Health workers pack dead chickens into trash bins at a wholesale poultry market in Hong Kong on Dec. 31 after bird flu was found in poultry imported from China. (Reuters/Tyrone Siu)

The virus causing a second wave of bird flu across China has mutated frequently and "should be considered as a major candidate to emerge as a pandemic strain in humans," researchers reported Wednesday.

While it is much too early to predict whether that might happen, one of the scientists said in an interview, there is cause for alarm because the H7N9 virus jumps to humans more quickly than its predecessors and previously has been found in mammals.

"This virus is more dangerous," said Yi Guan of the University of Hong Kong, one of the authors of a research letter published online Wednesday in the journal Nature.

It's not clear why the outbreak, which began in late 2013, has re-emerged after fading. But by September 2014, it had infected 318 people and killed more than 100 of them, twice as many as the first wave, the scientists reported. Many people suffer severe pneumonia if infected by this flu, which also has spread to China, Taiwan, Hong Kong, Malaysia and Canada, according to the World Health Organization.

"What we don't know from this paper is the significance of all these mutations that are accumulating as the virus persists and spreads," Wendy Barclay, an expert in flu virology at Britain's Imperial College London, told Reuters. "This is especially relevant for human health -- does any of this change the pandemic potential of the virus?"

Another variant of bird flu, H5N1, has resulted in nearly 650 human cases in 15 countries since 2003.

Also Wednesday, the U.S,. government said it had found a case of a different bird flu in Arkansas, part of an outbreak that appears to be spreading from commercial turkey flocks Missouri and Minnesota. The H5N2 strain of avian influenza has not been found in humans and the risk to people is considered low, according to the U.S. Department of Agriculture.

[First human case of bird flu found in North America]

In China, the researchers went to live poultry markets in 15 cities and determined that chickens were responsible for the rapid spread of the bird flu. Humans were infected in seven of the cities where H7N9 viruses were found in chickens, they reported. With the flu probably spreading along poultry trade routes, "it is probable that the H7N9 virus is now present across most of China," they wrote.

"Given the current pattern of dissemination, it will only be a matter of time before poultry movements spreads this virus beyond China by cross-border trade," they added.

Guan said that authorities must take steps quickly to slaughter infected poultry, close live poultry markets and prevent export of the birds. Unless that happens, the researchers wrote, "it is reasonable to expect the H7N9 and other viruses to persist and cause a substantial number of severe human infections."