The Netherlands is running tests on ducks, swans and gulls as well as bird manure in an effort to establish if avian flu is being spread by wild birds, junior economic affairs minister Sharon Dijksma said on Monday evening.

Test have already been carried out on some 500 wild ducks and a number of swans and gulls will also be caught for testing, Dijksma said. Bird excrement from a 10-kilometre radius around the farm where avian flu was first identified is also being tested for the virus.

Bird flu has been identified on four poultry farms in the Netherlands and the government has imposed a regional ban on the transport of eggs, birds and their manure.

Some experts said at the start of the outbreak the disease is probably being spread by migrating birds but others have disputed this.

‘The Netherlands and Germany are working closely together to try and find the cause of the outbreak,’ Dijksma said in her written briefing. Germany and the Netherlands are also to ask the European Commission for financial help in this, she said.