Two more people in China have died from a new strain of avian influenza

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The latest victims were from the commercial capital of Shanghai, where the majority of the 77 cases have been found, according to local reports.



Because the source of the infection has not been effectively controlled, it is possible that the number of cases of infections could continue to rise, China's National Health and Family Planning Commission warned.



Zeng Guang, the chief scientist in charge of epidemiology at the China Disease Prevention and Control Centre (CDPCC), has said about 40 percent of the victims have no clear history of poultry exposure.



"How did these people get infected? It's a mystery," Zeng said.



Human-to-human spread of the virus has not been confirmed, but samples have tested positive in some birds in poultry markets that remain the focus of investigations by China and the U.N.'s Food and Agriculture Organization (FAO).

The exact source of infection remains unknown

How did these people get infected? It's a mystery Zeng Guang, chief scientist

The panic over bird flu has caused many Chinese to shun eating chicken for fear of catching the virus.



China's poultry sector has recorded losses of more than 10 billion yuan (£1.0bn) since reports emerged of the new strain two weeks ago, an official at the country's National Poultry Industry Association said.



Authorities have slaughtered thousands of birds and closed live poultry markets in Shanghai and Beijing in an attempt to reduce the rate of human infection.



As fears have heightened over the possibility of a pandemic, an international team of flu experts will go to China this week to help with investigations into the virus, the World Health Organization (WHO) revealed.



China said on Sunday the virus had spread outside the Yangtze River delta region in eastern China, with cases reported in the capital Beijing and the central province of Henan.



Experts have said the deadly strain is still evolving, making it hard for scientists to predict how dangerous it might become.



Influenza experts say the H7N9 strain is probably still swapping genes with other strains, seeking to select ones that might make it fitter.



If it succeeds, the world could be facing the threat of a deadly flu pandemic, scientists fear.



Genetic analyses has also shown the virus – which before March had never been seen in humans – has already acquired some mutations that make it more likely be able to spread between mammals, and more able to spark a human pandemic, scientists revealed.



A study in the online journal Eurosurveillance by leading flu experts Yoshihiro Kawaoka of the University of Wisconsin and Masato Tashiro at the National Institute of Infectious Diseases in Tokyo, said the H7N9 sequences "possess several characteristic features of mammalian influenza viruses, which are likely to contribute to their ability to infect humans".



These features, the scientists wrote, "raise concerns regarding their pandemic potential".

The virus had previously only been found in people direcly exposed to poultry