PARIS—France has raised its bird flu alert level after a severe form of the virus was found among backyard poultry in the northeast, on top of cases in neighboring Belgium and Luxembourg, the farm ministry said on Friday. The highly contagious H5N8 strain of avian influenza was found this week among ducks, hens, turkeys, and pigeons belonging to a household in the Ardennes region, with all the animals slaughtered as a precaution, the ministry said in a statement. “The health situation regarding highly pathogenic avian influenza is worrying. Since Aug. 1, 25 cases have been detected in Europe among wild and captive birds,” it said. Two cases of H5N8 were reported last week in Belgium—one at a bird merchant and one at a private home—and another at a home in Luxembourg that has been linked to the merchant in Belgium, the French ministry added. The latest cases involving the H5N8 …