On January 15, India's agriculture officials reported the deadly H5N1 strain in two administrative districts in the state of West Bengal. The outbreak occurred at private farms in the Birbhum District, some 250 km (160 miles) from the state's capital, Calcutta, and at a state poultry farm in a nearby district.
Now the bird flu outbreak has spread to three more districts in West Bengal, the agriculture officials said.
About 100,000 birds have been culled in recent days and agriculture officials say a total of 800,000 poultry will be slaughtered in West Bengal to stem the bird flu outbreak.
In 2006, India reported a bird flu outbreak in the state of Manipur, in the northeast of the country. Hundreds of thousands of chickens were slaughtered at the time.
Although no cases of human-to-human transmission of avian influenza have been reported, scientists fear the virus could mutate into a strain that could pass easily from person to person, causing a global pandemic.
More than 200 people have died worldwide from bird flu since the first fatality was registered in 2003, according to the World Health Organization.