Culling began today in bird-flu hit Nadilag and its surrounding areas of West Tripura district as the state government said neighbouring Bangladesh could be the possible source of infection. As the H5N1 virus was confirmed in blood samples of dead birds from Nadilag village by the High Security Animal Diseases Laboratory in Bhopal, the authorities started the operation, setting a target of culling 20,000 chicken in eight gram panchayat areas surrounding Nadilag in four days. In the Mohanpur area of Sadar sub-division in the same district, culling continued for the fourth day today. Another day is left to complete the target of culling 60,000 birds there, official sources said. Meanwhile, samples were collected from seven other areas of Ranirbazar in Sadar subdivision and sent to the Bhopal laboratory yesterday following the death of about 1,000 birds at a farm there during the past one week, the sources said. On April 6, samples from the Malaya grampanchayat area in Dhalai district tested positive and 42,000 birds were culled. Import of poultry from outside the state, including Bangladesh with which the state shares an over 850-km-long border, has been banned. Of the 64 districts in Bangladesh, 47 are reported to be affected by the disease. "All the bird-flu hit areas are near the border and as the neighbouring country is badly hit by the avian influenza there is every reason to believe that the infection came from Bangladesh,' the Animal Resource Department said.