BEIJING, Dec 11 (Reuters) - China has warned of a "very high" possibility of outbreaks of bird flu over winter and spring as the country hunts for the causal link between a son and a father struck by the virus, state media said on Tuesday.

A 24-year-old man surnamed Lu from eastern Jiangsu province died last week of the H5N1 strain of bird flu in the first case in China since June.

Late last week, his 52-year-old father was also diagnosed with the disease, raising questions about how the man was infected.

China, with the world's biggest poultry population and millions of backyard birds, is seen as crucial in the fight against bird flu.

China's agriculture ministry was "not optimistic" about winter and spring, when the virus is at its most contagious, the China Daily quoted Yin Chengjie, a vice minister of agriculture, as saying.

Methods of poultry breeding, slaughter, delivery and processing still needed "radical changes", and prevention measures were not fully carried out in some regions, Yin said.

He called on local authorities to step up immunisation efforts and testing of birds at border areas and local wetlands.

China has now confirmed 27 human infections of bird flu, with 17 deaths.

The rare case of two family members struck by the disease has drawn urgent concern from health authorities, as humans almost always contract H5N1 from infected birds.

Xinhua earlier reported that the son had had no contact with dead poultry and there had been no reported bird flu outbreak in poultry in Jiangsu province.

Experts fear the virus could mutate into a strain that jumps easily from person to person.

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