show any symptoms that they were carrying the disease.

Whether cats can transmit the virus strain, A(H5N1), to humans is not known.

The World Health Organization has received no reports that cats played a role in afflicting the 35 people who have developed A(H5N1) infection, all in Thailand and Vietnam, said Dick Thompson, a spokesman for the agency in Geneva.

Those cases were traced chiefly to direct contact with sick birds.

Even so, the Dutch study has important implications for human and animal health, said Juan Lubroth, a senior animal health officer at another United Nations agency, the Food and Agriculture Organization.