LONDON - A new strain of bird flu that can infect and kill animals has been discovered in chickens in poultry markets in China, according to a new study published on Wednesday in the journal Nature.

Having studied samples from birds for the H7N9 virus, researchers at the University of Hong Kong said genetic tests suggested the virus entered domestic ducks from wild birds and then infected chickens, which are probably the origin of infection to humans.

The research team also discovered a new virus called H7N7 in chickens. Lab tests showed H7N7 was also able to cause severe pneumonia in ferrets, which are usually used as proxies for humans in flu research.

Zhu Huachen, one of the leading authors of the paper, told Xinhua that H7 viruses probably transferred from ducks to chickens on at least two independent occasions and that reassortment with H9N2 viruses generated the H7N9 outbreak lineage.

Although the H7N7 viruses carry only some of the molecular markers present in the human H7N9 isolates, the authors suggested the current pandemic threat could extend beyond H7N9 viruses, and that long-term influenza surveillance was essential for early warning of new viruses and interspecies transmission events.

H7N9 bird flu has killed 45 people on the Chinese mainland since the first human infection was confirmed in late March, a health official said last week.