Jan. 21: The scale of the bird flu outbreak in Bengal enlarged today with samples from a block in Malda testing positive and the state government raising the culling target to 20 lakh birds.

Bengal health minister Surjya Kanta Mishra said the culling target had been revised to 20 lakh — a five-fold increase over the original figure. Given the current scale of the outbreak — and the new culling target — nine-tenths of the task remains to be done.

State animal resource development Anisur Rahman underscored the challenge posed by the affliction among backyard poultry. “How can we finish culling so quickly? We have to look for them in every house, nook and corner,” he said in Nadia.

Central officials said Chanchol-I block in Malda had been affected but officials in Bengal said they were not aware of this.

The outbreak — now confirmed in seven Bengal districts — has baffled experts who have still been unable to determine whether the virus is spreading from one district to another or has simultaneously struck multiple sites.

“We can only speculate at this point in time,” an animal husbandry official in Delhi said. It is possible that wild birds such as ducks, which can serve as carriers of the virus, are mingling with backyard poultry and moving the virus from one place to another, the official said.

The officials said a ban on the movement of poultry from Birbhum and South Dinajpur — the first two districts to be recognised as affected — had been ordered soon after avian influenza had been suspected.

“We’ve ordered that no movement of poultry takes place but this has to be enforced, and there may be practical problems in enforcement,” a senior official said.

Scientists said they could not rule out the possibility that the virus was not spreading, but was only being detected at different sites as the mortality of poultry there became starkly visible.

“It’s easiest to detect unusually high number of deaths when poultry density is very high,” said a scientist. “In backyard poultry where the number of chickens is small, it could take time to recognise that several birds are dying.”

But the officials pointed out that poultry deaths have been reported from only a few blocks in the districts. “We have to ensure that the virus does not spread into any other state,” Pradip Kumar, the animal husbandry department secretary, said.