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Associated Press
Ducks swim near a warning sign at an amusement park in Beijing, China, Wednesday, April 3, 2013. Scientists taking a first look at the genetics of the bird flu strain that recently killed two men in China said Wednesday the virus could be harder to track than its better-known cousin H5N1 because it might be able to spread silently among poultry without notice. The virus also appears to have mutated into a form that enables it to more easily infect animals such as pigs, meaning they could serve as hosts that spread the virus more widely among humans. (AP Photo/Alexander F. Yuan)

BEIJING — In a worrisome sign, a bird flu in China appears to have mutated so that it can spread to other animals, raising the potential for a bigger threat to people, scientists said Wednesday.

So far the flu has sickened nine people in China and killed three. It's not clear how they became infected, but there's no evidence that the virus is spreading easily among people.

But the virus can evidently move through poultry without making them sick, experts said, making it difficult to track the germ in flocks.

The findings are preliminary and need further testing.

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In the wake of the illnesses, the Chinese Center for Disease Control and Prevention shared the genetic sequence of the H7N9 virus with other scientists to help study how the virus might behave in different animals and situations.

One scientist said the sequence raises concern about a potential global epidemic, but that it's impossible to give a precise estimate of how likely that is.

"At this stage it's still unlikely to become a pandemic," said Richard Webby, director of a World Health Organization flu center at St. Jude Children's Research Hospital in Memphis, Tenn.

"We should be concerned (but) there's no alarm bells ringing yet," he said.

The virus has genetic markers that would help it infect people, Webby said.