Chinese researchers have found that H7N9 influenza viruses are transmissible in ferrets by respiratory droplets, warning that it is possible the virus may be able to efficiently spread between humans eventually.

In a study published on July 19 on the website of the US journal Science, they reported that one virus isolated from humans was highly transmissible in ferrets, a common animal model for studying flu transmission, by respiratory droplets, saying their findings "indicate nothing to reduce the concern that these viruses can transmit between humans."

"The H7N9 virus still exists in animals and continues to evolve, and the elimination of the virus from nature is a huge and long-term challenge," Chen Hualan, a professor with Harbin Veterinary Research Institute of the Chinese Academy of Agricultural Sciences, who led the study, was quoted as saying by the Xinhua News Agency on Friday.

"We must have material and technical reserves, including policies and measures for possible reemergence of the H7N9 virus in the future, otherwise the virus could hit the world hard," said Chen.

The findings come after news that a 61-year-old woman from Langfang, Hebei Province, was confirmed to be suffering from H7N9 after a period of several weeks with no new infections.

The woman, a vegetable seller who worked in a market, is being treated in Beijing Chaoyang Hospital, according to a statement from Beijing Municipal Health Bureau.

Despite the latest case, Zhong Nanshan, a renowned respiratory disease expert in China who was instrumental in leading the treatment of SARS-infected patients in 2003, said that people should not be unduly worried about the risks, despite the new case.

"Bird flu occurs most often in spring, generally from March to May. However, it is not surprising to see a new case reported in Hebei at this time. Even sporadic new SARS cases were reported now and then. This is just an individual case, which could not be called a breakout of bird flu," said Zhong.

"Chen's findings were obtained in the laboratory. There is currently no evidence in epidemiology that the H7N9 virus can spread among people," he noted.

To investigate the possible origins of the H7N9 viruses that caused human infections, Chen's team collected over 10,000 samples from poultry markets, poultry farms, wild bird habitats, and poultry and swine slaughterhouses across China from March to May this year, Xinhua reported.

By systematically analyzing H7N9 viruses isolated from birds and humans, they found the viruses were genetically closely related. Viruses isolated from birds were nonpathogenic in chickens, ducks, and mice. However, the viruses isolated from humans caused up to 30 percent body weight loss in mice.

As of Sunday, 133 cases of H7N9 avian flu were reported on the Chinese mainland, 43 of whom have died.