DHAKA (Reuters) - Bird flu has spread to another districtin Bangladesh despite efforts by authorities to contain it,taking the number of affected districts to 40, officials saidon Sunday.

Health workers culled nearly 12,000 fowls after testsconfirmed some chickens had died from the avian influenza virusin the northeast, livestock officials said.

The H5N1 virus, first detected in Bangladesh in March lastyear, was quickly brought under control through aggressivemeasures, including culling. But follow-up monitoring eased inlater months prompting the disease to reappear, experts say.

So far, no human infections have been reported inBangladesh, a densely populated nation with millions ofbackyard poultry and thousands of chicken farms.

The interim government has enhanced compensation forpoultry farmers to encourage them to report and kill sickbirds.

More than half a million birds to be culled across thecountry, but the virus has spread to more than half the SouthAsian country's 64 districts partly due to a lack of awareness.

Media reports said many children were seen smiling andplaying with dead poultry. Even health workers have been seenburying dead birds without any protective gear.

The World Health Organization worries that the H5N1 strain,which has already killed more than 220 people worldwide since2003, could mutate into a form that passes easily betweenhumans and infect and kill millions.

(Reporting by Ruma Paul; Editing by Bill Tarrant)