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BEIJING — China is investigating four possible cases of human-to-human transmission of a deadly bird flu that has killed 17 people, but there has been “no sustained” evidence of transmission between people, the World Health Organization (WHO) said Thursday.

Three families in Shanghai and two children in Beijing were being examined as possible examples of human-to-human transmission, said Gregory Hartl, a WHO spokesman in Geneva.

“Even if two family members are positive, it is not necessarily the case they got it from each other. They may have gotten it from the same bird.”

As investigators looked at the possibility of human transmission, there was mounting concern that the new virus, H7N9, may not originate in birds but in other animals and in environmental sources, he said.

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To that end, some international influenza experts who were invited by China to help investigate the virus arrived in Beijing on Thursday.

The experts — from WHO and the U.S. Centers for Disease Control and Prevention (CDC) — would be looking at possible sources for the virus other than birds, Hartl said.

A Chinese expert on the disease, Feng Zijian, director of the health-emergency center at the Chinese Center for Disease Control and Prevention, said about 40 percent of people infected with the virus said they never had contact with poultry.

Seventeen people have died since China told WHO on March 3 of the bird-flu outbreak, according to China’s state-run news agency, Xinhua. There were 83 cases of infection, the news agency said.

Even as the international investigators would be seeking other sources of infection, China’s agricultural authorities were insisting the H7N9 virus was still confined to live poultry markets. The news agency said that 47,801 samples had been collected from 1,000 poultry markets, habitats and farms from across China, and that agricultural authorities said 39 tested positive for H7N9.

Hartl noted that the percentage of positives was very low.

Early suspicions that pigs might be the carrier of the virus have not been confirmed, Hartl added.

Chinese authorities had informed WHO about three families in Shanghai where more than one person was infected with the virus, Hartl said. In two, two people were infected, he said, and in the other, three were infected. In that case, an 87-year-old man and his 55-year-old son died, and his other son, 69, was sickened, Hartl said.

The two children infected in Beijing, a boy and a girl, were neighbors and often played together, Hartl said. It is possible, he said, that they may have picked up the virus from the same infected bird.

At a news conference, Feng played down the possibility of “effective” human-to-human transmission.

There was “currently no evidence showing that H7N9 carries continuous infecting power,” Feng said.

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