Mexico's top government epidemiologist said Wednesday that it is "highly improbable" that a farm in the Mexican state of Veracruz operated by Smithfield Foods Inc. is responsible for the nation's swine-flu outbreak.

Miguel Ángel Lezana, the government's chief epidemiologist, said in an interview that pigs at the farm are from North America, while the genetic material in the virus is from Europe and Asia.

Government...