(Updates death toll after WHO statement)
BEIJING, April 22 (Reuters) - A man in the northeastern Chinese province of Shandong has been infected by a new strain of bird flu, the first case found in the province, state news agency Xinhua said on Monday, bringing the total number of cases in China to 105.
The H7N9 virus has killed 21 people in China, according to the Geneva-based World Health Organization (WHO). Although it is not clear exactly how people are becoming infected, WHO experts say there is no evidence of the most worrying scenario - sustained transmission between people.
"Investigations into the possible sources of infection and reservoirs of the virus are ongoing," the WHO said in an update late on Monday. "Until the source of infection has been identified, it is expected that there will be further cases of human infection with the virus in China."
A 36-year-old man from the city of Zaozhuang in Shandong was being treated in hospital, while two more people were infected in eastern Zhejiang province, Xinhua said.
A total of nine people in close contact with the victim in Shandong were under observation but were showing no signs of infection, Xinhua said.
An international team of experts led by the WHO has carried out field investigations into the response to the virus in Shanghai, where many of the cases have occurred, said Keiji Fukuda, WHO's assistant director-general for health, security and the environment.
"Right now we are in the middle of our work. We have not come up with any final conclusions, and I think it is too early to say," Fukuda told reporters.
Other bird flu strains, such as H5N1, have been circulating for many years and can be transmitted from bird to bird and bird to human, but do not generally pass from human to human.
But Ho Pak-leung, an associate professor in the Department of Microbiology at the University of Hong Kong, noted in the British Medical Journal (BMJ) that in the two months since it was first detected, the H7N9 flu has already resulted in almost twice as many confirmed infections in China as H5N1 caused there in a decade.
"H7N9 is much more transmissible to humans, and it's much more difficult to track down," he told the BMJ, adding: "We don't understand why it's so difficult to find."
He said he did not believe the difficulties were related to sampling techniques or testing levels, but thought it could be that virus can only be detected in infected animals for a few days, meaning the timing of tests could be important.
"It (also) might be that they are not sampling enough animal species, and they may have to take a look at the less common species of birds being sold in Chinese markets," he said.
The WHO's China representative, Michael O'Leary, issued data on Friday showing that half of the patients analysed had no known contact with poultry, the most obvious potential source, but he said it appeared human-to-human transmission was rare.
Some bird samples have tested positive and China has culled thousands of birds and shut down some live poultry markets. (Reporting by Sui-Lee Wee in Beijing, Kate Kelland in London and Adam Jourdan in Shanghai.; Editing by Nick Macfie and Andrew Heavens)