KOLKATA, India, May 9 (Reuters) - Veterinary workers were getting ready to cull thousands of backyard poultry to contain an outbreak of bird flu in India's eastern state of West Bengal which has struggled to control the virus since January.

The communist-ruled state briefly contained the outbreak by culling more than four million birds in 14 of its 19 districts, but the virus has intermittently resurfaced.

Poultry sales in the state had fallen by about 70 percent in the January-March period, but traders said they were still struggling to overcome losses.

On Friday, officials said the virus has spread to the tea-growing Darjeeling district, the 15th to be hit by bird flu this year.

"Tests in a central laboratory confirmed the recent poultry deaths from the H5N1 strain of the virus in Darjeeling district," Rajesh Pande, a senior government official told Reuters.

Around 300 birds mysteriously died last week in Sukna area in the foothills of the Himalayas near the bustling northern town of Siliguri, he said.

"The deaths are all in backyard poultry and not in any farm," Pande added.

After a massive culling operation, authorities in West Bengal said in February that bird flu was under control.

The World Health Organization (WHO) has described the situation in West Bengal as India's worst bird flu outbreak in poultry. India's first outbreak of H5N1 was reported in 2006.

Officials in West Bengal said they were looking for people with flu-like symptoms.

India has so far not reported any human infections, but experts fear the H5N1 strain could mutate into a form easily transmitted from person to person, leading to a pandemic.

Since the virus resurfaced in Asia in late 2003, at least 241 people have died from bird flu in a dozen countries, the WHO says.

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