BEIJING (Reuters) - Chinese health authorities said onMonday they were hunting for the causal link between a son andhis father both struck by bird flu but have found no evidencethat the virus has mutated into a new strain.

The 52-year old father was diagnosed with the H5N1 strainof bird flu late last week in the eastern province of Jiangsu,days after his 24-year old son died from the disease.

This rare case of two family members struck by the diseasehas drawn urgent concern from health authorities, becausehumans almost always contract H5N1 from infected birds. Expertsfear the virus could mutate into a strain that jumps easilyfrom person to person, risking wider outbreaks.

Chinese Health Ministry spokesman Mao Qun´an said analysisof a sample taken from the dead son indicated the bird fluvirus has not mutated, but he could not exclude the possibilityof human-to-human infection in this case.

"The virus is still avian and has not undergone a mutationin its nature," he told a news conference.

Mao said one of the men might nonetheless have infected theother through close contact, or they might have become infectedfrom another source or separate sources.

"We can´t offer a final determination on these threepossibilities," he said, according to a government Web cast(www.china.com.cn). "There is a thorough investigation underway."

Joanna Brent, spokeswoman for the World Health Organizationin Beijing, said it had no further information on theinfections.

The official Xinhua news agency earlier reported that theson had had no contact with dead poultry and there had been noreported poultry outbreak in Jiangsu province.

With the world´s biggest poultry population and millions ofbackyard birds, China is at the centre of the fight againstbird flu. There have been other cases of human infectionwithout confirmed outbreaks among birds in the same area.

A Hong Kong newspaper controlled by the mainland said onMonday that the two men had both eaten chicken that was notfully cooked. The Ta Kung Pao paper cited unnamed sources assaying they had eaten the partly raw chicken in a restaurant inNanjing, capital of Jiangsu.

The latest cases bring the number of confirmed humaninfections of bird flu in China to 27, with 17 deaths.

Spokesman Mao said the father was now in a stable conditionand showing signs of improvement.

(Reporting by Chris Buckley; editing by Nick Macfie andRoger Crabb)