Indian health officials walk through Namopara Bazar village to cull birds to curb the spread of bird flu in Margram, north of Calcutta, 18 Jan 2008

Officials in India are closely monitoring health workers amid the country's worst outbreak of bird flu.

In the eastern state of West Bengal, hundreds of workers are being monitored for symptoms of the virus that has now spread to 13 of 19 districts in the state.

Veterinary and medical staff involved with the slaughtering of birds have been asked to quarantine themselves as a precautionary measure before they return home. Hospitals have set up isolation wards to deal with the outbreak.

State officials say more than 2.5 million birds have been killed since bird flu was first detected in the Birbhum district of West Bengal in early January.

There have been no confirmed human cases of bird flu in India, and health officials continue to send blood samples for testing to make sure the virus has not spread to people.

In the neighboring countries of Pakistan and Bangladesh, officials have ordered the slaughter of thousands of chickens after bird flu was detected.

The World Health Organization says 224 people around the world have died from the deadly H5N1 strain of bird flu since 2003.

Health experts fear it will mutate into a form that could be easily spread among humans, triggering a global pandemic.

Some information for this report was provided by AFP, AP and Reuters.