If confirmed it would be only the second case of H5N1 on a commercial farm in the European Union, said the European Commission.
The feared case was found in a mallard on a farm near the town of Oskarshamn, on the eastern coast of Sweden, where two wild ducks were confirmed earlier this week to have had H5N1, it said.
Initially the European Union’s executive issued a statement saying that H5N1 had been confirmed, but subsequently issued a correction saying only that it was “strongly suspected.”
The first case of the lethal strain on a farm – as opposed to in wild birds – was an outbreak on a turkey farm in France in late February, and came despite concerted efforts to keep H5N1 out of the human food chain.
All birds on the Swedish farm – around 500 mallards and 150 pheasants – were ordered to be immediately destroyed, and checks on other holdings in the area were stepped up.
“Swedish veterinary authorities appear to have the situation under full control,” it said, noting that no birds have been sent for slaughter or dispatched from the infected farm in the past weeks.
The farm involved is within a surveillance zone set up in response to a confirmed case of avian influenza in wild birds last month, said the EU commission.
On Wednesday this week Swedish authorities confirmed that the H5N1 virus had been confirmed in two wild tufted ducks found dead in Oskarshamn at the end of February.
In the latest case the virus was only found in one mallard on the farm. “In order to understand the situation better, further testing will be carried out on the birds on the farm following slaughter,” the commission said.
The H5N1 strain first arrived in the EU last month, and has now been confirmed in 11 EU countries: Austria, France, Germany, Greece, Hungary, Italy, Poland. Slovakia, Slovenia, Sweden and Denmark.
The H5N1 strain, in its most aggressive form, has killed nearly 100 people worldwide, mainly in Asia.
Philip Tod, spokesman for EU Health Commissioner Markos Kyprianou, said the new case vindicated the rapid response measures agreed by all EU member states in response to the growing bird flu threat.
“It justifies creating a surveillance zone around the area where a wild bird is detected with H5N1, in the same way as it did in France,” he told AFP.