Pigeon fanciers in eastern China are caging, cleaning and vaccinating tens of thousands of the birds in an attempt to contain the spread of avian flu as the death toll from the disease continues to mount.

"We've been cleaning a lot of cages," said a man from the Beijing Fengtai District Carrier Pigeon Association who gave his name as Mr Wang. He added that the Beijing government had required the association to cancel upcoming long-distance races and instruct members to keep their birds caged for at least a month.

State media reported four new cases of H7N9 avian influenza on Monday, including one death. Since last month, the virus has infected 24 people and killed seven of them; cases have been reported in Shanghai and throughout three eastern provinces. The World Health Organisation said it had not yet found evidence that the virus could be transmitted from person to person.

In Shanghai, authorities ordered the slaughter of at least 100,000 chickens, ducks and geese after a national laboratory found traces of the virus in two pigeons near one of the city's live poultry markets.

Now the country's pigeons have become a focus for concerns over the spread of the disease. The Hangzhou Carrier Pigeon Association said it planned to suspend races and vaccinate up to 90,000 pigeons, according to state media. "Right now, pigeons in [Shanghai's] squares and public parks have all been caged," reported the China Sports Daily website.

Pigeon racing is a popular pastime in China dating back to the Ming Dynasty, which ended in 1644. Almost 300,000 people across the country participate in the sport.

According to the news portal Zhejiang Online, a complaints hotline in the metropolis Hangzhou received a flurry of pigeon-related calls on Sunday, when local media reported that samples of pigeon feathers found at a local market had tested positive for the virus.

Chinese researchers have identified the H7N9 virus as a "genetic mix from South Korean migratory birds and ducks from Zhejiang province", the independent magazine Caixin reported on Tuesday.

Chinese health authorities have not detected any flu-like symptoms in 621 people who have had close contact with infected patients, the director of China's National Health and Family Planning Commission, Liang Wannian, announced on Monday.

The outbreak has had an effect on China's capital markets: shares of mainland poultry-meat producers have slumped since reports of the disease surfaced last week. Chinese airline and hotel stocks have also taken a hit, suggesting fears that the outbreak could affect the country's tourism industry.