China has confirmed that a father and son who were sickened with bird flu are the country's first infections within the same family, but said their cases showed no evidence that the virus has changed into a form that can easily be passed between humans, according to the World Health Organization.

The 24-year-old son from the eastern city of Nanjing died Dec. 2, becoming China's 17th fatality from the H5N1 bird flu virus. His 52-year-old father began showing symptoms a day later and was confirmed to have the disease.

State media said the man, identified only by his surname Lu, was released Dec. 26 after 20 days in a hospital.

"The outbreak was a confirmed family cluster of human infection with H5N1 avian influenza between blood relatives for the first time in mainland China," Hans Troedsson, WHO's representative in China, said Friday.

A man who answered the telephone at the Ministry of Health's press office confirmed the Nanjing cases were the first family infections but did not give any other details.

More than 80 people who had come in contact with the two men were monitored, but so far there have been no other reported infections.

India outbreakSeparately, India's worst bird flu outbreak spread to two new districts in eastern India, including an area close to Calcutta, raising fears that the disease could spread to the crowded city, authorities said Friday.

Officials in the state of West Bengal, the center of the outbreak, have struggled to contain the disease, which now has spread to 11 of the state's 19 districts. Their efforts to slaughter as many as 2 million birds have been severely hampered by stifling bureaucracy, a shortage of qualified personnel, ignorance among villagers and bad weather.

Officials have killed roughly 1.3 million birds, mainly chickens, over the past 10 days and will continue slaughtering birds that may have been infected over the weekend, said state Animal Husbandry Minister Anisur Rahman.

While the outbreak has so far been largely confined to rural areas, officials worry the disease will spread to Calcutta, one of the country's biggest cities with a population of 14 million. India has successfully contained two previous outbreaks, which both were in large poultry farms.

This outbreak has largely struck chickens kept by peasants in their small yards, and many villagers were unaware of the danger.

Bird flu has killed at least 221 people worldwide, according to WHO. Scientists have warned that if outbreaks among poultry are not controlled, the virus may mutate into a form more easily passed between people, potentially resulting in millions of deaths.

No evidence of genetic changesWhile the Ministry of Health in China "has not ruled out the possibility that the second case might have acquired infection from the first case, there was no evidence ... that there were any changes in the genetic sequences that make the virus more efficient in human-to-human transmission," Troedsson said.

Six days before the onset of his illness, the son visited a market where live poultry were slaughtered and sold, possibly exposing him to the virus, Troedsson said.

While the father had direct contact with his son's respiratory secretions and waste, "the Ministry of Health could not completely rule out the possibility of his separate exposure" to the market, he said.

The Chinese mainland has not confirmed any cases of human-to-human infection, although the sister of a Chinese boy who was diagnosed with H5N1 in 2005 later became sick and died. Authorities were not able to confirm whether the girl had been infected with H5N1.

Possible human-to-human transmission of the hard-to-treat H5N1 virus has been reported in Hong Kong, Vietnam and Indonesia, but officials determined there was no epidemiological significance because the spread was not sustained.