(Removes reference to WHO in paragraph 6, making clear that the federal government criticised West Bengal notification delay)

By Bappa Majumdar

KOLKATA, India, Jan 16 (Reuters) - Veterinary workers began killing thousands of chickens in eastern India on Wednesday following what the World Health Organisation (WHO) said was the worst outbreak of bird flu in the country.

Officials said it could take up to a week to cull about 350,000 birds in three districts of West Bengal.

The WHO's India office said the fourth bird flu outbreak in India since 2006 presented its toughest challenge yet.

"More serious risk factors are associated with this current outbreak than previously encountered, including that the affected areas are more widespread," Indrani Dasgupta, a WHO India official, wrote in an email.

But Pradeep Kumar, India's animal husbandry secretary, emphasised that the outbreak was "localised" and under control. "There is no need for a scare," he told a news conference in New Delhi.

Kumar acknowledged some of the difficulties containing the threat. He said the West Bengal government took too long to tell New Delhi about suspicious bird deaths in the state.

And in Birbhum, the worst-affected district, where nearly half the adult population is illiterate and few own a television, it has been difficult to get villagers to observe stringent hygiene practices that might limit the spread of infection.

Local men wearing only cotton wraps around their waist were seen carrying dead birds with their bare hands and dropping them in shallow pits.

In some villages inside the quarantine zone, a Reuters photographer saw backyard poultry roaming in farmlands and mud roads as villagers said they were unaware about a culling operation and had let them loose.

"It is a very densely populated area ... it is not easy to create awareness," Kumar said.

The WHO said that while the central government was very well-prepared to deal with the disease, that may not always be the case at the state and district level.

Preliminary tests showed the highly contagious H5N1 strain had infected birds in the area, health ministry officials have said. H5N1 is the strain that scientists worry could mutate into a form easily passed between humans.

However, India's animal husbandry department said its laboratory confirmed only that it was an H5 strain, and establishing the N-type would take a few more days.

India has never reported a case of human infection, but health workers were checking people in affected areas for flu symptoms. In some villages, people queued up at local health centres for a check-up.

Experts from the West Bengal's animal husbandry department were also investigating the southern fringes of the state capital, Kolkata, after reports of backyard poultry deaths in the area.

West Bengal has sealed a stretch of its border with Bangladesh, which has been fighting to contain the spread of bird flu since March last year.

Bird flu has spread to another district in Bangladesh, forcing authorities to cull more than 1,500 birds, officials said on Wednesday.

With the latest outbreak, 72 farms in 23 of Bangladesh's 64 districts have been infected with the virus. (Additional reporting by Jonathan Allen in New Delhi and Parth Sanyal in Margram; Writing by Jonathan Allen)

AlertNet news is provided by