An aggressive virus, avian flu has been plaguing poultry and wild birds in about a dozen countries in Asia over the past two years. Scientists say it's only a matter of time until a virulent new strain of the virus, known as H5N1, begins to spread among humans.

The belated announcement came amid fresh criticism that China's disease-surveillance system is inadequate to deal with an avian flu virus that scientists say may turn into a global pandemic among humans.

An outbreak of hoof-and-mouth disease, which causes livestock to waste away, was detected in cattle in five regions of China, leading animal-health experts to slaughter 4,383 head, the nation's chief veterinarian, Jia Youling, said at a news conference.

Rumors had spread in rural areas about the outbreak as animal-health agents began culling cattle and spreading disinfectant along roads. But the matter was subject to a news blackout, and journalists were barred from affected regions. As recently as this week, officials said they knew nothing about a hoof-and-mouth contagion.

Jia defended the belated announcement about the epidemic among dairy and beef cattle.

"There is nothing strange about the delay of 20 days," Jia said. "It takes quite a while after discovering a disease to confirm it. ... We have controlled the epidemic."

1,000 Migratory Wild Birds

Highly contagious hoof-and-mouth disease affects cloven-hoofed animals, such as pigs, cattle, sheep, goats and deer. It doesn't affect humans.

China came under fierce criticism in 2003 for its slow and secretive response to the outbreak of severe acute respiratory syndrome, or SARS, which sprang up in southern Guangdong province and spread to 30 countries, killing more than 770 people, mostly in Asia. Since then, China has pledged to be more open about outbreaks of infectious disease.

In another announcement, Jia said avian flu had killed 1,000 migratory wild birds in the northwestern province of Qinghai, but that it hadn't spread to poultry or humans. The toll marked a fivefold increase from previous reports China had offered to the Paris-based World Organization for Animal Health.

Earlier this week, authorities dispatched experts to vaccinate 3 million chickens, ducks and other poultry in Qinghai, a sparsely populated province in the Himalayan steppes that's a migratory route for birds between Central Asia and India. Jia said the wild birds found dead include bar-headed geese, cormorants and great black-headed gulls.

Only a Matter of Time

An aggressive virus, avian flu has been plaguing poultry and wild birds in about a dozen countries in Asia over the past two years. While the virus hasn't yet mutated to allow human-to-human transmission, it's killed about 40 people who came in contact with poultry. Scientists say it's only a matter of time until a virulent new strain of the virus, known as H5N1, begins to spread among humans.

An international weekly science journal, Nature, devoted a special issue this week to the threat of a global avian-flu pandemic among humans. It criticized China's lack of transparency and preparedness to deal with avian flu.

"There is little doubt that China will be in deep trouble if the flu pandemic were to strike in the next few years. It has a moral obligation to its own people, and to the world, to rectify the situation as soon as possible," wrote a virologist, David Ho, of the Aaron Diamond AIDS Research Center at Rockefeller University in New York City.