A deadly strain of avian influenza is starting to emerge in Asia, alarming international public health officials who say there is no vaccine for the disease. Their fear is compounded by the discovery reported this week in the journal Science that cats can become ill with the flu. Cats could provide another souce of viral illness, making it much harder to contain a human influenza outbreak.

International public health officials are bracing for what they fear may be a global influenza pandemic which they say could lead to widespread illness and death, and for which there is, as yet, no vaccine.

The problem is an aggressive A strain of the avian flu virus, which originates in Asia in birds and is passed to humans through live poultry usually bought at market for consumption. Pigs can also harbor the virus and pass it to humans.

Virologist Thijs Kuiken of Erasmus Medical Center in Rotterdam, Netherlands says researchers have now discovered another source of the avian flu virus: cats. "It's another potentional host, yes," he said.

Scientists had long suspected cats could harbor the avian flu but they hadn't been able to prove it.

In the latest study, researchers fed six cats with baby chicks infected with a sample of a virus taken from someone in Viet Nam who died of avian flu. Within days, the cats became ill with severe respiratory infection. They then passed the disease on to two healthy cats kept in close quarters.

Dr. Kuiken doesn't think house cats are much cause for concern. Instead, he worries about farm cats establishing a new source of infection.

"Cats could form an opportunity for the virus to adapt itself to mammals, and that could increase the possibility of a human influenza pandemic occurring," he noted.

The World Health Organization (WHO) is on high alert after receiving recent reports of sporadic deaths caused by the avian flu.

Klaus Stohrer of the WHO says virologists need to broaden their investigation into the sources of the illness.

"Now, what we don't know, for instance, is what other farm animals, mammals, what role they play," said Mr. Stohrer. "This virus had had very many surprises. And we should be proactive and get ahead of the curve now, not wait until the next animal species [is being] affected, and look at the large array of animals so that we can better anticipate what this virus might be doing in the future."

Dr. Stohrer and other international health officials are keeping a close watch on this year's A strain, which has been described as a ticking time bomb. It has so far struck provinces in Malaysia, Viet Nam, Thailand and China. While the number of deaths to date is small, many have occurred at top-notch hospitals with state-of-the-art care.