DHAKA, Jan 4 (Reuters) - Bird flu has spread to another district in Bangladesh, forcing health and veterinary workers to cull around 1,500 birds, officials said on Friday.

The latest H5N1 infection was reported in Kurigram, 380 km (240 miles) north from the capital, said Salehuddin Khan, director of the government's livestock department.

Bird flu was first reported near the capital in March last year and has since spread mainly to northern districts, forcing authorities to kill more than 300,000 chickens.

With the latest outbreak, 21 of Bangladesh's 64 districts have been affected by the deadly virus.

There are around 150,000 poultry farms in Bangladesh, with an annual turnover of $750 million, officials said.

About 4 million Bangladeshis are directly or indirectly associated with poultry farming, but so far there have been no cases of human infection in the densely populated country, government and health officials say.

Experts fear the bird flu virus might mutate or combine with the highly contagious seasonal influenza virus and spark a pandemic that could kill millions of people.

Bird flu has killed more than 210 people in 12 countries since 2003, the World Health Organisation says. (Reporting by Ruma Paul)

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