New Delhi: Scientists have claimed that the prevalent bird flu virus in China may be more widespread and extensive than thought.

The deadly virus that has killed more than 100 people this winter is hard to detect in chickens and geese, animal health experts say.

Poultry that have contracted the H7N9 strain of the avian flu virus show little or no sign of symptoms. That means any infection is only likely to be detected if farmers or health authorities carry out random tests on a flock, the experts said.

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That`s different to other strains, such as the highly pathogenic H5N6 that struck South Korean farms in December, prompting the government to call in the army to help cull some 26 million birds.

But that strain didn`t kill any people.

There have been multiple outbreaks of bird flu around the world in recent months, with at least half a dozen different strains circulating.

The scale of the outbreaks and range of viral strains increases the chances of viruses mixing and mutating, with new versions that can spread more easily between people, experts say.

For now, H7N9 is thought to be relatively difficult to spread between people. China`s Center for Disease Control and Prevention has said the vast majority of people infected by H7N9 reported exposure to poultry, especially at live markets.

As many as 79 people died from H7N9 bird flu in China in January alone, up to four times higher than the same month in past years.

(With Reuters inputs)